WO2005025726A2

WO2005025726A2 - Preparation of liquids for a medical procedure

Info

Publication number: WO2005025726A2
Application number: PCT/SE2004/001298
Authority: WO
Inventors: Göran Andersson; Mats Brink; Thomas Hertz; Lennart JÖNSSON; Henrik Lindgren
Original assignee: Gambro Lundia Ab
Priority date: 2003-09-12
Filing date: 2004-09-10
Publication date: 2005-03-24
Also published as: WO2005025726A3

Abstract

In a method for preparing a liquid for a medical procedure by mixing at least two liquid components, batches of liquid are successively prepared for continuous withdrawal for use in the medical procedure. The pro-portioning is done using a continuous measurement method A mixing chamber (7), a system (1) and a device (2) for preparing a liquid for a medical procedure are also disclosed, as well as use of such system (1) and device (2). Further a method and a mixing chamber for proportioning components of a liquid for a medical procedure are disclosed.

Description

PREPARATION OF LIQUIDS FOR A MEDICAL PROCEDURE

Technical Field of the Invention The present invention relates to a method for preparing a liquid for a medical procedure by mixing at least two liquid components, wherein batches of liquid are successively prepared for continuous withdrawal for use in the medical procedure, the method comprising: proportioning the liquid components for a batch of the liquid for the medical procedure, checking whether there is room for the batch in a storage vessel wherefrom liquid is withdrawable for use in the medical procedure, if there is room, passing the batch to the storage vessel and proportioning the liquid components for a successive batch of the liquid for the medical procedure, if there is not enough room in the storage vessel, waiting while liquid is withdrawn for use in the medical procedure until there is room in the storage vessel, passing the batch to the storage vessel and proportioning the liquid components for a successive batch of the liquid for the medical procedure. The invention further relates to a device, a system and a mixing chamber for preparing a liquid for a medical procedure. Background Art In haemodialysis operations, the blood of a patient suffering from impaired kidney function is conducted along one side of a permeable membrane in a dialyser device, while at the same time a dialysis fluid is conducted along the opposite side of the same membrane. The poisons or other waste substances that are to be removed from the blood pass with the help of diffusion and/or convection from the blood of the patient to the dialysis fluid through the permeable membrane. The dialysis fluid is normally prepared from dialysis concentrates diluted with suitably pre-treated water. These concentrates may be a so called A-concentrate which includes acetic acid, sodium chloride, potassium chlo- ride, calcium chloride and magnesium chloride, and a so called B-concentrate including bicarbonate. Dialysis concentrates are often prepared in centralised plants and delivered in liquid form in large drums . These bulky containers pose a logistic problem in terms of transportation as well as storage. Methods and systems have been developed wherein the dialysis concentrates are delivered in powder form and dissolved in suitably pre-treated water in the clinic. Examples of such methods and systems are described in e.g. EP-A-278 100. In this system water from a reservoir is passed through a vessel containing concentrate in powder form. The concentrate is thus dissolved to produce a liquid concentrate. Known systems for preparing dialysis concentrates use different methods for reaching the prescribed composition of the liquid, e.g. gravimetric, feedback or volumetric proportioning. Gravimetric proportioning, i.e. weighing, is potentially the most accurate method. However, it is difficult to apply in automated preparation of dialysis liquids. Feedback proportioning involves measuring the concentration of substances in the resulting liquid, and controlling the addition of the specific substance by closed loop feedback. The concentration of the resulting liquid may be determined by measuring e.g. the conductivity of the resulting liquid. The accuracy of this method is dependent on that the added substance contributes significantly to the conductivity and it is therefore not suitable for proportioning a substance added to water if the added substance has a conductivity close to the one of the liquid already present. Feedback proportioning also places high requirements on the accuracy of the conductivity sensors used. Volumetric proportioning is based on measuring the volume of liquid components to be added. This is gener- ally done using volumetric flow meters. However, it is difficult to achieve the accuracy necessary for obtaining the desired accuracy of composition in the resulting liquid. Combinations of these methods may also be used, but they still suffer from the abovementioned drawbacks. Preparation of dialysis concentrates may be done batchwise, wherein the volume used for one treatment session for a patient is prepared at a time, using one or more of the above-mentioned proportioning methods. A drawback of batchwise preparation of dialysis is that the composition of the dialysis concentrate cannot be altered during treatment of the patient. In some cases it is beneficial to have one concentration of a certain ion at the beginning of the dialysis session and another concen- tration at the end of the session, but this is not practicable when the dialysis concentrate is prepared batch- wise . Some systems effect on-line preparation of dialysis concentrate. One such system is marketed by the Applicant under the name BiCart Select®. This system includes a salt cartridge and a bag of ion solution. Water is passed through the salt cartridge to form a saturated salt solution and the salt solution, the ion solution and suitably pre-treated water are combined at a first mixing point in a conduit. At a second mixing point, bicarbonate is added. The resulting dialysis concentrate is passed to a consumer point in the dialysis machine where it is diluted to a physiologically acceptable dialysis fluid. In general in on-line preparation of dialysis concentrate, since the different components are added in a continuous water stream the sensors used for metering the amount added and for checking the concentrations of the differ- ent components have to be very accurate, as no corrections can be made in the resulting concentrate. Possible deviations, e.g. in the salt concentration, have to be quickly and accurately corrected by adjustment of the amounts added of the other components. However, this way of preparing dialysis concentrate affords a high degree of flexibility, since the composition of the concentrate being prepared may be altered at any given moment. US-A-3, 653, 640 describes an apparatus for preparing a dialysis solution, in which electrodes are used for measuring the level of components added. With this apparatus it is difficult to change the proportions of the components of the liquid to be prepared. Summary of the Invention The object of the present invention is to overcome or at least partly alleviate the above-mentioned drawbacks of prior art methods and systems for preparing a liquid for a medical procedure. Another object is to provide a method and a system which are more flexible than prior art methods and systems employing batchwise preparation. Yet another object is to provide a method and a system which may use more cost-effective sensors for proportioning than known methods and systems employing on-line preparation, while maintaining a high accuracy of the proportions in the resulting liquid. A specific object of the invention is to provide an accurate method for preparing an A-concentrate for the preparation of a haemodialysis fluid. According to the invention these objects are achieved by means of a method as defined in claim 1, preferred variants being defined in dependent claims 2-16. The objects of the invention are also achieved by means of a mixing chamber as claimed in claim 17 with preferred embodiments as defined in claims 18-32. The above-mentioned objects are further achieved by means of a device as defined in claim 33, preferred embodiments being defined in claims 34-55. According to the invention, these objects are also achieved by means of a system as defined in claim 56. The above-mentioned objects are also achieved by means of a use as claimed in claim 57, a preferred variant being defined in claim 58, as well as by means of a use as defined in claim 59. Further, these objects are achieved by means of a method according to claims 60-72, a mixing chamber according to claims 73-84 and a device according to claims 85-101. In the method of the invention, batches of liquid are successively prepared for continuous withdrawal for use in the medical procedure. The method comprises: proportioning the liquid components for a batch of the liquid for the medical procedure, checking whether there is room for the batch in a storage vessel wherefrom liquid is withdrawable for use in the medical procedure, if there is room, passing the batch to the storage vessel and proportioning the liquid components for a successive batch of the liquid for the medical procedure, if there is not enough room in the storage vessel, waiting while liquid is withdrawn for use in the medical procedure until there is room in the storage vessel, passing the batch to the storage vessel and proportioning the liquid components for a successive batch of the liquid for the medical procedure. The proportioning is done using a continuous measurement method. In this manner, a flexible preparation of liquid is achieved. According to a variant of the method of the invention, regulation of the proportions of the liquid components is achieved by gravimetric measurement of the liq- uid components. This is a convenient way of achieving an accurate proportioning. In another variant of the method, regulation of the proportions of the liquid components is achieved by volumetric measurement of the liquid components. This is another convenient way of achieving an accurate proportion- ing. In one variant of the method of the invention, regulation of the proportions of the liquid components is achieved by filling each liquid component to a separate predetermined level in a mixing chamber, the predeter- mined levels being based on predetermined volumes to be added and the cross sectional area of the mixing chamber. This is a convenient manner of achieving an accurate proportioning. In one variant, the level of each liquid component added is measured by an optical measurement method. This is one useful method of level measurement. In another variant, the level of each liquid component added is measured by a conductivity measurement method. This is another useful method of level measure- ment . In yet another variant, the level of each liquid component added is measured by an acoustic method. This is also a useful method of level measurement. The level of each liquid component added in a mixing chamber may be measured by ultrasonic measurement. The volume of each added component may thus in a simple manner be accurately monitored and the proportions of the added components easily adapted in accordance with any recipe . Variations in the velocity of sound in the liquid in the mixing chamber are preferably compensated for when measuring the liquid level. The accuracy of the measurement may thus be maintained even though there are changes in the velocity of sound as the different liquid compo- nents are added. In a first method of compensating for changes in velocity of sound ultrasound may be transmitted from a transmitter towards a first echo-generating surface at a first known distance from the transmitter and the time from transmission of the ultrasound to receipt of an echo by a receiver at a second distance from the echo- generating surface may be used for compensating for the velocity of sound in the liquid in the mixing chamber. This is an expedient manner of determining the current velocity of sound in the liquid in the mixing chamber. In another method of compensating for changes in ve- locity of sound, ultrasound is transmitted from a transmitter towards a first and a second echo-generating surface, said echo-generating surfaces being arranged at different known distances from the ultrasonic transmitter. The difference in time between echoes from said echo-generating surfaces received by a receiver is used for compensating for the velocity of sound in the liquid in the mixing chamber. In this manner, the accuracy of the compensation for the velocity of speed may be improved. In one variant of the method according to the invention, any variations in the cross sectional area along the height of the mixing chamber is compensated for for each liquid component by calculating adjusted levels to which the liquid components are filled. Thus, the accu- racy in the level measurement may be maintained even though the mixing chamber does not have a constant cross sectional area along its height. For instance, if a portion of the mixing chamber has a known smaller cross sectional area than the rest of the mixing chamber, a higher level is calculated for filling the same volume. The concentration of each liquid component may be measured and any discrepancies as compared to an expected concentration may be compensated for by calculating adjusted levels to which the liquid components are filled in the mixing chamber. The desired proportions in the resulting liquid may thus be assured, even though the con- centration of one or more of the added components is lower or higher than expected. In a variant of the method of the invention, the adjusted levels are based on known data on the velocity of sound for different properties of the liquid components. Corrections may thus be made easily and quickly. In a specific variant of the inventive method, a saturated sodium chloride solution is filled in the mixing chamber to a first level, water is subsequently added to a second level, the saturated sodium chloride solution and water are mixed in the mixing chamber to a homogeneous diluted sodium chloride solution of a required intermediate sodium chloride concentration, an electrolyte solution is subsequently added to a third level, and the diluted sodium chloride solution and the electrolyte solution are mixed to a homogeneous solution with a required final sodium chloride concentration and a required final electrolyte concentration. This is an expedient manner of preparing a liquid for a medical procedure, the liquid preferably being a dialysis concentrate. The mixing chamber according to the invention has means for successively preparing batches of the liquid for continuous withdrawal to the medical procedure, said means comprising means for continuous measurement of the liquid components. With such a mixing chamber, a flexible preparation of liquid can be achieved. The means for continuous measurement of the liquid components may comprise means for volumetric measurement of the volume of the liquid components. With continuous volumetric measurement a flexible and accurate measurement may be ensured. According to the invention, the means for continuous volumetric measurement of the volume of the liquid components may comprise means for continuous measurement of liquid levels in the mixing chamber. Continuous level measurement provides a simple and reliable way of proportioning the liquid components. In a specific embodiment of the invention, the means for continuous measurement of the liquid levels in the mixing chamber comprise an ultrasonic transmitter/receiver unit for measuring liquid levels^' in the ix- ing chamber. Ultrasonic measurement is a reliable means of accurately measuring liquid levels. In one embodiment of the invention, the mixing chamber further comprises means for calculating, for each liquid component, a level to which the liquid component is to be filled based on a predetermined volume to be added and a cross sectional area of the mixing chamber. Knowing the cross sectional area of the mixing chamber, this makes it possible to accurately control the proportions of the liquid components added. The mixing chamber of the invention may further comprise at least one echo-generating surface at a distance from the transmitter/receiver unit. This echo-generating surface may be used for calculations for compensating for the velocity of sound in the liquid in the mixing chamber in order to maintain the accuracy of level measurements. In one embodiment of the invention, the mixing chamber comprises two echo-generating surfaces arranged at a first and a second distance respectively from the transmitter/receiver unit. This arrangement enables an im- proved compensation for the velocity of sound in the liquid in the mixing chamber, allowing an even better accuracy of level measurements. The inventive mixing chamber may comprise means for compensating for the velocity of sound in the liquid in the mixing chamber by means of detection of at least one ultrasonic echo from said at least one echo-generating surface. In this manner, an automated compensation for changes in the velocity of sound may be achieved. The mixing chamber preferably has a constant cross section as measured along a height of the mixing chamber, thus simplifying the calculation of the levels to which each liquid component is to be filled. In one embodiment of the invention, said at least one echo-generating surface is arranged to occupy a constant portion of the cross section of the mixing chamber as measured along a height of the mixing chamber. A con- stant cross section of the mixing chamber may thus be maintained, simplifying the calculation of the levels to which each liquid component is to be filled. The mixing chamber of the invention may comprise means for calculating adjusted levels for filling the liquid components for compensating for variations in the cross section of the mixing chamber. Thus, the desired accuracy in the proportioning of the liquid components added may be maintained even though the cross section of the mixing chamber is not constant. According to one embodiment of the invention, the mixing chamber may comprise means for measuring the concentration of each liquid component and means for calculating an adjusted level to which the liquid component is filled in the mixing chamber for compensating for any discrepancies as compared to an expected concentration. In this manner, the desired composition of the resulting liquid may be obtained even though the concentration of one or more of the added liquid components differs from what is initially intended. The inventive mixing chamber may comprise means for storing data on the velocity of sound for different properties of the liquid components. Corrections for changes in the velocity of sounds may thus easily be made and the current velocity of sound need not necessarily be meas- ured. The mixing chamber of the invention may advantageously be adapted for preparation of a dialysis concentrate . The device according to the invention comprises a first source of a first liquid component, a second source of a second liquid component, a mixing chamber for mixing the liquid components and means for successively prepar- ing batches of the liquid for continuous withdrawal to the medical procedure, said means comprising means for continuous measurement of the liquid components. Thus, a system for flexible preparation of a liquid for a medical procedure is achieved. In one embodiment of the invention, the device further comprises a storage vessel and means for checking a liquid level in the storage vessel. The storage vessel may be used as a buffer in the preparation of the liquid for the medical procedure, whereas the means for checking the liquid level constitute a safety device for avoiding too low or too high levels in the storage vessel. The device of the invention preferably comprises means for volumetric measurement of the volume of the liquid components. This is a convenient manner of ensuring a flexible proportioning. The mixing chamber of the inventive device may be flexible or rigid. It may be advantageous to choose a flexible or rigid mixing chamber depending on other equipment used in the medical procedure. The means for volumetric measurement of the volume of the liquid components may comprise a piston pump or other volumetric pump. This is one convenient way of measuring the volume of the liquid components. In one embodiment, the means for volumetric measurement of the volume of the liquid components comprise means for continuous measurement of liquid levels in the mixing chamber. Level measurement is a simple and reliable way of measuring volumes . According to a specific embodiment of the invention, the mixing chamber comprises an ultrasonic transmitter/ receiver unit for measuring liquid levels in the mixing chamber. Ultrasonic measurement is an accurate method of measuring liquid levels. The device of the invention may further comprise means for calculating, for each liquid component, a level to which the liquid component is to be added based on a predetermined volume to be added and a cross sectional area of the mixing chamber. The proportions of the different liquid components may thus easily be controlled by measuring the levels to which they are filled. The mixing chamber of the inventive device may further comprise at least one echo-generating surface at a distance from the transmitter/receiver unit. The echo- generating surfaces may be used for determining the velocity of sound in the liquid in the mixing chamber. In one embodiment of the invention, the mixing chamber of the device comprises two echo-generating surfaces arranged at a first and a second distance respectively from the transmitter/receiver unit. This arrangement affords an improved accuracy in the determination of the velocity of sound in the liquid in the mixing chamber. The device of the invention may comprise means for compensating for the velocity of sound in the liquid in the mixing chamber by means of detection of at least one echo from said echo-generating surface. The level meas- urements may thereby be automatically compensated for changes in the velocity of sound in the liquid in the mixing chamber. The mixing chamber of the device preferably has a constant cross section as measured along a height of the mixing chamber, thereby simplifying the calculation of the levels to which the liquid components are to be filled. The at least one echo-generating surface may be arranged to occupy a constant portion of the cross section of the mixing chamber as measured along a height of the mixing chamber. A constant cross section of the mixing chamber may thus be maintained. In one embodiment, the device of the invention comprises means for compensating for variations in the cross sectional area along the height of the mixing chamber. The accuracy of the level measurements may thus be main- tained even though the cross section of the mixing chamber is not constant. The inventive device may comprise means for measuring the concentration of each liquid component and means for calculating an adjusted level to which the liquid component is filled in the mixing chamber for compensating for any discrepancies as compared to an expected concentration. This arrangement makes it possible to obtain the required accuracy in the proportioning of the liquid components despite variations in the concentration of one or more of the liquid components added. Means for storing data on the velocity of sound for different concentrations of the liquid components may be included in the device of the invention. In this manner, corrections for changes in velocity of sound may easily be performed without the need for actual measurements of the current velocity of sound. The inventive device may comprise a safety mechanism for checking the concentration of the resulting liquid mixed in the mixing chamber and for discarding the contents of the mixing chamber whenever a discrepancy is detected in the concentration of the resulting liquid. According to different embodiments of the invention,the means for continuous measurement of liquid levels in the mixing chamber use an acoustic measurement method, an optical measurement method, a capacitive measurement method or a gravimetric measurement method. These are all convenient alternatives that may be used for measuring liquid levels. In the device of the invention, the first source of liquid component may comprise a reservoir of dry component and means for passing water through said dry component for forming the first liquid component. The first liquid component may thus be easily obtained, while han- dling and transportation are simplified. The dry component may be a powder, a granulate or similar. The system of the invention comprises a dialysis machine and means for preparation of dialysis fluid. It is convenient to prepare the dialysis concentrate near the dialysis machine, instead of transporting large contain- ers of ready-made concentrate. The means for preparation of dialysis fluid preferably comprise a bicarbonate cartridge and a device as described above. Thus, a highly flexible system for dialysis treatment may be obtained. It is advantageous to use a device according to the invention for preparing a liquid for a medical procedure. In a preferred variant of such a use, the liquid for the medical procedure is a dialysis concentrate. It is also advantageous to use a system according to the invention for performing dialysis treatment. In an another method of the invention regulation of the proportions of the liquid components is achieved by filling each liquid component to a separate predetermined level in a mixing chamber, the predetermined levels being based on predetermined volumes to be added and the cross sectional area of the mixing chamber and the level of each liquid component added being measured by a continuous measuring method. This is a convenient manner of achieving an accurate and flexible proportioning. In one variant, the level of each liquid component added is measured by an optical measurement method. This is one useful method of level measurement. In another variant, the level of each liquid component added is measured by a conductivity measurement method. This is another useful method of level measurement . In yet another variant, the level of each liquid component added is measured by an acoustic method. This is also a useful method of level measurement. The level of each liquid component added in a mixing chamber may be measured by ultrasonic measurement. The volume of each added component may thus in a simple man- ner be accurately monitored and the proportions of the added components easily adapted in accordance with any recipe . Variations in the velocity of sound in the liquid in the mixing chamber are preferably compensated for when measuring the liquid level. The accuracy of the measurement may thus be maintained even though there are changes in the velocity of sound as the different liquid components are added. In a first method of compensating for changes in velocity of sound ultrasound may be transmitted from a transmitter towards a first echo-generating surface at a first known distance from the transmitter and the time from transmission of the ultrasound to receipt of an echo by a receiver at a second distance from the echo- generating surface may be used for compensating for the velocity of sound in the liquid in the mixing chamber. This is an expedient manner of determining the current velocity of sound in the liquid in the mixing chamber. In another method of compensating for changes in velocity of sound, ultrasound is transmitted from a transmitter towards a first and a second echo-generating surface, said echo-generating surfaces being arranged at different known distances from the ultrasonic transmit- ter. The difference in time between echoes from said echo-generating surfaces received by a receiver is used for compensating for the velocity of sound in the liquid in the mixing chamber. In this manner, the accuracy of the compensation for the velocity of speed may be im- proved. In one variant of the method according to the invention, any variations in the cross sectional area along the height of the mixing chamber is compensated for for each liquid component by calculating adjusted levels to which the liquid components are filled. Thus, the accuracy in the level measurement may be maintained even though the mixing chamber does not have a constant cross sectional area along its height. For instance, if a portion of the mixing chamber has a known smaller cross sectional area than the rest of the mixing chamber, a higher level is calculated for filling the same volume. The concentration of each liquid component may be measured and any discrepancies as compared to an expected concentration may be compensated for by calculating adjusted levels to which the liquid components are filled in the mixing chamber. The desired proportions in the re- suiting liquid may thus be assured, even though the concentration of one or more of the added components is lower or higher than expected. In a variant of the method of the invention, the adjusted levels are based on known data on the velocity of sound for different properties of the liquid components. Corrections may thus be made easily and quickly. In a specific variant of the inventive method, a saturated sodium chloride solution is filled in the mixing chamber to a first level, water is subsequently added to a second level, the saturated sodium chloride solution and water are mixed in the mixing chamber to a homogeneous diluted sodium chloride solution of a required intermediate sodium chloride concentration, an electrolyte solution is subsequently added to a third level, and the diluted sodium chloride solution and the electrolyte solution are mixed to a homogeneous solution with a required final sodium chloride concentration and a required final electrolyte concentration. This is an expedient manner of preparing a liquid for a medical procedure, the liquid preferably being a dialysis concentrate. In one mixing chamber according to the invention, the means for continuous volumetric measurement of the volume of the liquid components comprise means for continuous measurement of liquid levels in the mixing cham- ber. Continuous level measurement provides a simple and reliable way of proportioning the liquid components. In a specific embodiment of the invention, the means for continuous measurement of the liquid levels in the mixing chamber comprise an ultrasonic transmitter/receiver unit for measuring liquid levels in the mix- ing chamber. Ultrasonic measurement is a reliable means of accurately measuring liquid levels. In one embodiment of the invention, the mixing chamber further comprises means for calculating, for each liquid component, a level to which the liquid component is to be filled based on a predetermined volume to be added and a cross sectional area of the mixing chamber. Knowing the cross sectional area of the mixing chamber, this makes it possible to accurately control the proportions of the liquid components added. The mixing chamber of the invention may further comprise at least one echo-generating surface at a distance from the transmitter/receiver unit. This echo-generating surface may be used for calculations for compensating for the velocity of sound in the liquid in the mixing chamber in order to maintain the accuracy of level measurements. In one embodiment of the invention, the mixing chamber comprises two echo-generating surfaces arranged at a first and a second distance respectively from the transmitter/receiver unit. This arrangement enables an im- proved compensation for the velocity of sound in the liquid in the mixing chamber, allowing an even better accuracy of level measurements . The inventive mixing chamber may comprise means for compensating for the velocity of sound in the liquid in the mixing chamber by means of detection of at least one ultrasonic echo from said at least one echo-generating surface. In this manner, an automated compensation for changes in the velocity of sound may be achieved. The mixing chamber preferably has a constant cross section as measured along a height of the mixing chamber, thus simplifying the calculation of the levels to which each liquid component is to be filled. In one embodiment of the invention, said at least one echo-generating surface is arranged to occupy a constant portion of the cross section of the mixing chamber as measured along a height of the mixing chamber . A con- stant cross section of the mixing chamber may thus be maintained, simplifying the calculation of the levels to which each liquid component is to be filled. The mixing chamber of the invention may comprise means for calculating adjusted levels for filling the liquid components for compensating for variations in the cross section of the mixing chamber. Thus, the desired accuracy in the proportioning of the liquid components added may be maintained even though the cross section of the mixing chamber is not constant. According to one embodiment of the invention, the mixing chamber may comprise means for measuring the concentration of each liquid component and. means for calculating an adjusted level to which the liquid component is filled in the mixing chamber for compensating for any discrepancies as compared to an expected concentration. In this manner, the desired composition of the resulting liquid may be obtained even though the concentration of one or more of the added liquid components differs from what is initially intended. The inventive mixing chamber may comprise means for storing data on the velocity of sound for different properties of the liquid components. Corrections for changes in the velocity of sounds may thus easily be made and the current velocity of sound need not necessarily be meas- ured. The mixing chamber of the invention may advantageously be adapted for preparation of a dialysis concentrate . In one device according to the invention, the means for volumetric measurement of the volume of the liquid components comprise means for continuous measurement of liquid levels in the mixing chamber. Level measurement is a simple and reliable way of measuring volumes. According to a specific embodiment of the invention, the mixing chamber comprises an ultrasonic transmitter/ receiver unit for measuring liquid levels in the mixing chamber. Ultrasonic measurement is an accurate method of measuring liquid levels. The device of the invention may further comprise means for calculating, for each liquid component, a level to which the liquid component is to be added based on a predetermined volume to be added and a cross sectional area of the mixing chamber. The proportions of the different liquid components may thus easily be controlled by measuring the levels to which they are filled. The mixing chamber of the inventive device may further comprise at least one echo-generating surface at a distance from the transmitter/receiver unit. The echo- generating surfaces may be used for determining the velocity of sound in the liquid in the mixing chamber. In one embodiment of the invention, the mixing chamber of the device comprises two echo-generating surfaces arranged at a first and a second distance respectively from the transmitter/receiver unit. This arrangement affords an improved accuracy in the determination of the velocity of sound in the liquid in the mixing chamber. The device of the invention may comprise means for compensating for the velocity of sound in the liquid in the mixing chamber by means of detection of at least one echo from said echo-generating surface. The level meas- urements may thereby be automatically compensated for changes in the velocity of sound in the liquid in the mixing chamber. The mixing chamber of the device preferably has a constant cross section as measured along a height of the mixing chamber, thereby simplifying the calculation of the levels to which the liquid components are to be filled. The at least one echo-generating surface may be arranged to occupy a constant portion of the cross section of the mixing chamber as measured along a height of the mixing chamber. A constant cross section of the mixing chamber may thus be maintained. In one embodiment, the device of the invention comprises means for compensating for variations in the cross sectional area along the height of the mixing chamber. The accuracy of the level measurements may thus be main- tained even though the cross section of the mixing chamber is not constant. The inventive device may comprise means for measuring the concentration of each liquid component and means for calculating an adjusted level to which the liquid component is filled in the mixing chamber for compensating for any discrepancies as compared to an expected concentration. This arrangement makes it possible to obtain the required accuracy in the proportioning of the liquid components despite variations in the concentration of one or more of the liquid components added. Means for storing data on the velocity of sound for different concentrations of the liquid components may be included in the device of the invention. In this manner, corrections for changes in velocity of sound may easily be performed without the need for actual measurements of the current velocity of sound. The inventive device may comprise a safety mechanism for checking the concentration of the resulting liquid mixed in the mixing chamber and for discarding the con- tents of the mixing chamber whenever a discrepancy is detected in the concentration of the resulting liquid. According to different embodiments of the invention, the means for continuous measurement of liquid levels in the mixing chamber use an acoustic measurement method, an optical measurement method, a capacitive measurement method or a gravimetric measurement method. These are all convenient alternatives that may be used for measuring liquid levels. In the device of the invention, the first source of liquid component may comprise a reservoir of dry compo- nent and means for passing water through said dry component for forming the first liquid component. The first liquid component may thus be easily obtained, while handling and transportation are simplified. The dry component may be a powder, a granulate or similar. Brief Description of the Drawings The invention will be described in more detail with reference to the appended schematic drawings, which show examples of currently preferred embodiments of the invention, and in which: Fig. 1 is a diagram of a system according to the invention; Fig. 2 is a diagram of a device according to the invention; Fig. 3 is a sectional view of a mixing chamber according to the invention; Figs 4a-d show alternative embodiments of the assembly of echo-generating surfaces of Fig. 3, Fig. 5 is a flow chart showing the preparation of a liquid according to the method of the invention; and Fig. 6 is a diagram of an alternative embodiment of the invention including four dialysis machines. Description of Preferred Embodiments of the Invention With reference to Fig. 1, the system 1 of the invention comprises a device 2 for preparation of A- concentrate, a bicarbonate cartridge 3 for preparation of B-concentrate and a dialysis machine 4. Turning to Fig. 2, the device 2 of the invention is shown in closer detail. The device 2 for preparation of A-concentrate comprises a salt cartridge 5, an electro- lyte bag 6 and a mixing chamber 7 for preparing a liquid. The salt in the salt cartridge 5 is dry and may be in the form of a powder, a granulate or similar. A water source 8 and piping 9 are provided for passing water suitable for use in the medical procedure through the salt cartridge 5 for preparing a saturated salt solution. The salt cartridge 5 with the water source 8 and the piping 9 constitute a first source of a first liquid component and the electrolyte bag 6 constitutes a second source of a second liquid component. As can be seen in Fig. 1, the water source 8 is also connected via piping 10 to the bicarbonate cartridge 3. Thus, water may be passed through the bicarbonate cartridge 3 for forming a bicarbonate solution or so-called B-concentrate . The mixing chamber 7 is shown in greater detail in Fig. 3. At the bottom of the mixing chamber 7 an ultra- sonic transmitter/receiver unit 11 is arranged for measuring the liquid level h in the mixing chamber 7. For calibrating or compensating the level measurement for changes in the velocity of sound, an echo assembly 12 providing two echo-generating surfaces 13a, 13b is ar- ranged in the mixing chamber 7. In the embodiment shown in Fig. 3, the mixing chamber 7 is a circular cylinder having a diameter D. The cross sectional area A of the mixing chamber 7 is constant as measured along the height H of the mixing chamber 7, as is the cross sectional area A' of the echo assembly 12. The echo assembly 12 thus occupies a constant portion of the cross sectional area A of the mixing chamber 7. With reference again to Fig. 1, the system 1 of the invention further comprises a storage vessel 14 for stor- ing the liquid prepared. The storage vessel 14 has a connector 15 for connection with the dialysis machine 4. The system 1 also includes a drain 16 for discarding liquid, should an error in the preparation be detected. When preparing a liquid for a medical procedure, e.g. a dialysis concentrate, the concentrations of the different components are very important and therefore the proportions of the components added have to be very accu- rately monitored. In the preferred variant, the method of the invention utilizes a volumetric proportioning technique. The volumes of the components to be added are determined based on the required concentrations of their constituent elements in the resulting liquid and the concentrations in the added liquid components. Knowing the cross sectional area A of the mixing chamber 7, the level to which each liquid component is to be filled is calculated as h = ^^L+ h n, -\ l " A wherein h_n is the level to which component n is to be added, h_n-ι is the level to which previous components have been filled, V_n the required volume of component n and A the cross sectional area of the mixing chamber 7. The level h to which an added component has been filled is measured by means of the ultrasonic transmitter/receiver unit 11. An ultrasonic pulse is transmitted by the transmitter of the unit 11 at the bottom of the mixing chamber 7 and an echo is generated as the pulse reaches the liquid surface 17. The time t from transmission of the ultrasonic pulse to receipt of the echo by the receiver of the unit 11 at the bottom of the mixing chamber 7 is measured. If the transmitter and receiver of the transmitter/receiver unit 11 are arranged at essentially the same point, i.e. at the same distance from the liquid surface 17, the liquid level h in the mixing chamber can be calculated as c-t h = wherein h is the liquid level in the mixing chamber 7, c is the velocity of sound and t is the time from transmission of the pulse to receipt of the echo. When a liquid component has been added in the mixing chamber 7, the liquid level is checked. Should the level deviate somewhat from what was initially intended, the level to which the next liquid component should be filled is recalculated, so that the correct proportions of the different liquid components are obtained in the resulting liquid. It should be noted that the velocity of sound is not the same in all liquids. For instance, the velocity of sound in pure water is approximately 1,500 m/s, while in a saturated sodium chloride solution it is approximately 1,790 m/s. Therefore, for improving the accuracy of the level measurement the correct velocity of sound should be determined and used in the calculations . In the embodiment shown, the mixing chamber 7 is therefore equipped with an echo assembly 12 having two echo-generating surfaces 13a, 13b, which may be used for compensating the level measurements in accordance with the current velocity of sound. When using such an echo assembly 12, the calculations of the levels to which the liquid components are to be filled are adjusted to compensate for the volume occupied by the echo assembly 12:

V " A - A ^{n X}

When calibrating the level measurement, an ultrasonic pulse is transmitted by the transmitter of the unit 11 towards the two echo-generating surfaces 13a, 13b and the time ti, t₂ from transmission to receipt of the two echoes respectively is measured. The velocity of sound is calculated as

wherein c is the velocity of sound, Di is the distance between the two echo-generating surfaces 13a, 13b, and ti and t₂ are the times from transmission of ultrasound to receipt of the echo from the first and second echo-generating surfaces 13a, 13b respectively. When diluting a first liquid component having a con- centration K_conc with water to a required final concentration K_fi_nai the following equation applies:

Tζ Tζ V cone τζ A - h cone __ tζ h con ^•**- final ~ "-cone _Tr ~ ^J^co c , » ^— -""COHC , ' ^V final ^{A ' n} final ^H final wherein V_conc and V_fi_nal are the volumes of the concentrated component and the diluted component, respectively, A is the cross sectional area of the mixing chamber 7, and h_conc and h_fi_naι are the levels of the concentrated and diluted components, respectively. Since the liquid level h, as mentioned above, is calculated as

2 the equation for K_fi_naι can be expressed as

1 2 — t con c cone f p _ jζ ^l ccoonnee ccone ^■"^■ final cone i cone , - t _r ^T final⁰ final final final

With the velocity of sound c calculated as c = 2 t₂ t_j

Kfinai can be expressed as

and it can thus be seen that the concentration K_fi_naι of the diluted component is independent of the distance Di between the echo-generating surfaces 13a, 13b. The manufacturing tolerances for the distance Di need therefore not be very tight, as long as this distance Di is constant during the preparation of a batch of liquid. In a simplified embodiment of the invention, as shown in Fig. 4a, only one echo-generating surface 13 need be used, since the concentration K_fi_naι of the resulting liquid is independent of the distance between the transmitter/receiver unit 11 and the echo-generating surface 13. An ultrasonic pulse would then be sent towards the echo-generating surface 13 and the time from trans- mission of the pulse to receipt of an echo would be measured and used for calculating the current velocity of sound. The velocity of sound would then be calculated as c = 2— ²- , wherein c is the velocity of sound, D₂ is the distance between the ultrasonic transmitter/receiver unit 11 and the echo-generating surface 13, and ti is the time between transmission of an ultrasonic pulse and receipt of an echo from the echo-generating surface 13. In comparison with the embodiment using two echo- generating surfaces, this is a more economical solution. Fig. 5 shows a flow chart outlining the method of the invention. First, a recipe is chosen for the liquid to be prepared. Based on the recipe the volumes Vi, V₂, V₃, ..., v_n of saturated sodium chloride solution, water and electrolyte solution, respectively, are calculated. Knowing the cross sectional area A of the mixing chamber 11 and the cross sectional area A' of the echo assembly 12, the nominal levels hi, h₂, h₃, ..., h_n to which each liquid component is to be added is calculated. Filling of the first component, the saturated sodium chloride solution emanating from the salt cartridge 5 and the water source 8, commences and as soon as the liquid level has passed the echo-generating surfaces 13a, 13b calibration can be performed. An ultrasonic pulse is transmitted by the transmitter/receiver unit 11 from the bottom of the mixing chamber 7 towards the liquid surface 17. A first and a second echo are produced as the ultrasonic pulse hits the first and second echo-generating surface 13a, 13b, respectively. These echoes are received by the transmitter/receiver unit 11. The time ti from transmission of the ultrasonic pulse to receipt of the echo from the first echo-generating surface 13a is measured, as well as the time t₂ from transmission to receipt of the echo from the second echo-generating surface 13b. The control unit (not shown) of the system 1 and device 2 calculates the compensation necessitated by the current velocity of sound in the sodium chloride solution in the mixing chamber 7 and filling continues until the predetermined level hi is reached. The current velocity of sound need not be explicitly calculated. Knowledge of the distance D_x between the first and second echo-generating surfaces 13a, 13b and data on the times ti and t₂ from transmission of the ultrasonic pulse to receipt of the echoes from the first and second echo-generating surfaces 13a, 13b, respectively, suffice to compensate the level measurements for changes in the velocity of sound. The distance Di does not have to be known, as long as it is constant during the measurement. The same applies in the case where only one echo-generating surface 13 is used as long as the distance D₂ from the ultra-sonic transmitter/receiver unit 11 to the echo-generating surface 13 does not change during the measurement. Filling of the second component, suitably pre- treated water from the water source 8, then commences and calibration is performed as before. Water is added until the predetermined level h₂ (adjusted for possible deviations from the nominal value of level hi) is reached. To achieve a homogeneous liquid in the mixing chamber 7, the liquid is thoroughly mixed using circulation piping 18 connected to the mixing chamber 7. Finally, addition of the third component, electrolyte solution from the electrolyte bag 6, is performed. Again, the level measurement is calibrated by determining the velocity of sound in the liquid in the mixing chamber 7. Filling of the electrolyte solution continues until the predetermined liquid level h₃ is reached. The liquid in the mixing chamber 7 is mixed to a homogeneous solution having the desired composition. Instead of measuring the velocity of sound during preparation, data on known velocities for different compositions with varying concentrations, components and temperatures of the liquid may be stored in the control unit and used for calibrating level measurements. In such case, the echo assembly 12 may be dispensed with. As a safety precaution, the system 1 and device 2 of the invention include a first conductivity meter 19 for checking the concentration of the resulting liquid leaving the preparation device 2. If the conductivity of the resulting liquid corresponds to the required concentra- tion, the liquid is passed to the storage vessel 14. Via connector 15 the storage vessel 14 is connected to the dialysis machine 4 which withdraws liquid from the storage vessel 14 for preparation of a dialysis fluid at a required rate. The storage vessel 14 thus constitutes a buffer in the production of dialysis fluid. If the conductivity of the resulting liquid from the device 2 does not correspond to the required concentration, the liquid is shunted to the drain 16 and not used for preparation of dialysis fluid. As a further safety precaution, the system 1 and device 2 of the invention may be equipped with a second conductivity meter 20 for checking the concentration of the first liquid component as it enters the mixing chamber 7. The second conductivity meter 20 may also be used for checking the conductivity of the resulting liquid leaving the mixing chamber 7, thus replacing the first conductivity meter 19. A piston pump 21 used for pumping electrolyte solution from the electrolyte bag 6 may also be used as a safety mechanism. The strokes performed by the piston pump 21 for pumping the electrolyte solution to the mix- ing chamber 7 are counted during filling to the predetermined level h₃. Knowing the volume pumped in one stroke, the volume pumped into the mixing chamber 7 may be calculated and used for verifying the volume added as measured by ultrasonic measurement in the mixing chamber 7. Another safety mechanism in the system 1 and device 2 includes a low level sensor 22 near the bottom of the storage vessel 14 for alerting when there is too little liquid in the storage vessel 14. The low level sensor is preferably placed at a sufficient distance from the bot- tom of the storage vessel 14 to allow preparation of a new batch before the dialysis machine has used all remaining liquid. The low level sensor 22 may for instance be placed at a level corresponding to two batches of liquid prepared in the mixing chamber 7. If the liquid level in the storage vessel is above the low level sensor 22, liquid prepared in the mixing chamber 7 will not be allowed to pass to the storage vessel 14. Instead, preparation of a new liquid batch will not start until the liquid level in the storage vessel 14 has again fallen below the level of the low level sensor 22. Instead of fixed level sensors, an ultrasonic transmitter/receiver unit (not shown) similar to the unit 8 used in the mixing chamber 7 may be arranged in the storage vessel 14 for continuous measurement of the liquid level in the storage vessel. Other level measurement means may also be used. The dialysis machine 4 also withdraws bicarbonate solution prepared in the bicarbonate cartridge 3. The A- concentrate prepared in the device 2 and the bicarbonate solution or B-concentrate are combined in the dialysis machine 4 and diluted with water (e.g. one part A- concentrate and 34 parts water) to a physiological solu- tion for use in dialysis treatment. In order to avoid overfilling the storage vessel 14 a high level sensor 23 may optionally be arranged near the top of the storage vessel 14. In the embodiment shown in Fig. 3, the echo assembly 12 occupies a constant portion of the cross sectional area A of the mixing chamber 7 and the levels to which each liquid component are to be filled are therefore easily calculated from the predetermined volume. However, if the echo assembly 12 occupies a non-constant portion of the cross sectional area A of the mixing chamber 7, such as in the embodiments shown in Figs 4b-d, or if the cross sectional area A of the mixing chamber 7 varies along the height H of the mixing chamber 7, the calculations of the levels to which the liquid components are to be filled have to be adjusted. This is conveniently achieved using appropriate software in the control unit (not shown) of the inventive system 1 and device 2. The levels to which the liquid components are to be filled may also be adjusted because of deviations in the concentration of the added components. If, e.g., the sodium chloride solution deriving from the salt cartridge 5 is not saturated, but rather has a 90 % saturation, the volume added should be increased by 11 % and the added volume of water reduced accordingly. Adjusted levels to which the components should be filled may conveniently be calculated using appropriate software in the control unit (not shown) of the inventive system 1 and device 2. The skilled person will realise that a number of modifications of the embodiments of the invention de- scribed herein are possible without departing from the scope of the invention, as defined in the claims. For instance, the device and system of the invention may be fitted as an adapter for an existing dialysis machine of almost any type. Specifically, it may be used as an adapter for a dialysis machine using at least two sources of liquid component, such as the BiCart Select® system described above. However, the invention is equally suitable for integration in a new dialysis machine. In the preferred embodiment, the mixing chamber 7 is a circular cylinder. Other shapes may, however be used, e.g. a rectangular or triangular cylinder. As mentioned above, the cross sectional area A of the mixing chamber need not necessarily be constant, but may vary along the height H of the mixing chamber 7. In the description above, level measurement is done from the bottom of the mixing chamber 7 through the liquid. Level measurements could instead be performed from the top of the mixing chamber 7 through the air/gas above the liquid level (not shown) . The echo-assembly would in such case not necessarily have to occupy any space below the liquid surface. Regardless of through which fluid, liquid or air, in the mixing chamber 7 measurements are performed, the echo-generating surfaces used for compensating for changes in velocity of sound should be entirely within the fluid in question. In more general terms, the system 1 and device 2 work according to what could be called a semi-batch principle. In contrast to known batchwise preparation methods, the entire batch for one treatment session is not prepared at once. However, the method employed in the system 1 and device 2 does not place the same high requirements of speed and accuracy of measurements as known on-line methods would. Yet, an improved flexibility is achieved as compared to batchwise preparation. Small ^λ,sub-batches" (e.g. 40 sub-batches for one treatment session) are successively prepared in the mixing chamber 7 and passed to the storage vessel 14. The dialysis machine 4 then withdraws liquid from the storage vessel 14 continuously for use in the dialysis treatment of a patient. As soon as the preparation device 2 has prepared and discharged a sub-batch of liquid, preparation of a new sub-batch commences. Should a need to change the composition of the liquid prepared arise, e.g. in the case of profiling the dialysis treatment session with one concentration of a certain ion at the beginning of the treatment session and another concentration at the end of the session, the composition may be changed from one sub-batch to the next. The fact that the mixing chamber 7 is separate from the storage vessel 14 contributes to the flexibility and safety of the system 1 and device 2 of the invention. Should there be an undesirable deviation in the composi- tion of the liquid prepared in the mixing chamber 7 this does not immediately affect the liquid withdrawn by the dialysis machine 4. A sub-batch not complying with the requirements may easily be discharged through the drain 16 and a new sub-batch prepared. If, when using a batch- wise method for preparation, a mistake were made in the preparation, the whole batch would have to be discarded or calculations would have to be made for correcting the resulting composition by supplemental addition of some of the components. A number of methods for continuous level measurement in the mixing chamber 7 are possible to use in the inventive system 1 and device 2. For instance, instead of ultrasonic measurement, capacitive or optical measurement methods could be used. It would also be possible to use a floating body indicating the level in the mixing chamber 7. Changes in resonance in the mixing chamber could also be used for determining the liquid level. Other continuous volumetric measurement methods are also conceivable, other than level measurement in the mixing chamber 7. For example, piston pumps may be used. One piston pump having one cavity for each component to be added could also be employed. In the preferred embodiment, the mixing chamber 7 is rigid. However, a flexible mixing chamber in the form of a bag could also be used. Depending on whether the mixing chamber is rigid or flexible different proportioning methods are suitable. The method employing level measurement as described above may be used in a rigid mixing chamber, whereas it is unsuited in a flexible mixing chamber. Volumetric measurement methods employing piston pumps are equally suitable for rigid and flexible mixing chambers. Gravimetric proportioning may also be used. The liquid components are in such case added sequentially in the mixing chamber and for each liquid component the mixing chamber and its contents are weighed. Knowing the weight of the mixing chamber an accurate proportioning may be achieved. Gravimetric proportioning may be used with rigid as well as flexible mixing chambers. The device and system of the invention need not necessarily include a separate mixing chamber. Instead, the storage vessel could be used both as mixing chamber and as interface with the dialysis machine. This might be convenient e.g. when measuring vessels or piston pumps are used for proportioning the liquid components. In such case, each liquid component may be measured out and added in the storage vessel. However, if a mixing chamber separate from the storage vessel is used, an increased flexibility may be achieved as relates to errors in the proportioning, since there is a possibility of detecting errors in the composition of the liquid in the mixing cham- ber before this liquid is passed to the storage vessel for use in the medical procedure. Resulting liquid not complying with the requirements may thus be shunted to the drain. Fig. 6 shows an alternative embodiment of the inven- tion. This embodiment may be used on sites where several dialysis machines are used and A- and B-concentrates are centrally prepared. In the embodiment of Fig. 6, four dialysis machines 4, 4a, 4b, 4c are shown by way of example. Each dialysis machine 4, 4a, 4b, 4c is connected to a multiposition valve 24 via a connector 15, 15a, 15b, 15c and a storage vessel 14, 14a, 14b, 14c. Each dialysis machine 4, 4a, 4b, 4c is also connected to a control unit CPU for input of information on the requested composition. The CPU controls the composition to be prepared by the device 2 and controls the multiposition valve 24 to transfer the pre- pared A-concentrate to the dialysis machine that requests it. In this embodiment it is possible to supply individualized A-concentrate to each dialysis machine. In one embodiment of the present invention (not shown) several electrolyte bags 6 are used in order to allow different compositions in each electrolyte bag. This embodiment allows further individualization of A- concentrate for each batch to be prepared. It is also conceivable to achieve an individualization by using several cartridges of dry components. Although the invention has mainly been described in connection with preparation of a dialysis concentrate, it is also suited for preparation of any other liquid for medical procedures where the composition of the resulting liquid needs to be closely controlled. For instance, the invention would be useful also for preparing the finished dialysis fluid for haemodialysis from the concentrate. It should also be appreciated that the invention is applicable to mixing any number of different liquid components. The invention may also be used for preparation of dialy- sis fluid for peritoneal dialysis. Other examples of medical fluids for the preparation and proportioning of which the present invention would be applicable are infusion fluids, e.g. physiological saline solution which is prepared from water and saturated saline solution, and lactated Ringer' s solution, which is a physiologically balanced electrolyte solution used as a resuscitation fluid.

Claims

1. A method for preparing a liquid for a medical procedure by mixing at least two liquid components, wherein batches of liquid are successively prepared for continuous withdrawal for use in the medical procedure, the method comprising: proportioning the liquid components for a batch of the liquid for the medical procedure, checking whether there is room for the batch in a storage vessel (14) wherefrom liquid is withdrawable for use in the medical procedure, if there is room, passing the batch to the storage vessel (14) and proportioning the liquid components for a successive batch of the liquid for the medical procedure, if there is not enough room in the storage vessel (14), waiting while liquid is withdrawn for use in the medical procedure until there is room in the storage vessel (14), passing the batch to the storage vessel (14) and proportioning the liquid components for a successive batch of the liquid for the medical procedure, c h a r a c t e r i s e d i n that the proportioning is done using a continuous measurement method.

2. A method as claimed in claim 1, wherein regula- tion of the proportions of the liquid components is achieved by gravimetric measurement of the liquid components .

3. A method as claimed in claim 1, wherein regulation of the proportions of the liquid components is achieved by volumetric measurement of the liquid components .

4. A method as claimed in claim 3, wherein regulation of the proportions of the liquid components is achieved by filling each liquid component to a separate predetermined level in a mixing chamber, the predetermined levels being based on predetermined volumes to be added and the cross sectional area of the mixing chamber.

5. A method as claimed in claim 4, wherein the level of each liquid component added is measured by an optical measurement method.

6. A method as claimed in claim 4, wherein the level of each liquid component added is measured by a conductivity measurement method.

7. A method as claimed in claim 4, wherein the level of each liquid component added is measured by an acoustic measurement method.

8. A method as claimed in claim 4, wherein the level of each liquid component added is measured by ultrasonic measurement .

9. A method as claimed in claim 8, wherein variations in the velocity of sound in a fluid in the mixing chamber (7) are compensated for when measuring the liquid level (h) .

10. A method as claimed in claim 9, wherein ultrasound is transmitted from a transmitter (11) towards a first echo-generating surface (13) at a first distance from the transmitter (11) and the time from transmission of the ultrasound to receipt of an echo by a receiver (11) at a second distance from the echo-generating surface (13) is used for determining the velocity of sound in the liquid in the mixing chamber (7) .

11. A method as claimed in claim 9, wherein ultrasound is transmitted from a transmitter (11) towards a first and a second echo-generating surface (13a, 13b) , said echo-generating surfaces (13a, 13b) being arranged at different distances from the ultrasonic transmitter, and wherein the difference in time between echoes from said echo-generating surfaces received by a receiver (11) is used for determining the velocity of sound in the liquid in the mixing chamber (7) .

12. A method as claimed in any one of claims 4-11, wherein for each liquid component any variations in the cross sectional area along the height of the mixing chamber (7) are compensated for by calculating an adjusted level to which the liquid component is filled in the mixing chamber (7) .

13. A method as claimed in any one of claims 4-12, wherein the concentration of each liquid component is measured and any discrepancies as compared to an expected concentration are compensated for by calculating an adjusted level to which the liquid component is filled in the mixing chamber (7) .

14. A method as claimed in claim 13, wherein the ad- justed levels are based on known data on the velocity of sound for different properties of the liquid components.

15. A method as claimed in any one of claims 4-14, wherein a saturated sodium chloride solution is filled in the mixing chamber (7) to a first level, water is subse- quently added to a second level, the saturated sodium chloride solution and water are mixed in the mixing chamber (7) to a homogeneous diluted sodium chloride solution of a required intermediate sodium chloride concentration, an electrolyte solution is subsequently added to a third level, and wherein the diluted sodium chloride solution and the electrolyte solution are mixed to a homogeneous solution with a required final sodium chloride concentration and a required final electrolyte concentration.

16. A method as claimed in any one of the preceding claims, wherein the liquid for the medical procedure is a dialysis concentrate.

17. A mixing chamber for preparing a liquid for a medical procedure by mixing at least two liquid components, comprising means (7, 11) for successively prepar- ing batches of the liquid for continuous withdrawal to the medical procedure, ch a r a c t e r i s e d i n that the means (7, 11) for successively preparing batches of the liquid comprises means for continuous measurement of the liquid components.

18. A mixing chamber as claimed in claim 17, wherein the means for continuous measurement of the liquid compo- nents comprise means (11) for volumetric measurement of the volume (V) of each of the liquid components .

19. A mixing chamber as claimed in claim 18, said mixing chamber being flexible.

20. A mixing chamber as claimed in claim 18, said mixing chamber being rigid.

21. A mixing chamber as claimed in claim 20, wherein the means for volumetric measurement of the volume (V) of each of the liquid components comprise means (11) for continuous measurement of liquid levels (h) in the mixing chamber (7) .

22. A mixing chamber as claimed in claim 21, wherein the means for continuous measurement of liquid levels (h) in the mixing chamber (5) comprise an ultrasonic trans- mitter/receiver unit (11).

23. A mixing chamber as claimed in claim 22, further comprising means for calculating, for each liquid component, a level to which the liquid component is to be filled based on a predetermined volume to be added and a cross sectional area of the mixing chamber (7) .

24. A mixing chamber as claimed in claim 23, further comprising at least one echo-generating surface (13; 13a, 13b) at a distance from the ultrasonic transmitter/receiver unit (11) .

25. A mixing chamber as claimed in claim 23, comprising two echo-generating surfaces (13a, 13b) arranged at a first and a second distance respectively from the ultrasonic transmitter/receiver unit (11) .

26. A mixing chamber as claimed in claim 24 or 25, further comprising means for calculating the velocity of sound in the liquid in the mixing chamber (5) by means of detection of at least one ultrasonic echo from said at least one echo-generating surface (13; 13a, 13b) .

27. A mixing chamber as claimed in any one of claims 22-26 wherein the mixing chamber has a constant cross section as measured along a height (H) of the mixing chamber (7 ) .

28. A mixing chamber as claimed in claim 27, wherein said at least one echo-generating surface (13; 13a, 13b) is arranged to occupy a constant portion of the cross section of the mixing chamber (7) as measured along a height (H) of the mixing chamber (7) .

29. A mixing chamber as claimed in any one of claims 23-28, further comprising means for calculating adjusted levels for filling the liquid components for compensating for variations in the cross sectional area along the height of the mixing chamber (7) .

30. A mixing chamber as claimed in any one of claims 23-29, further comprising means for measuring the concentration of each liquid component and means for calculating an adjusted level to which the liquid component is filled in the mixing chamber (7) for compensating for any discrepancies as compared to an expected concentration.

31. A mixing chamber as claimed in claim 30, further comprising means for storing data on the velocity of sound for different properties of the liquid components.

32. A mixing chamber as claimed in any one of claims 17-31, which is adapted for preparation of a dialysis concentrate .

33. A device for preparing a liquid for a medical procedure, comprising a first source (5, 8, 9) of a first liquid component, a second source (6) of a second liquid component, a mixing chamber (7) for mixing the liquid components, and means (7, 11) for successively preparing batches of the liquid for continuous withdrawal to the medical procedure, c h a r a c t e r i s e d i n that the means for successively preparing batches of the liquid comprises means for continuous measurement of the liquid components .

34. A device as claimed in claim 33, further comprising a storage vessel (14) and means (22, 23) for checking a liquid level in the storage vessel (14) .

35. A device as claimed in claim 33 or 34, further comprising means (11) for continuous volumetric measurement of the volume (V) of the liquid components.

36. A device as claimed in any one of claims 33-35, wherein the mixing chamber is flexible.

37. A device as claimed in any one of claims 33-35, wherein the mixing chamber is rigid.

38. A device as claimed in any one of claims 35-37, wherein the means for volumetric measurement of the vol- ume (V) of the liquid components comprise a piston pump.

39. A device as claimed in claim 37, wherein the means for volumetric measurement of the volume (V) of the liquid components comprise means (11) for continuous measurement of liquid levels (h) in the mixing chamber (7).

40. A device as claimed in claim 39, wherein the mixing chamber (7) comprises an ultrasonic transmitter/receiver unit (11) for measuring liquid levels (h) in the mixing chamber (7) .

41. A device as claimed in claim 39 or 40, further comprising means for calculating, for each liquid component, a level to which the liquid component is to be filled based on a predetermined volume to be added and a cross sectional area of the mixing chamber (7) .

42. A device as claimed in claim 40 or 41, wherein the mixing chamber (7) further comprises at least one echo-generating surface (13; 13a, 13b) at a distance from the ultrasonic transmitter/receiver unit (11) .

43. A device as claimed in claim 42, wherein the mixing chamber (7) comprises two echo-generating surfaces (13a, 13b) arranged at a first and a second distance respectively from the ultrasonic transmitter/receiver unit (11) •

44. A device as claimed in claim 42 or 43, further comprising means for calculating the velocity of sound in the liquid in the mixing chamber (7) by means of detec- tion of at least one ultrasonic echo from said at least one echo-generating surface (13; 13a, 13b) .

45. A device as claimed in any one of claims 41-44, wherein the mixing chamber (7) has a constant cross sec- tion as measured along a height (H) of the mixing chamber (7) .

46. A device as claimed in claim 45, wherein said at least one echo-generating surface (13; 13a, 13b) is arranged to occupy a constant portion of the cross section of the mixing chamber (7) as measured along a height (H) of the mixing chamber (7).

47. A device as claimed in any one of claims 40-46, further comprising means for calculating adjusted levels for filling the liquid components for compensating for variations in the cross section of the mixing chamber (7) .

48. A device as claimed in any one of claims 40-47, further comprising means for measuring the concentration of each liquid component and means for calculating an ad- justed level to which the liquid component is filled in the mixing chamber (7) for compensating for any discrepancies as compared to an expected concentration.

49. A device as claimed in claim 40-48, further comprising means for storing data on the velocity of sound for different properties of the liquid components.

50. A device as claimed in any one of claims 33-49, further comprising a safety mechanism (16, 19) for checking the concentration of the resulting liquid mixed in the mixing chamber (7) and for discarding the contents of the mixing chamber (7) whenever a discrepancy is detected in the concentration of the resulting liquid.

51. A device as claimed in claim 39, wherein the means for continuous measurement of liquid levels (h) in the mixing chamber (7) include means for acoustic meas- urement.

52. A device as claimed in claim 39, wherein the means for continuous measurement of liquid levels (h) in the mixing chamber (7) include means for optical measurement .

53. A device as claimed in claim 39, wherein the means (8) for continuous measurement of liquid levels (h) in the mixing chamber (7) include means for capacitive measurement .

54. A device as claimed in claim 39, further comprising means for gravimetric measurement of each of the liquid components.

55. A device as claimed in any one of claims 33-54, wherein the first source (2, 6, 7) of the first liquid component comprises a reservoir (2) of dry component and means (6, 7) for passing water through said dry component for forming the first liquid component.

56. A system for dialysis treatment, comprising a dialysis machine and means for preparation of dialysis fluid including a bicarbonate cartridge (3) and a device (2) as claimed in any one of claims 33-55.

57. A use of a device as claimed in any one of claims 33-55 for preparing a liquid for a medical procedure .

58. A use as claimed in claim 57, wherein the liquid for the medical procedure is a dialysis concentrate.

59. A use of a system as claimed in claim 56 for performing dialysis treatment.

60. A method for proportioning components of a liquid for a medical procedure, wherein regulation of the proportions of the liquid components is achieved by filling each liquid component to a separate predetermined level in a mixing chamber, the predetermined levels being based on predetermined volumes to be added and the cross sectional area of the mixing chamber, c h a r a c t e r i s e d i n that the level of each liquid component added is measured by a continuous measuring method.

61. A method as claimed in claim 60, wherein the level of each liquid component added is measured by an optical measurement method.

62. A method as claimed in claim 60, wherein the level of each liquid component added is measured by a conductivity measurement method.

63. A method as claimed in claim 60, wherein the level of each liquid component added is measured by an acoustic measurement method.

64. A method as claimed in claim 60, wherein the level of each liquid component added is measured by ultrasonic measurement.

65. A method as claimed in claim 64, wherein variations in the velocity of sound in a fluid in the mixing chamber (7) are compensated for when measuring the liquid level (h) .

66. A method as claimed in claim 65, wherein ultra- sound is transmitted from a transmitter (11) towards a first echo-generating surface (13) at a first distance from the transmitter (11) and the time from transmission of the ultrasound to receipt of an echo by a receiver (11) at a second distance from the echo-generating sur- face (13) is used for determining the velocity of sound in the liquid in the mixing chamber (7) .

67. A method as claimed in claim 65, wherein ultrasound is transmitted from a transmitter (11) towards a first and a second echo-generating surface (13a, 13b), said echo-generating surfaces (13a, 13b) being arranged at different distances from the ultrasonic transmitter, and wherein the difference in time between echoes from said echo-generating surfaces received by a receiver (11) is used for determining the velocity of sound in the liq- uid in the mixing chamber (7) .

68. A method as claimed in any one of claims 60-67, wherein for each liquid component any variations in the cross sectional area along the height of the mixing chamber (7) are compensated for by calculating an adjusted level to which the liquid component is filled in the mixing chamber (7) .

69. A method as claimed in any one of claims 60-68, wherein the concentration of each liquid component is measured and any discrepancies as compared to an expected concentration are compensated for by calculating an ad- justed level to which the liquid component is filled in the mixing chamber (7) .

70. A method as claimed in claim 69, wherein the adjusted levels are based on known data on the velocity of sound for different properties of the liquid components.

71. A method as claimed in any one of claims 60-70, wherein a saturated sodium chloride solution is filled in the mixing chamber (7) to a first level, water is subsequently added to a second level, the saturated sodium chloride solution and water are mixed in the mixing cham- ber (7) to a homogeneous diluted sodium chloride solution of a required intermediate' sodium chloride concentration, an electrolyte solution is subsequently added to a third level, and wherein the diluted sodium chloride solution and the electrolyte solution are mixed to a homogeneous solution with a required final sodium chloride concentration and a required final electrolyte concentration.

72. A method as claimed in any one of claims 60-71, wherein the liquid for the medical procedure is a dialysis concentrate.

73. A mixing chamber for proportioning components of a liquid for a medical procedure comprising means for volumetric measurement of the volume (V) of each of the liquid components, c h a r a c t e r i s e d i n that the means for volumetric measurement comprise means (11) for continuous measurement of liquid levels (h) in the mixing chamber (7) .

74. A mixing chamber as claimed in claim 73, wherein the means for continuous measurement of liquid levels (h) in the mixing chamber (5) comprise an ultrasonic trans- mitter/receiver unit (11) .

75. A mixing chamber as claimed in claim 74, further comprising means for calculating, for each liquid compo- nent, a level to which the liquid component is to be filled based on a predetermined volume to be added and a cross sectional area of the mixing chamber (7) .

76. A mixing chamber as claimed in claim 75, further comprising at least one echo-generating surface (13; 13a,

13b) at a distance from the ultrasonic transmitter/receiver unit (11) .

77. A mixing chamber as claimed in claim 75, comprising two echo-generating surfaces (13a, 13b) arranged at a first and a second distance respectively from the ultrasonic transmitter/receiver unit (11) .

78. A mixing chamber as claimed in claim 76 or 77, further comprising means for calculating the velocity of sound in the liquid in the mixing chamber (5) by means of detection of at least one ultrasonic echo from said at least one echo-generating surface (13; 13a, 13b).

79. A mixing chamber as claimed in any one of claims 76-78 wherein the mixing chamber has a constant cross section as measured along a height (H) of the mixing chamber (7 ) .

80. A mixing chamber as claimed in claim 79, wherein said at least one echo-generating surface (13; 13a, 13b) is arranged to occupy a constant portion of the cross section of the mixing chamber (7) as measured along a height (H) of the mixing chamber (7) .

81. A mixing chamber as claimed in any one of claims 75-80, further comprising means for calculating adjusted levels for filling the liquid components for compensating for variations in the cross sectional area along the height of the mixing chamber (7) .

82. A mixing chamber as claimed in any one of claims 75-81, further comprising means for measuring the concentration of each liquid component and means for calculating an adjusted level to which the liquid component is filled in the mixing chamber (7) for compensating for any discrepancies as compared to an expected concentration.

83. A mixing chamber as claimed in claim 82, further comprising means for storing data on the velocity of sound for different properties of the liquid components.

84. A mixing chamber as claimed in any one of claims 73-83, which is adapted for preparation of a dialysis concentrate .

85. A device for preparing a liquid for a medical procedure, comprising a first source (5, 8, 9) of a first liquid component, a second source (6) of a second liquid component, a mixing chamber (7) for mixing the liquid components and means for volumetric measurement of the volume (V) of the liquid components, c h a r a c t e r i s e d i n that the means for volumetric measurement comprise means (11) for continuous measurement of liquid levels (h) in the mixing chamber (7) .

86. A device as claimed in claim 85, wherein the mixing chamber (7) comprises an ultrasonic transmitter/receiver unit (11) for measuring liquid levels (h) in the mixing chamber (7) .

87. A device as claimed in claim 85 or 86, further comprising means for calculating, for each liquid component, a level to which the liquid component is to be filled based on a predetermined volume to be added and a cross sectional area of the mixing chamber (7).

88. A device as claimed in claim 86 or 87, wherein the mixing chamber (7) further comprises at least one echo-generating surface (13; 13a, 13b) at a distance from the ultrasonic transmitter/receiver unit (11) .

89. A device as claimed in claim 88, wherein the mixing chamber (7) comprises two echo-generating surfaces (13a, 13b) arranged at a first and a second distance respectively from the ultrasonic transmitter/receiver unit

(11) • 90. A device as claimed in claim 88 or 89, further comprising means for calculating the velocity of sound in the liquid in the mixing chamber (7) by means of detec- tion of at least one ultrasonic echo from said at least one echo-generating surface (13; 13a, 13b) . 91. A device as claimed in any one of claims 87-90, wherein the mixing chamber (7) has a constant cross sec- tion as measured along a height (H) of the mixing chamber (7) . 92. A device as claimed in claim 91, wherein said at least one echo-generating surface (13; 13a, 13b) is arranged to occupy a constant portion of the cross section of the mixing chamber (7) as measured along a height (H) of the mixing chamber (7) . 93. A device as claimed in any one of claims 86-92, further comprising means for calculating adjusted levels for filling the liquid components for compensating for variations in the cross section of the mixing chamber (7) . 94. A device as claimed in any one of claims 86-93, further comprising means for measuring the concentration of each liquid component and means for calculating an ad- justed level to which the liquid component is filled in the mixing chamber (7) for compensating for any discrepancies as compared to an expected concentration. 95. A device as claimed in claim 86-94, further comprising means for storing data on the velocity of sound for different properties of the liquid components. 96. A device as claimed in any one of claims 85-95, further comprising a safety mechanism (16, 19) for checking the concentration of the resulting liquid mixed in the mixing chamber (7) and for discarding the contents of the mixing chamber (7) whenever a discrepancy is detected in the concentration of the resulting liquid. 97. A device as claimed in claim 85, wherein the means for continuous measurement of liquid levels (h) in the mixing chamber (7) include means for acoustic meas- urement . 98. A device as claimed in claim 85, wherein the means for continuous measurement of liquid levels (h) in the mixing chamber (7) include means for optical measurement . 99. A device as claimed in claim 85, wherein the means (8) for continuous measurement of liquid levels (h) in the mixing chamber (7) include means for capacitive measurement. 100. A device as claimed in claim 85, further comprising means for gravimetric measurement of each of the liquid components. 101. A device as claimed in any one of claims

85-100, wherein the first source (2, 6, 7) of the first liquid component comprises a reservoir (2) of dry component and means (6, 7) for passing water through said dry component for forming the first liquid component.