GB2256478A

GB2256478A - Flow measurement device

Info

Publication number: GB2256478A
Application number: GB9112351A
Authority: GB
Inventors: Malcolm Charles Brown; Paul Edwin Hammond
Original assignee: Masar Ltd
Current assignee: Masar Ltd
Priority date: 1991-06-08
Filing date: 1991-06-08
Publication date: 1992-12-09
Anticipated expiration: 2011-06-08
Also published as: GB2256478B; DE4218899A1; GB9112351D0

Description

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FLOW MEASUREMENT DEVICE FIELD OF INVENTION

The present invention concerns flow measurement devices for testing medical infusion means.

BACKGROUND OF THE INVENTION

In British Patent 2200445, we describe a flow measurement device wherein liquid is directed into an upright tube with a plurality of optical sensors and with a logic unit which accepts signals from those sensors, and if the time difference is within acceptable limits calculates flow rate, but if not calls up another time difference derived from another pair of sensors and checks that, and if that is within limits calculates flow rate. Whilst that device is an improvement on previously known devices, there are circumstances when the performance is not ideal. Thus when an infusion device does not have a steady flow such as a small diaphragm cassette infusor, which delivers a succession of small volumes of say 0.03m1, the fluid pulses can move the meniscus past the sensors and the calculated flow rate, which relies on the time difference sensed and the volume between the sensors, can be inaccurate. Also the smallest volume measured between the first two sensors is intentionally small to give a short elapsed time before the calculation of flow rate, but this is at the expense of accuracy which is improved in proportion to the volume (length of tube) used in the measurement. Moreover the testing of f low rate is f rom a series of volume samples after which the meniscus is lowered below the sensors and the process repeated. outside the sample volumes the flow is unmeasured. The aim of the present invention is to provide an improved measurement device. SUMMARY OF THE PRESENT INVENTION

The present invention concerns a flow measurement device for testing medical infusion means wherein the flow of an aqueous liquid to be measured is directed: into-- a vertical tube or container with associated means-.for sensing the height of the liquid, essentially along the entire length of the tube, and substantially continuously, and a logic means for setting an initial fluid height sensed and for calculating flow rates from the rate of rise of the liquid.

Since monitoring of the height of the fluid is substantially continuous it is possible to calculate and display an accurate average flowrate and/or ( with less accuracy) current flow rates. Measurement of the current flow rate enables the operator, with the assistance of the logic means, to identify repeating cycles of the infusion device, and thus better calculate the longer term average flowrate of the device.

The sensor system may be a continuous strip of photo- sensitive material, such as a photo-diode, mounted externally and parallel to the central axis of the tube and illuminated through the tube or container by a light source, such that the output from the sensor is proportional to the height of fluid in the- tube, or the sensor may conveniently be an array of optical sensors so close together that effectively the meniscus is continually tracked. Such effect is provided by a linear charge coupled device (CCD) as is found in FAX machines. Alternatively the sensor may be a capacitance device which measures capacitive reactance between, say, an outer metallised coat on the tube or container and an electrode in or near the centre of the tube. In the latter case the actual capacitance depends on the level of liquid in the tube. A capacitance measurement may be temperature dependant or otherwise liable to drift, but suitable automatic calibration and drift correction can be provided by a pair of optical sensors near the bottom and top of the tube.

The logic means can be arranged to sense the start of a liquid movement at any point along the tube and, in appropriate circumstances, the start and end points can be used to give a reading of the bolus after occlusion.

Preferably the device also measures other related performance parameters, in particular bolus (that is the initial surge of fluid when a blockage in the fluid path of the infusion means is released) and flow performance when the device is operated against constant back pressure (negative or positive).

Further, the use of the entire length of the tube permits less frequent emptying of the tube with consequent reduction in the volume of fluid delivered during non-measuring periods.

A further optional variant on the measuring system uses two measuring tubes, used alternately so that substantially all the liquid delivered is measured. One tube is used for measurement while the other tube is being emptied.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic vertical section of a measuring device according to the present invention using an optical method.

Figure 2 is a logic circuit in block form suitable for use with the device shown in Figure 1.

Figure 3 is a detail of a schematic vertical section of the measuring device according to the present invention using a capacitance method.

Figure 4 is a logic circuit in block f orm suitable for use with the device as shown in Figure 3.

Figure 5 is a detail of a two tube arrangement which may be used with either the optical or capacitive methods shown in Figures 1 and 3.

DESCRIPTION OF EXEMPLARY EMBODIMENT

In Figure 1 a f low to be measured enters a f low measuring device, according to the invention, through a multipath valve ( or combination of valves achieving the same result) and is directed into a vertical transparent tube 11. This tube is illuminated from the side by a strip light 12 or collimated light from a lamp and lens arrangement, so that the light passes through the tube to the opposite side where a closely spaced array of sensors 14 (e.g. a Charge Coupled Array Device - CCD) or a continuous strip of photo- sensitive material, such as a photo-diode is placed. The valve arrangement is a multipath arrangement so that, under the control of a logic unit 17, flow can be blocked, passed to a drain 15 and the tube can be discharged to the drain. The drain can be connected to the top of the tube or container so as to form a pressurisable close loop. Thus the top end of the tube is formed as a convergent nozzle 18 which enters a wide bore tube 19. The nozzle breaks up the size of liquid drops leaving the top of the tube 11 and the tube 19 has a bore of say 8 millimetres so the drops cannot bridge this tube, but run down the sides minimising the risk of an airlliquid mixture re- entering the tube 11. The tube 19 leads down to a sump 20 into which the drain 15 empties. This sump is of a sealed type and can be pressurised by a pump 21.

Figure 2 is a simplified block diagram of the logic unit 5 17. This receives inputs from the charge coupled device 14 and employs a pulse generator or other means 23 of converting the output of the ccd optical array or photo-sensitive strip into an electrical signal which is proportional to the height of the liquid in the tube or container. A central processing unit 24 receives inputs from a timer 25 and the signal from the means 23, and employs calibration factors to compensate for tube and other possible causes of inaccuracy to generate a suitable output to send to a display device 26. The logic in the central processing unit senses whether there is a change of signal, denoting that the height of the liquid is changing positively, and only when the height is changing permits, possibly after a slight delay, measurement of motion and time to occur and a calculation to start. The central processing unit also provides control signals to the valve 10 on line 27 and receives position signals back from the valve on line 28. These position signals can also be used to identify a starting level of liquid and initiate a measuring operation. The valve will normally be operated when the liquid is approaching either the top of the tube 11, to dump the contents of that tube into the drain, or at the bottom to start flow into the tube 11. In addition, a switch 29 can be operated so that the valve can be closed to occlude flow into the tube and then opened so that there is a bolus or surge and the processing unit would then merely measure the change of level. The switch can be part of an operator control console 29a allowing the operator to request the logic unit to display:

1) average flow 2) current (incremental) flow 3) the bolus volume and 4) occlusion pressure and/or to send results to another display device, such as a printer, and possibly to control the back pressure in the sump. Display of average flow can be inhibited for a predetermined time and/or a certain flow volume to ensure z acceptable accuracy for average flow rates.

Figure 3 shows an alternative arrangement in which the tube or container is sheathed in a conductive coat 31 and has a central metal electrode 32 and a capacitance measurement device 33. Two optical sensors 34 and 35 are also arranged to verify the position of the meniscus near the bottom of the tube and the other near the top. Small holes are made in the outer conductive coat and the central electode is placed eccentrically so that the optical paths for the sensors are not obstructed. The valve arrangements are similar to that in Figure 1 above, and with the same functions. Different capacitance arrangements are possible, for example the tube can be of rectangular section with opposite large sides metallised, but the illustrated arrangement is preferred.

Figure 4 is a simplified block diagram of the logic unit used with the device of Figure 3. The level of the liquid is determined from the capacitance between the outer metallised coat and the inner electrode. The capacitance measured, neglecting end effects, may be closely approximated by the f ormula:

C= Ll(Cg + Cw)/(Cg x Cw) + (Lt - Ll)(Cg + Ca)/(Cg x Ca) Where: Cg = capacitance/unit length of glass tube Ca = capacitance/unit length of air space Cw = capacitance/unit length of water However the dielectric constant of the liquid (aqueous) is dependent on temperature, and the impurity of the aqueous solution (e.g. saline of unknown concentration). The device is calibrated by measuring by means of the capacitance measuring device 33 the contents of the tube 11 between the two optical sensors, and then comparing this measurement with the accurately known and premeasured contents, and introducing a parameter into the calculations of flow rates and bolus measurements. The volume of liquid infused between these two points is established during a manufacturing calibration process. The volume is related to the capacitance change to calibrate the unit for the liquid used before each use of the instrument. Further, every time liquid traverses the distance between the optical sensors during normal use, the calibration of the capacitance change is corrected to improve the accuracy of the displayed result before the liquid reaches the top sensor on the following measurement cycle. In practice the capacitance change will not be a linear function of the length of the fluid column and predetermined calibration tables will be available to the central processing unit to adjust for this. often it is desired to maintain a flow even when the tube is emptying and this can be achieved by simultaneously dumping the inlet flow to the drain whilst emptying the tube. However, no measurements are taken during this time and sometimes it is useful to observe flow rates over a long period. This can be achieved by the modification shown in Figure 5 which can be used with the devices of Figures 1 and 2, or Figures 3 and 4. In this modification there are two tubes 11a and 11b and valve arrangements comprising a main valve 10a for directing flow into one or other tube 11 a and 11 b, and a valve 1 Ob, and a valve 10c, for controlling discharge to the drain.

In the Figures all valves are shown as rotary valves but in practice the functions described may be achieved with other types of valves or combinations of valves.

Z.

Claims

C L A I M S 1. A flow measurement device for testing medical infusion

means wherein the flow of an aqueous liquid to be measured is directed into a vertical tube or container with associated means for sensing the height of the liquid, essentially along the entire length of the tube and substantially continuously, and logic means for setting an initial liquid height sensed and for calculating flow rates from the rate of rise of the liquid.
2. A flow measurement device according to claim I wherein the sensing means is a strip of photo-sensitive material mounted externaly and parallel to the central axis of the tubb and illuminated through the tube by a light source, such that the output from the sensing means is a function of the height of liquid.
3. A flow measurement device according to claim I wherein the sensing means is an array of closely spaced optical sensors.
4. A flow measurement device according to claim I wherein the sensing means comprises capacitor electrodes disposed so that the liquid will form a dielectric and a capacitance measurement device.
5. A flow measurement device according to any one of claims I to 4 wherein the logic means senses the start of a liquid movement at any point along the tube and then calculates a flow rate from subsequent movement.
6. A flow measurement device according to claim 5 wherein the logic means can be set to measure the bolus volume.
7. A flow measurement device according to any one of the preceding claims whe I rein the logic means can calculate and display average flow, current incremental flow, bolus volume and occlusion pressure.
8. A flow measurement device according to any one of the preceding claims wherein the tube is part of a closed loop.
9. A flow measurement device according to any one of the preceding claims wherein there are two such tubes - 8 and valve means for directing flow to one tube whilst the other is emptying and vice versa.
10. A flow measurement device substantially as herein described with reference to Figures 1 and 2 or Figures 3 and 4 and possibly as modified as shown in Figure 5.