GB2181494A

GB2181494A - Piston dosing pump for a fluid

Info

Publication number: GB2181494A
Application number: GB08623782A
Authority: GB
Inventors: Martin Ryschka
Original assignee: Draegerwerk AG and Co KGaA
Current assignee: Draegerwerk AG and Co KGaA
Priority date: 1985-10-03
Filing date: 1986-10-03
Publication date: 1987-04-23
Also published as: GB2181494B; GB8623782D0; DE3535329A1; FR2588320A1; SE8604186D0; SE8604186L

Abstract

A pump has a compensating vessel 7 off the inlet duct 2 leading to the pumping chamber 6 between the pistons 4,5. In operation, liquid will find its way in to vessel 7 and some will automatically be given up when not enough to fill chamber 6 is supplied directly down duct 2. This is particularly useful when pumping low-boiling liquids such as anaesthetics which can vaporise under the low pressure of the inlet stroke. Preferably the vessel 7 is in the form of metallic bellows. <IMAGE>

Description

SPECIFICATION Piston dosing pump for a fluid This invention relates to a piston dosing pump, which is for use with a fluid which is sucked into a pump space through adjustable piston stroke movements by way of a feed and a suction inlet.

One pump of this kind is described in DE OS-30 38 525 and the corresponding British Patent Application, Publication No.

2,086,993A. This known pump has two pistons, of which the one, as a fixed piston, executes a regular full stroke while the other, as a free, or floating, piston, executes only a part of the stroke on account of a brake force acting thereon. The difference in the stroke paths of the two pistons determines the pump space, which is also limited by the common cylinder bore for the pistons. During the intake stroke, the medium to be conveyed is sucked into the pump space via a feed line and a suction inlet. During the following pressure stroke, it is displaced, through a corresponding outlet, out of the pump space, as the fixed piston is forced into contact with the braked free piston and thus forces the medium out of the pump space.

Known piston dosing pumps of this kind are advantageously used for dosing liquid anaesthetics. In such a situation, it is required to add very small quantities of a medium, in the microlitre range, in a constant and reproducible manner to the respiratory gas which is to be supplied to the patient. The dosing of the quantity of anaesthetic determined by means of the adjustable piston stroke movement is then of great significance. Due to the small quantities to be conveyed, the piston movements are required to be fast and with short strokes. Then, during the suction process, a low pressure develops in the pump space which is to be filled, as a result of the dynamic resistance of the filled feed lines. This low pressure is then encountered by the anaesthetic which has been sucked in.As a result of this, and because of the high vapour pressure of anaesthetic liquids, gas bubbles may form during the suction stroke, preventing the pump space from being completely filled and causing a serious error in the dosed volume.

Although the disadvantage of gas bubble formation during the suction process applies to liquids, and particularly to liquid anaesthetics, a similar problem might exist when pumping a gaseous medium, namely if the subsequent flow into the pump space of the gas, which has been sucked in, is restricted so much by the dynamic flow resistance of the feed lines that the pump space is not completely filled in the preset time.

The present invention seeks to improve a piston dosing pump of the kind described above, so that the medium which is sucked in can flow on into the pump space without hindrance from the dynamic resistance of the feed lines.

According to the present invention, there is provided a piston dosing pump for a fluid which is to be sucked, via a feed duct, through a suction opening into a pump space by piston stroke movements, the pump having a compensating vessel for containing such fluid, which vessel is connected by a duct to the feed duct close to the suction opening.

Using pumps according to the present invention, during the suction process, the medium to be conveyed, for example a liquid anaesthetic, can be sucked out of the compensating vessel into the pump space at a high speed along short feed routes with low resistance. The dynamic resistance of these feed lines is now so small that the development of low pressure, which might encourage gas bubble formation, may be avoided. During the suction, the volume of the compensating vessel is reduced by the amount of fluid sucked in, and the compensating vessel can then be filled up again from a storage container connected to the feed. There is sufficient time for the compensating vessel to be refilled from the storage container during the following piston stroke, when the fluid located in the pump space is transferred to the outlet.

The dynamic resistance of the filled pipelines then no longer matters.

The compensating vessel can be filled from the storage container in different ways. For example, it might be achieved by pressure compensation, using the hydrostatic pressure exerted on the contents of the compensating vessel by a liquid medium in the storage container, or by using a compensating container which is elastic in form and which, after the quantity to be dosed has been withdrawn and its volume has been reduced accordingly, automatically returns to its original shape. This development of the compensating container is suitable for both gaseous and liquid media.

Advantageously, the compensating container is formed as a metallic bellows.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, in which the single Figure shows a section through a dosing pump according to one embodiment of the invention.

In the single Figure, there is shown a section of the pump body 1, which is known from DE-OS-30 38 525, where it is shown in full. A fixed piston 4 and a free piston 5 run in a cylinder bore 11, and are driven, as described in DE-OS-30 38 525, by a drive mechanism, which is not shown. The two pistons 4, 5 define a pump space 6 which is in communication with an inlet duct 2. A storage container 9 is connected to the inlet duct 2 by a pipeline 8. A compensating vessel 7 in the form of a bellows is connected by a connecting line or duct 10 to the inlet opening 2.

The outlet duct 3 is continued as an outlet line (not shown) to the consumer.

In the manner shown, the piston dosing pump executes a suction stroke during which, as a result of the low pressure prevailing in the pump volume 6, a liquid anaesthetic, for example, is sucked out of the compensating vessel 7 into the pump space 6 along the short connecting line 10, which has a low resistance. Subsequently, both the fixed piston 4 and the free piston 5 move in the direction of the outlet duct 3. When the pump space 6 is adjacent the outlet duct 3, the free piston 5 stops moving, so that, as the return stroke of the fixed piston 4 begins, the volume of the piston space 6 is reduced and hence its contents are ejected out through the outlet duct 3. When the end surfaces 12, 13 of the two pistons 4, 5 come into contact, the free piston 5 is forced into the original position shown in the Figure and described earlier. Between suction strokes, the compensating vessel 7 is refilled from the storage container 9 by way of the pipeline 8. The elastic bellows 7 then expands until it has reached its original volume again.

Clearly, by using compensating container 7 with a relatively short connection between it and the inlet opening 2, the resistance to flow is very low as compared to that of feed path 8. Formation of gas bubbles in the pump chamber may be avoided therefore, by appropriate dimensioning.

Claims

1. A piston dosing pump for a fluid which is to be sucked, via a feed duct, through a suction opening into a pump space by piston stroke movements, the pump having a compensating vessel for containing such fluid, which vessel is connected by a duct to the feed duct close to the suction opening.

2. A piston dosing pump as claimed in claim 1, wherein the volume of the compensating vessel may be varied in volume and the compensating vessel is elastic.

3. A piston dosing pump as claimed in claim 2, wherein the compensating vessel is a metallic bellows.

4. A piston dosing pump for a fluid, comprising: a hollow cylinder; first and second pistons, slidably located in the cylinder and defining a pump space there between; an inlet duct, to which a supply of the fluid may be connected; an outlet duct; and a compensating vessel, the compensating vessel being connected to, and located adja cent, the inlet duct, such that it may be filled from the supply of the fluid and may supply fluid to the inlet duct.

5. A piston dosing pump, substantially as herein described with reference to, or as shown in, the accompanying drawings.