WO1985001993A1

WO1985001993A1 - Metering device

Info

Publication number: WO1985001993A1
Application number: PCT/SE1984/000357
Authority: WO
Inventors: Kent Lennartsson
Original assignee: Ab Rovac
Priority date: 1983-11-01
Filing date: 1984-10-24
Publication date: 1985-05-09
Also published as: SE8305997L; EP0200722A1; SE8305997D0

Abstract

A metering pump comprises a drive unit (1) and a pump head (11), into and from which fluid which is to be metered can be sucked from an input line via a first non-return valve (17) or can be expelled to an output line (20) via a second non-return valve (19). The drive unit comprises a control organ (2) which, on the basis of electrical control signals, controls a force-applying fluid or an actuating flow inside the drive unit so as to regulate movements of the pump. The pump head comprises a working space (16) and an expulsion organ (12) capable of interacting with the fluid which has been sucked into the space, said expulsion organ in the course of a space-reducing movement in the pump head expelling the sucked-in fluid into the output line. Part of the total space-reducing movement of the expulsion organ is reserved for compressing the fluid sucked into the space before the expulsion movement begins. At the metering of fluid including gas bubbles, the metering pump makes accurate metering processes possible.

Description

TITLE OF THE INVENTION Metering device.

TECHNICAL FIELD

The present invention relates to a metering device which comprises a metering pump with a drive unit and a pump head, into and from which a fluid which is to be metered can be sucked from an input line via a first non-return valve device or can be expelled to an output line via a second non-return valve device. The drive unit in this case comprises a control organ (control valve, electric amplifier or some other equivalent organ) which, on the basis of electric control signals, controls a force-applying fluid or an actuating flow inside the drive unit so as to regulate movements of the pump, for example positions of the piston and piston speeds, etc., inside the pump. The pump head in turn comprises a working space (or pump space) for sucking in and expelling the fluid and an expulsion organ capable of interacting with the fluid which has been sucked in, said expulsion organ in the course of a space- reducing movement in the pump head expelling the sucked-in fluid into the output line. The metering device is also of the kind which comprises a control or regulating unit which provides the aforementioned electric control signals for the aforementioned control organ.

DESCRIPTION OF THE PRIOR ART

Previously disclosed is the use of a pump head in conjunction with a drive unit in a metering context. The length of stroke and the piston speed can be so arranged in a previously disclosed fashion as to be capable of being regulated from a control unit so as to permit the desired quantity of fluid to be metered with the pump in question.

DESCRIPTION OF THE PRESENT INVENTION

TECHNICAL PROBLEM

A need associated with metering pumps of this type is the ability to meter even those fluids which have mixed in with them compressible elements such as gas bubbles. The presence of such elements makes metering very much more difficult, in particular if the quantity of the elements is comparatively large and/or varies. Problems are associated with maintaining accuracy of metering, and in the very worst cases metering may be completely impossible.

SOLUTION The object of the present invention is to propose an arrangement which will solve, amongst other things, the problem outlined above. What may be regarded in this respect as being essentially characteristic of the novel arrangement is that part of the total space-reducing movement of the expulsion organ is reserved for compressing the fluid sucked into the space before an expulsion movement begins, in which case the effects on the metering of the fluid of any compressible elements such as air bubbles present in the fluid are minimized.

In a first, preferred embodiment the effects on metering of any compressible elements mixed in with the metered fluid are reduced by modifying the movement of the pump to suit the content of the aforementioned elements in the fluid. The volume of gas in the fluid sucked into the working space of the pump should preferably be measured or reduced in this way.

In a further embodiment the expulsion organ is stationary for a period between the compression movement and the expulsion movement. There should also preferably be arranged inside the working space a sensor organ connected to the control organ which provides an indication during the aforementioned period of a state, for example a state of pressure, existing inside the working space.

A sensor organ of the type in question may also be arranged inside the working space for the purpose of measuring the pressure of the fluid sucked into the working space.

It is also possible to measure the degree of mixing of the compressible elements for a given expulsion pressure. Measurements in this case are taken of the pressure at the aforementioned expulsion pressure for different lengths of piston displacement, on the basis of which the gas volume can be pre-determined in accordance with the appropriate gas laws. In the case of a fluid which is able to tolerate a higher pressure, it is also permissible to utilize a return movement of the piston between the compression movement and the expulsion movement. This can be of interest when the expulsion pressure is comparatively low and occurs against a controlled non-return valve device.

In the case of a viscous fluid, for example a paste, the input line is in the form of a comparatively short line from a container used for the fluid.

ADVANTAGES

Thanks to the subject matter of invention the effect of, for example, air bubbles in a metered fluid can be eliminated to a significant degree. In the case of a fluid which contains, for example, 50% of air and which is to be metered by means of a pump not equipped with the subject matter of invention, the metering error will be about 50%, and the question must be asked whether metering is, in fact, possible. With the help of the pre- compression movement the fluid in question can be compressed to, for example, 100 bar inside the working space of the pump head, thereby reducing the error to 0.5%. As an alternative, the pre- compression movement may be used to measure or determine the gas volume at the expulsion pressure, whereupon the pump movement may be modified to suit the gas content. Advantages are also to be gained when metering a fluid containing a compressible gas, the quantity of which is dependent upon the temperature conditions at the time of metering. The subject matter of invention enables such temperature-dependence largely to be eliminated. The basis of the invention in one embodiment is that the pump shall be designed for a constant length of stroke. The range of settings for a pump equipped with the subject matter of invention can be extended considerably in relation to previously disclosed variable-stroke pumps. Thus, for example, a range of settings of between 1 and 10 litres per unit of time can be extended to between 1 and 100 litres per unit of time by the application of the invention. The error in the movement of the piston in relation to the total volume remains unaffected by the capacity of the pump, and the error in the volume which is not filled during sucking-in (referred to here as the dead volume), is also reduced.

In the aforementioned further developments of the invention the use is also proposed of a sensor unit inside the working space. By causing the sensor organ preferably to indicate, for example, the fluid pressure in the sucked-in fluid, it is possible to control the degree of filling in a simple fashion. In the event of a stationary period being used for the expulsion organ between the compression movement and the expulsion movement, it is also possible by the use of the sensor organ in question to check in a simple fashion whether, for example, any tendency to leakage is present in or on the metering pump.

DESCRIPTION OF THE DRAWINGS

A preferred embodiment of an arrangement which exhibits the significant characteristic features of the invention is described below with reference to the accompanying drawings, in which:

Figure 1 shows in the form of a block diagram and a basic circuit diagram a pump with a drive unit and pump head which is also capable of interacting with a control unit; Figure 2 shows in the form of a graph the movements of the piston inside the pump;

Figure 3 shows in the form of a graph the pressure inside the working space of the pump;

Figure 4 shows in the form of a graph the piston speeds; Figure 5 shows in the form of a block diagram the design of the control unit; Figures 6a-6c show in the form of a graph the curves for the position, pressure and speed for a modified operating plan for the pump in accordance with Figure 1; and Figures 7a-7b show in the form of a graph the curves for the position and pressure for a third operating plan for the pump in accordance with Figure 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A previously disclosed drive unit for a pump is indicated by 1 in Figure 1. The drive unit may be of the type sold by Bofors ELektronik under the model reference COP9-SE. The drive unit comprises a control organ in the form of a proportional valve or servo-valve 2 provided with an electric control input 3 and is connected to the pressure line 4 and the return line 5 leading respectively from and to a pressure source 6 , for example for hydraulic oil. The drive unit also comprises an actuating cylinder 7 , the piston of which is indicated by 8 . The position of the piston is monitored continuously by means of a position sensor 9 provided with an electric outlet 10 . The valve 2 is so arranged on the basis of electric control signals via the input 3 as to determine the effects of the force-applying fluid (the hydraulic oil) on the piston 8 in such a way as to achieve the desired positions and speeds for the piston. Alternatively, the drive unit may comprise an electric motor, and the control organ may be in the form of an electric amplifier.

The pump also comprises a pump head 11 which may be of a previously disclosed type, for example the pump head sold by Metering Pumps Limited as Type E1 or E2 (Plunger Pumpheads). The length of stroke of the drive unit is modified to suit the length of stroke of the pump head. The pump head comprises a piston 12 which is connected to the piston rod 14 of the drive unit via a connecting arrangement 15 . The aforementioned piston rod is connected to the piston 8 in the hydraulic cylinder 13 , which is connected to the pump head. The connection between the pistons 8 and 12 is in this case so executed that the movements of the pistons are co-ordinated.

The pump head also comprises a working space 16 which via a first non-return valve 17 is connected to a suction line 18 and via a second non-return valve 19 is connected to an output line 20 . The connection of the pump head to the aforementioned suction and output lines 18 and 20 is executed in a previously disclosed fashion. The suction side 18 leads to a tank 21 for the actual fluid which is to be metered, said fluid possibly containing compressible elements such as gas bubbles which may vary in their quantity and/or may account for a comparatively large part of the fluid volume.

The piston 12 describes a reciprocating motion. One end position of the piston is indicated by 12' and the other end position by 12" . The piston may be regarded as describing a working space-reducing movement from the aforementioned position 12' to the aforementioned position 12" . The piston 12 also describes a sucking-in movement from the position 12" to the position 12' , during which sucking-in movement fluid is sucked into the working space 16 from the incoming line 18 via the first non-return valve 17 . During the aforementioned working space-reducing movement, the piston 12 executes in accordance with the present invention a compression movement by means of which fluid which has been sucked into the working space 16 is compressed. The compression movement is followed by an expulsion movement which follows a possible interjacent rest period for the piston 12 . During the aforementioned expulsion movement the compressed fluid in the working space is forced out into the outgoing line 20 via the second non-return valve. During the aforementioned stationary phase for the piston, measuring or checking of the working pressure inside the working space containing the compressed fluid takes place. The pressure in the fluid compressed in this way is measured by means of a sensor organ 22 which is equipped with an electric sensor output 22a . The sensor organ of the sensor element extends into the working space 16 . The pressure indide the working space can be checked with the help of the aforementioned sensor organ. The adoption by the pressure of a pre-determined value for a compression movement of a pre-determined length during the measurement phase indicates that the correct degree of filling with sucked-in fluid has been reached, and thus that the quantity of fluid which is to be forced out during the expulsion movement will be correct. The aforementioned control phase can also be used to check for the presence of leakage tendencies in the pump head, for example between the cylinder of the pump head and the piston 12 , or at the first and second non-return valves, etc. A constant pressure during the measurement phase indicates the absence of leakage, and vice versa. An additional sensor organ 23 can be connected to the outgoing line 20 in such a way as to permit checking of, for example, the conditions of pressure also to take place in the outgoing line. The control output for the last- mentioned sensor is indicated by 23a .

The suction line between the pump head 11 and the tank 21 should preferably be short, at least in the case of viscous fluids or fluids in the form of a paste, and should have a length L of not more than about 1 m. The internal cross-sectional area of the line is also important.

In accordance with the invention the pump 1, 11 shall be capable of being controlled by means of a control unit 24 which is provided with an outlet 25 which, via a cable 26 , is connected to the control input on the valve 2 . The control unit also receives status information from the pump and the outgoing line 20 . The control unit is provided in this way with an input 27 for the sensor signals via the cable 28 from the position sensor 9 . Also provided are inputs 29 and 30 from the sensor organs 22 and 23 which are connected via cables 31 and 32 . The control unit may consist of a microcomputer-based control unit which comprises a microprocessor and its necessary peripheral equipment. The control unit 24 may in turn be connected to a supervisory system 33 . Figure 2 shows the positions of the piston 12 . The piston is stationary and its position thus remains unchanged during the period between t₀ and t₁. The piston describes its expulsion movement during the period between t₁ and t₂ and reaches the position. 12" at the time t₂ (see Figure 1). The piston returns to its second end position 12' between the times t₂ and t₃ (see

Figure 1). The piston remains stationary at the aforementioned end position 12' until the time t₄, when the piston describes a compression movement which terminates at t₅. The piston is again stationary betw the times t₅ and t₆ and recommences its expulsion movem at the t ime t₆, and so on. o

The same time a used in Figure 2 is applicable to Figures 3 and

4. Between the times t₀ and t₁ in Figure 3, a fluid is present in the working space 16 which has been pre-compressed at a pressure which has a value immediately below a utilized pumping-out pressure. The pressure is at its maximum value after the time t. and remains constant until the time t₂, when the pressure inside the working space falls momentarily as the piston reverses its direction of travel. The pressure inside the working space assumes a minimum value from the time t₂ until the time t₄, when the compression movement is resumed. The compression movement is complete at the point t₅, and a pre-compression pressure immediately below the pumping-out pressure will continue to exist until t₆, at which point a new pumping-out phase begins.

Figure 4 shows positive speed values +V for movements from the position 12' to the position 12" and negative speed values -V when the piston 12 moves from the position 12" to the position 12' . As may be appreciated from the speed diagram in accordance with Figure 4, the return (negative) speed in the typical embodiment shown is about three times as high as the positive speed. Sufficient time is available between the times t₃ and t₄ to equalize the pressure on the suction side. The speed is 0 between the times t₀ and t₁, and after the time t₁ the speed increases rapidly until maximum speed in a forward direction is reached, with the speed then being retarded immediately before the end of the expulsion movement until the value 0 is assumed at the time t₂. The piston 12 is accelerated rapidly until it reaches its maximum value on the return movement and is then retarded once more immediately before the time t₃ so that it can assume the value 0 at the time t₃. The piston then remains stationary during the period between t₃ and t₄. The piston begins to describe its compression movement at the time t₄, which it completes in the period to t₅. The aforementioned control phase takes place between the time t₅ and the time t₆, and after the time t₆ the piston 12 is once more accelerated rapidly in order to reach its maximum value in a forward direction, and so on. The control unit 24 is shown in more detail in Figure 5, in which the connections to the control valve 2 and the sensor organs 9, 22 and 23 are also shown. Communication is assumed to exist in this case between the unit 24 and the supervisory system 33 , said communication taking place via connections 34, 35 . The control unit is in the form of a card with printed circuits. The card comprises a microprocessor (CPU) 36 with the associated necessary peripheral equipment in the form of, amongst other things, the ROM memories 37, 37a, 37b and 37c and the RAM memories 38, 38a, 38b and 38c . The ROM memories are in the form of fixed programs, and the RAM memories can be loaded during a start-up phase from the supervisory system 33 . The last-mentioned memories also serve as working memories. The CPU is connected to a clock circuit 39 , to a transmit/receive circuit 40 , to a servo outlet 41 for the connection of the aforementioned control valve 2 , to a pulse generator input 42 for connection to the sensor organ 9 , to a first analogue input 43 for connection of the sensor organ 22 , and to a second analogue input 44 for connection of the sensor organ 23 . The printed circuit on the card connects the different circuits and is symbolized by the reference 45 . The connection of the terminals 33 and 34 and the terminals of the control valve 2 and the aforementioned sensor organs 9, 22 and 23 is made via a contact organ which includes a female contact component 46 and a male contact component 47 . From the upper surfaces of the female contact component project contact pins 46a with which contact and connection can be established by the terminals 34, 35 and the aforementioned valve and sensor organs.

The principle of controlling a pump by means of software is previously disclosed. The specific control described above can be achieved by means of software which can be designed in a previously disclosed fashion by the average specialist in this area once he has familiarized himself with the nature of the problem and its structure in accordance with what is described above.

The unit 40 contains so-called SIO circuits. The unit 24 can receive information from the supervisory system 33 and/or transmit information relating to the control in question of the pump. The signal II (Figure 1) transmitted via the control cable 26 is of a type which is suitable for the control of the valve 2 in question.

The length of stroke of the compression movement can be determined from and by means of the control unit which emits a value (a signal) which is determined either independently of the conditions existing inside the working space and/or the line or lines 18 or 20 or 18 and 20 , or depending on the conditions, for example the pressure ratio, existing inside the working space and/or the aforementioned line or lines. In the event of variations occurring in the air bubbles or gas bubbles, the compression movement can be modified whilst in operation so that a recorded variation in a previous pump stroke can be compensated for in a subsequent pump stroke. The length of stroke of the compression movement will be dependent upon the quantity of compressible elements present in the fluid. The length of the compression strokes should preferably be selected so as to have a minimum value of about 1%, for example 2 - 5 %, of the total working space-reducing movement. The pressure inside the working space should preferably be selected so as to have a value of between 5 - 300 bar, for example 10 - 100 bar. A typical example of the type of fluid to be metered is a plastics material such as polyester containing air.

The invention also makes it possible, by varying the pump speed. to compensate for the effect on the metering of the gas contained in the fluid. Figures 6a, 6b and 6c are intended to show examples of this. The aforementioned Figures all have a common time axis t". The Figures show that the fluid is pumped out at a constant rate and at comparatively low pressure, for example 1 - 10 bar. At the time t₁" the piston is drawn backwards rapidly so that new fluid is drawn into the working space of the pump. The fluid is caused to flow into the working space during the period between t₂" and t₃".

Between t₃" and t₄" the fluid is compressed to a high pressure against a closed outlet valve (= a modified non-return valve 17 ). The pressure is selected in such a way that the gas volume may be assumed to be negligible in this context. Between t₄" and t₅" a check is made to verify the tightness of the pump and the valves. The piston is then caused to describe a return movement, after which a pre-determined pumping-out pressure will exist; see the time between t₅" and t₆". The increase in the volume thus achieved is attributable to the expansion of the compressible element in question. The volume which is lost between the end position and the actual volume present is attributable to the so-called dead volume and to movements in the non-return valve or outlet valve. In Figure 6a the dead volume is indicated by a , whilst b is used to indicate the gas volume during the actual period.

The pumping-out pressure inside the working space is then maintained during the period between t₆" and t₇" so that the gas will stabilize.

Assuming that the gas volume V_gas , the dead volume V_{dead volume} and the original pump volume V_chamber are of known value, the dependence of the gas on the metering can be compensated for with the help of the pump speed. V_fluid = V_chamber - V_{dead volume} - V_gas 1

V_{pump volume} = V_fluid + V_gas 2

Pump speed = Vpump volume t

By inserting 1 into 2 .and 3 , and when the following are of known value: a) the value of V_chamber is known; b) the fluid flow has been set to the desired level; c) the values of V_gas and V_{dead volume} have been measured, it is possible to calculate the pump speed required in order to compensate for the gas by inserting 2 and 3 into 4 .

It is possible in this way to compensate by means of the pump speed after t7" in such a way that the fluid flow will be unaffected by the quantity of gas mixed in. After t₇" the valve (of. non-return valve 17 ) which is situated in the outlet is opened, and the contents are pumped out at the speed which has been compensated to take into account the presence of the gas.

Figures 7a and 7b show an example in which it is possible, by modifying the pump speed, to compensate for dynamic and static levels of gas in the fluid without increasing the pressure beyond the pumping-out pressure.

In the present case the pump is so executed as to exhibit a stroke volume of 0.6 litre and a chamber volume of 0.8 litre. It is also assumed that 0.1 litre of the chamber volume is occupied by gas at 1 atmosphere, and that 0.05 litre of the chamber volume was not filled, that is to say that the dead volume is equal to 0.05 litre.

The fluid is pumped out at a constant rate initially. At t₁ ' ' ' the piston is drawn backwards rapidly so as to enable new fluid to be sucked in from the tank.

The pump is filled for a short period between t₂'" and t₃''', which is particularly important in the case of viscous fluids. Between t₃''' and t₄''' the pressure is increased to a good 1 atmosphere in order to eliminate the dead volume and any condensible substances in the sucked-in fluid. The non-return valve on the suction side is also closed. Stabilization of the fluid, as outlined above, takes place during the period between t₄''' and t₅'''.

At the time t₅''' the volume inside the pump is measured at a good 1 atmosphere in respect of the stabilized fluid before the volume of the pump is reduced. At two different pressures P₂ and P₃ V₂, and V₃ are recorded respectively by the control unit. The results are utilized in conjunction with an appropriate formula for the gas laws. It is assumed in this case that the most suitable formula will be one having adiabatic form, that is to say:

By measuring the curves, it will be found in the present case that V_g = 0.11 litre (the curves were plotted on the basis of the assumption that V_g = 0.1 litre).

The dead volume in accordance with the above is 0.05 litre, for which reason the quantity of fluid is V_fluid = 0.8 · 0.05·0.11 = 0.64 litre.

If the pump has been set to pump 0.1 litre/s at a pressure of 2.25 bar, then compensation will have to be made by means of the pump speed in order to avoid the dependence on the gas.

At a pressure of 2.25 bar, V_g = 0.11 P1 1/1.4 = 0.0616 litre, in P2 accordance with gas law based on adi abati c theory.

V_chamber = V_flow + V_gas = 0.64 + 0.616 = 0.7016 li tre

pump speed = f luid + gas + 0.7016 t t t f luid pumping speed = ^Vf luid <=> 0.64 = 0.1 <=> t = 6.4 t pump speed = 0.7016 = 0.1096 l itre/s

0.4

The pump speed can be calibrated in this way for each stroke of the pump, thereby providing compensation for the static and dynamic levels of gas present in the fluid without the pressure requiring to be increased beyond the pumping-out pressure. This may be necessary when metering of the fluid is required and/or if a pump is used which is sensitive to high pressures.

The invention is not restricted to the embodiment described above by way of example, but may undergo modifications within the context of the following Patent Claims and the idea of invention. Thus, for example, one may avail oneself of double pump heads or double-acting pumps connected to a drive unit. In this way the compression movement or pumping-out movement for one pump volume is utilized in order to fill the other pump volume with new fluid from the tank. It is also possible to use two or more pump heads in accordance with the above for the purpose of pumping into a common line. An arrangement of this kind thus comprises pump organs having two or more pump heads connected in parallel and leading into a common line for metered fluid, with each of the aforementioned pump heads being connected to its pressure side and its suction side via nonreturn valve organs (see above). Each pump head is connected to its own drive unit which operates in a mechanically independent fashion in relation to the drive unit or drive units for the other pump head or pump heads. Each drive unit is allocated a control organ, for example a servo-valve or an electric amplifier, etc., which, on the basis of electrical control signals, controls a force-applying fluid or an actuating flow in such a way as to achieve the desired pump movements. The control organs for the drive units are connected to a common control unit, said control unit being so arranged as to provide, via the aforementioned control units, the co-ordinated actuation of the pump heads in such a way as to cause these to apply to the outgoing line a pressure or an actuating flow during the metering period which is free of significant variations from the desired pressure or flow or variations in the pressure or flow in the Line. It is possible to arrange in one or more pump heads a sensor organ which monitors a state, for example a pressure, a temperature or a fluid density, etc., inside the working space of the pump head, with each sensor organ being connected to the control unit. The pump piston will preferably cause the actuation of a position sensor, with this too being connected to the control unit.

Each pump head with its associated drive unit and pump head should preferably be of identical construction to that of the other pump or pumps. The suction sides of the pump heads may be connected to the same or to different fluid containers, and a sensor organ arranged in the outgoing line may similarly be connected to the control unit.

Claims

PATENT CLAIMS

1. A metering device comprising on the one hand a metering pump with a drive unit (1) and a pump head (11), into and from which a fluid which is to be metered can be sucked in from an input line (18) via a first non-return valve device (17) or can be expelled to an outlet line (20) via a second non-return valve device (19), in which case the drive unit also includes a control organ (2), for example a control valve or an electric amplifier, etc., which, on the basis of electric control signals, controls a force-applying fluid or a flow inside the drive unit so as to regulate movements of the pump, for example positions of the piston and piston speeds, etc., inside the pump, and the pump head contains a working space (16) for sucking in and expelling the fluid and an expulsion organ (12) capable of interacting with the fluid which has been sucked in, said expulsion organ in the course of a space-reducing movement in the pump head expelling the sucked-in fluid into the output line, and on the other hand a control unit (24) which produces the aforementioned electric control signals, c h a r a c t e r i z e d in that it is so arranged, by making use of a part (t₄ - t₅) of the total space-reducing movement of the expulsion organ as a compression movement for the fluid sucked into the space before a movement

(t₁- t₂) to expel the fluid from the working chamber begins, as to provide metering which is precisely controlled in its entirely irrespective of any compressible elements and/or viscosity present in the fluid.

2. Arrangement in accordance with Patent Claim 1, c h a r a c t e r i z e d in that the length of stroke of the compression movement is pre-determined during metering with the help of the control unit (24).

3. Arrangement in accordance with Patent Claims 1 or 2, c h ar a c t e r i z e d in that the length of stroke of the compression movement is controlled by the control unit in relation to the state, preferably a state of pressure, existing inside the working space and/or one or both of the aforementioned lines (18, 20).

4. Arrangement in accordance with Patent Claim 3, c h a r ac t e r i z e d in that the sucking into the working space is done from a fluid container (21) via a short input line (18), in particular in the case of a viscous fluid such as paste.

5. Arrangement in accordance with any of the preceeding Patent Claims, c h a r a c t e r i z e d in that the elimination of the effects on the metering of the fluid from the aforementioned compressible elements is achieved by modifying the movement of the pump to suit the content of compressible elements in the sucked-in fluid.

6. Arrangement in accordance with any of the Patent Claims 1-5, c h a r a c t e r i z e d in that the quantity of compressible elements is measured and the speed of the piston is varied in such a way as to increase for a large quantity of mixed-in compressible elements, and vice versa.

7. Arrangement in accordance with any of the preceeding Patent Claims, c ha r a c t e r i z e d in that the expulsion organ is stationary for a period (t₅ - t₆) between the compression movement and the expulsion movement, and in that inside the working space is arranged a sensor organ (22) connected to the control unit which provides an indication during the aforementioned period of the state, for example a state of pressure, existing inside the working space.

8. Arrangement in accordance with any of the Patent Claims 1-8, c h a r a c t e r i z e d in that inside the working space (16) is arranged a sensor organ (22) connnected to the control unit which provides an indication after the compression movement and before the expulsion movement of the pressure existing in the fluid sucked into the working space.