Method for detecting a liquid level in a container in a circuit and a dialysis machine for actuating the method

A container of a dialysis machine has a determined shape and a known volume occupied in part by a volume of a mass of blood; a remaining part is occupied by a volume of a mass of gas. A disturbance is induced in a combined mass of the liquid and the gas in the container and an entity of the disturbance is measured. Measurements of pressure in the container before and after the disturbance are taken, the volume of gas is calculated using a function correlated to a gas law, and the volume and level of the liquid in the container are derived after the volume occupied by the gas is calculated.

BACKGROUND OF THE INVENTION

The invention relates to a method for detecting a liquid level in a container of a circuit.

In particular, the invention relates to a method for detecting a level of blood in a container of a circuit of a dialysis machine, to which the present application will make specific reference without in any way limiting the scope of the invention.

A known-type dialysis machine comprises a first blood circulation circuit and a second circulation circuit for the dialysate liquid. The first circuit and the second circuit are connected to a filter for conveying, respectively, the blood and dialysate liquid through the filter, which is provided with a semi-permeable membrane separating the blood from the dialysate liquid. The first circuit is provided with a container, known as a drip chamber, into which the blood is supplied from a first tract of the first circuit, and drips and collects on the bottom of the container, thence to enter a second tract of the first circuit. The container has the function of preventing air from becoming trapped in the blood in the form of bubbles, which might cause embolisms once the treated blood was returned to the cardio-vascular system of the patient. To guarantee the safest possible treatment the blood level in the container must be maintained within an optimum range of values, below which the possibility of creating air bubbles in the blood returning to the patient exists, and above which the pressure increases to unacceptable values which are dangerous for the patient.

To solve this problem, the prior art teaches blood level detection devices, comprising an optical emitter arranged on one side of the container and an optical reader arranged on another side of the container at an optimal level. This sensor device detects only if the level of blood is above or below the optimal level and is therefore unable to provide an accurate level reading. To obtain a more accurate blood level reading, the above-described sensor device has been modified to include two optical emitters and two optical readers suitably arranged, which provide an acceptability interval parameter of the blood level.

Still more accurate readings can be achieved with a plurality of optical emitters and a plurality of optical readers, which define a plurality of intervals and detect the interval which the blood level is at.

The above-described sensor devices are based on the principle of emission and reception of a signal and become progressively more complicated as the need for more accurate blood level readings increases, since the number of emitters and readers increases together with the need for accuracy.

SUMMARY OF THE INVENTION

The main aim of the present invention is to provide a level sensor method in a container of a circuit, which method is without the drawbacks inherent in the prior art and which, in particular, provides a high degree of accuracy and requires the use of simple and economical equipment.

The present invention provides a method for detecting a level of liquid in a container connected to a circuit, the container being of a determined shape and having a known volume occupied in part by a volume of a mass of liquid and, in a remaining part, by a volume of a mass of gas, the method being characterised in that it determines the volume of the mass of gas in order to calculate the volume of the liquid and the level thereof.

The present invention also relates to a dialysis machine.

The present invention provides a dialysis machine for actuating the method characterized in that it comprises a pressure sensor for detecting the pressure of the mass of gas in the container.

DETAILED DESCRIPTION

With reference toFIG. 1,1denotes in its entirety a dialysis machine comprising a blood circulation circuit2, which during operation is connected up to the cardio-vascular system of a patient, in order to convey the patient's blood during a dialysis treatment. At the end of the treatment the circuit2is eliminated as it is disposable as special waste after one use only.

The dialysis machine1comprises a control unit3and a peristaltic pump4for circulating the blood in the circuit2.

The circuit2comprises a container5, a supply branch6for taking blood to the container5which is trained about a peristaltic pump4, and a return branch7taking the blood from the container5. The container5comprises an upper wall8, through which the supply branch6is connected to the container5, a lower wall, to which the return branch7is attached, and a lateral wall10.

The machine1comprises a pressure sensor11arranged along the supply branch6directly upstream of the container5, a temperature sensor12arranged along the lateral wall10of the container5, and a pressure sensor13arranged along the return branch7. Alternatively, the pressure sensor13can be substituted by a flow rate sensor14, which is illustrated in a broken line inFIG. 1, and detects the flow rate Qoutof blood exiting from the container5. The pressure sensor13reads the pressure Pbat a predetermined point in the branch7and enables calculation of flow rate Qoutby means of a constant H of known loss of head along the return branch7comprised between the container5and the pressure sensor13. The pressure sensor11, the temperature sensor12, and the pressure sensor13are connected to the control unit3, which is connected in turn to the peristaltic pump4for controlling and commanding the peristaltic pump4and for reading, at the same time, the flow rate of the blood Qinintroduced into the container5by the peristaltic pump4.

During operation, the blood removed from the patient is fed into the container5through the supply branch6, where it drips and is collected on the bottom of the container5from which it is removed through the return branch7.

The container5is hermetically sealed and is connected only to the supply branch6and the return branch7. The container5exhibits a constant volume VC, which is in part occupied by a mass M of blood corresponding to a volume V of blood and a mass N of air which corresponds to a volume VAat a predetermined pressure P.

The pressure sensor11monitors the air pressure P (which corresponds to the blood pressure in the container5) present in the upper part of the container5and transmits the values detected to the control unit3, while the temperature sensor14monitors the temperature T of the air contained in the container5and transmits the read values to the control unit3, which receives the measurements of the flow rate at inflow Qinand the flow rate at outflow Qout.

Determination of the level of blood in the container5is done by means of a calculation of the volume V of blood, which is determined as the difference between the volume VCof the container5and the volume VAof the air, which volume VAis determined by means of a function correlated to a law relating to gases on the basis of the values transmitted to the control unit3. As the calculation of the volume of air done using a gas law, such as Perfect Gas Law, (also known as Boyle-Mariotte's Law) namely PV=JRT, also requires measurement of the number J of moles of air present in the container5, as well as two easily measurable amounts i.e. pressure P and temperature T, the method is based on the principle of disturbance of the overall mass contained in the container5.

This in effect means inducing a change in the mass M of blood in the container5, calculating the entity of the ensuing disturbance corresponding to the variation in the volume V of blood, which can be calculated from the integral of the balance of the blood inflowing flow rate Qinand the outflowing flow rate Qout, and detecting the effects of the disturbance, which correspond to a change in the pressure P of the air, the temperature T remaining practically constant. With the disturbance caused to the mass M, the volume VAoccupied by the air can be calculated and so can the volume V and level of the blood.

The present invention presupposes that the blood is a non-compressible liquid and that the measurement of the level will be more accurate according to how true the non-compressible aspect is. Tests have shown that blood at the machine working pressures in dialysis machines does in fact behave as a non-compressible liquid: there therefore exists a directly proportional relationship between the mass M of blood and the volume V of blood in the container5.

In order to explain the invention in more detail, there follows an example relating to the calculation of the level following creation of a disturbance in the mass M of blood.

The container5contains, at a determined moment t0, a volume V of blood and the blood inflows at a flow rate Qinby means of the peristaltic pump4, while outflowing blood from the container5occurs at a flow rate Qout. The overall volume of the container5is VC, thus the volume VAoccupied by the air at t0is VC−V, while the air pressure at t0is equal to P0. The peristaltic pump4operation modes cause a variation in the inflow flow rate Qinand the outflow flow rate Qoutand therefore cause a cyclic variation in the volume V of blood in the container5. Thus, at a determined moment t1the change in the blood volume is VDand the following expression of the relations results:
P0(VC−V)=J R Tat momentt0
P1(VC−V−VD)=J R Tat momentt1
in which the number J of moles of air remains constant, R is a constant, and the temperature T is considered to be constant. From the above expressions the following can be derived:
P0(VC−V)=P1(VC−V−VD)
in whichVD=∫t0t1⁢(Qi⁢⁢n-Qout)⁢ⅆt
from which the volume V at moment t0is derived asV=Vc-P1·VDP1-P0
and the volume at t1is V+VD.

The value of V can be derived from the inflow flow rate Qinand the outflow flow rate Qout, i.e. the disturbance caused, and from the pressure P change, i.e. the effect of the disturbance. From the value of V the level of blood contained in the container can be determined. In this case, the circuit2must be equipped with the flow rate sensor14in order to detect the outflowing flow rate Qout, from the container5; and the control of the peristaltic pump4r.p.m. must provide the inflowing flow rate Qin.

Alternatively, on reading the outflowing flow rate Qout, the pressure Pbread at a determined point along the return branch7enables the outflow flow rate to be determined using the following equation:
Qout=H·(P−Pb),
in which H is the loss of head in the return branch7comprised between the container5and the pressure sensor13.

The temperature T is monitored only for the purpose of determining if there occur any relevant changes in the temperature T and, therefore, for the purpose of evaluating whether the measurement taken is valid. However, it has generally been the case that the change in temperature T is not appreciable and the temperature sensor12can be left out. The function PV=JRT can be rewritten as PV=NK in which K is a constant that comprises the value of the temperature T and the constant R of the gas, while the number J of moles of air is related to the mass N of air.

In the variant ofFIG. 2, the peristaltic pump4is left out and the machine1comprises a pump15connected to the upper wall8of the container5by a conduit16and controlled by the control unit3. The pump15is a positive displacement pump supplying an air flow rate QAwhich varies according to the number of pump15revolutions per minute.

In this case, the function PV=NK is used to evidence the variation in the mass of gas determined by the pump15. During operation, the pump15sends a determined mass DN of air into the container5to calculate the volume VAof air on the basis of the disturbance in the pressure P. To clarify this calculation process, a further example is now given.

The container5contains a determined volume V of blood which corresponds to a mass M of blood and both the blood supply and evacuation are interrupted. The volume occupied by the air is VA, which corresponds to a mass N of air. The overall volume of the container5is VC, therefore the volume occupied by the air is VC−V, while the pressure detected at a determined moment t0is P0. The slight pressure variations P lead to establish that the temperature T can be considered constant.

In the above established state, at moment t0the following is a valid expression:
P0(VC−V)=N0Ka)

The pump15induces a disturbance in the container5, which is a variation in the mass N0of air, by injecting a mass DN of air into the container5, to bring the mass of air up to a level expressed by:
N1=N0+DN.b)

The disturbance in the mass of air determines a variation in pressure P inside the container5. Following the variation in mass, at moment t1the following is a valid expression:
P1(VC−V)=N1K.c)

Putting the three expressions together (a, b, c), unknowns N0, N1and V are derived, while P0and P1are measured, VCis known from the geometry of the container5and DN is derived from the following equation:DN=∫t0t1⁢QA⁢ⅆt
in which QAis the flow rate of the pump4.

Once V has been determined, as the geometry of the container5is known, the level of blood in the container can be deduced. In this case the control unit3receives the values of the pressure P0before the disturbance, the values of flow rate QAof the peristaltic pump4for determining the entity of the disturbance, and the values P1after the disturbance. The start of the disturbance enables a relatively easy measurement to be made, namely the flow rate of air QAto obviate the measurement of the mass N0contained in the container5. The flow rate of air QAin terms of mass can be derived from the measurement of the flow rate QAin terms of volume, the compression ratio of the pump4and the fact that the air is taken in at room temperature.

In examples 1 and 2, reference has been made to Perfect Gas Law, though the method of the present invention is valid for determining the level even when other gas laws are used that relate the volume VA, the pressure P, the mass N and the temperature T and other properties that can be considered constant.

Tests carried out by the applicant have demonstrated that the reading of the pressure P before and after the disturbance is vital in calculating the volume of air VAand, therefore, the level, while monitoring the temperature T is not necessary for the calculation of the level, as it is supposed that variations in mass M and N induce isothermal transformation. The reading of the temperature can be considered constant and, therefore, provides an evaluation parameter regarding the reliability of the measurement.