Method and device for measuring the gas content in a liquid, and use of such a device

Abstract

The gas content of a liquid is quickly and reliably controlled by delivering at least one sub-quantity of the liquid into a measurement cell in which a negative pressure is set. A wave-shaped measurement signal is applied to the sub-quantity, and the wave-shaped measurement signal is measured by at least one detector after coming into contact with the sub-quantity of the liquid. A turbidity value of the liquid is determined and compared with a threshold. If the turbidity value is greater than or equal to the threshold, a pressure and a temperature in the measurement cell are measured, and the gas content in the liquid is ascertained using stored characteristics for the solubility of the gas in the liquid dependent on the pressure and temperature.

Claims

1. A method of measuring a gas content in a liquid, the method which comprises: conveying at least one sub-quantity of the liquid into a measurement cell; setting a vacuum in the measurement cell; subjecting the sub-quantity to a measurement signal selected from the group consisting of acoustic signals and electromagnetic wave signals; measuring the measurement signal after coming into contact with the sub-quantity; determining a turbidity value of the liquid and comparing the turbidity value with a threshold; if the turbidity value is greater than or equal to the threshold, measuring a pressure and a temperature in the measurement cell; and determining the gas content in the liquid based on stored characteristic data for a solubility of the gas in the liquid as a function of the pressure and the temperature.

2. The method according to claim 1, wherein said measurement signal is an electromagnetic wave signal in the form of a visible light signal.

3. The method according to claim 1, wherein said measurement signal is an acoustic measurement signal.

4. The method according to claim 3, which comprises subjecting the sub-quantity to an acoustic measurement signal in the form of ultrasound.

5. The method according to claim 1, which comprises controlling one or both of the pressure and the temperature in the measurement cell to remain above a boiling point of the liquid.

6. The method according to claim 1, wherein the threshold is specified as a numerical value.

7. The method according to claim 1, which comprises, prior to setting the vacuum, carrying out a measurement with the measurement signal for defining a reference value.

8. The method according to claim 7, which comprises comparing the reference value with a threshold.

9. The method according to claim 7, which comprises computing the threshold on a basis of the reference value.

10. The method according to claim 1, which comprises using an intensity of the measurement signal after coming into contact with the liquid in the measurement cell to determine the turbidity value.

11. The method according to claim 1, which comprises measuring a proportion of the measurement signal transmitted through the liquid.

12. The method according to claim 1, which comprises measuring a proportion of the measurement signal reflected by the liquid.

13. The method according to claim 1, which comprises controlling a flow speed of the liquid such that gas bubbles are flushed away from a wall of the measurement cell.

14. A device for measuring a gas content in a liquid, the device comprising: a transparent measurement cell for receiving at least a sub-quantity of the liquid; a vacuum system for generating a vacuum in the measurement cell; a measurement signal emitter for applying a measurement signal selected from the group consisting of acoustic signals and electromagnetic wave signals to the sub-quantity; at least one detector for measuring the measurement signal after coming into contact with the sub-quantity in the measurement cell; sensors for determining a pressure and a temperature in the measurement cell; and an analysis and control unit configured to determine a turbidity value of the liquid and to compare the turbidity value with a threshold and, if the turbidity value is greater than or equal to a threshold, to determine the gas content in the liquid on a basis of stored characteristic data for the solubility of the gas in the liquid as a function of the pressure and the temperature.

15. In combination with an electrochemical cell, the device according to claim 14 for measuring a gas content in a coolant of the electrochemical cell.

16. The combination according to claim 15, wherein the electrochemical cell is a fuel cell.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) An exemplary embodiment of the invention is explained in greater detail on the basis of the drawing, in which:

(2) FIG. 1 shows a device for discontinuous monitoring of the gas content of a coolant of a fuel cell,

(3) FIG. 2 shows a device for continuous monitoring of the gas content of a coolant of a fuel cell, and

(4) FIG. 3 shows a third embodiment variant of a device for measuring the proportion of gas in a liquid, and

(5) FIG. 4 shows an arrangement of a measurement cell.

DESCRIPTION OF THE INVENTION

(6) The same reference characters have the same meaning in the different figures.

(7) FIG. 1 shows a first device 2 for measuring the gas content of a liquid coolant K, here cooling water for a fuel cell not shown in any greater detail. The device 2 comprises an inlet valve 4, a measurement signal transmitter 6 for a wave-shaped measurement signal, which is a light source in the exemplary embodiment shown, two detectors 8, 10 which are shown symbolically as open eyes, and also a measurement cell 12 transparent for light of the light source 6. Suitable light sources 6 are LEDs, incandescent lamps, gas discharge lamps etc. Each of the light detectors can comprise photo transistors, photodiodes or photo resistors for example.

(8) As an alternative to light measurement the measurement signal transmitter 6 can be an ultrasound transmitter not shown in any greater detail here, the detector 8, 10 then accordingly being an ultrasound detector.

(9) In the exemplary embodiment shown, with the aid of the light detectors 8, 10, a light from the light source 6 is measured after coming into contact with the cooling water K in the measurement cell 12. A first light detector 8 in this case is provided for measurement of the light transmitted through the cooling water K in the measurement cell 12 and is disposed in this case such that the measurement cell 12 is located between it and the light source 6. A second light detector 10 is disposed on the same side of the measurement cell 12 as the light source 6 and in this case measures the light reflected from the sub-quantity M on the cooling water K.

(10) The device 2 is assigned in parallel to a main line 14 for the cooling water K. In the open state of the inlet valve 4 a sub-quantity M of cooling water K is conveyed via the ancillary line 16 from the main line 14 into the measurement cell 12. When the measurement cell 12 is especially completely filled up with cooling water K the inlet valve 4 is closed. Subsequently a reference measurement is carried out, in which a reference value RW for the turbidity of the cooling water K is obtained. A change of the transmission or reflection behavior of the liquid as a result of the formation of gas bubbles in the measurement cell is referred to as turbidity here. The reference value RW is stored in an analysis and control unit 18 which is likewise part of the device 2. In the exemplary embodiment shown only one unit is provided for analysis of the measurement results and for controlling the components of the device 2, as an alternative two separate units can also assume these two functions.

(11) In order to minimize the influence of colorations or contaminations in the cooling water K or the influence of an ageing or attenuation of the light source 6 on the measurement or even to suppress it, the reference value RW is compared in the analysis and control unit 18 with a predetermined, fixed limit value GW. If the reference values RW corresponds to the limit value GW or exceeds said value, the functionality of the device 2 is checked. In this way a self-monitoring of the device 2 takes place, in which contaminations or device outages are recognized at an early stage. Normally the reference value RW is smaller than the limit value GW, then the cooling water K is clear enough to carry out the measurement.

(12) After the reference measurement a pressure p.sub.m is lowered in the measurement cell 12. The pressure p.sub.m and also the temperature T.sub.m in the measurement cell 12 are monitored, in this case with the aid of a pressure sensor 20 and the temperature sensor 22. If the cooling water K contains dissolved gas, this gas occurs in the expansion of the cooling water K in the measurement cell 12 in the form of small bubbles, which lead to a turbidity of the cooling water K. Since however the cooling water K, on reaching the boiling point (boiling pressure and boiling temperature) itself vaporizes in the form of small bubbles, which influence the measurement results for the turbidity, the pressure p.sub.m and if necessary the temperature T.sub.m are regulated in the measurement cell 12 such that they constantly remain above the boiling point, especially at least 50 mbar above the boiling point.

(13) On the basis of the light measurement in a vacuum, a turbidity value TW for the cooling water K in the measurement cell 12 is determined. For determining the turbidity value TW it is sufficient for only one light detector 8, 10 to be provided, which detects either the light let through or the reflected light. For determining the turbidity value TW in particular the intensity of the light after it has come into contact with the cooling water K in the measurement cell 12 is included.

(14) The determined turbidity value TW is supplied to the analysis and control unit 18 and compared with a threshold SW. The threshold SW can be predetermined as a numerical value, as an alternative however the threshold SW can be defined for a specific measurement in that it is dependent on the reference value RW. If the turbidity value TW remains within a predetermined tolerance range and does not reach the threshold SW, it can be concluded from this that no gas bubbles are contained in the cooling water K. In this case the sub-quantity M of cooling water K, which was measured in the measurement cell 12, is fed back via a pump 24 into the main flow line 14. The pump 24 is especially controlled by the analysis and control unit 18.

(15) If however the turbidity value TW reaches the threshold SW or exceeds said threshold, then this is a sign that gas is contained in the cooling water K. To obtain information about the quantity of gas contained in the cooling water K, account is taken in such cases of the pressure p.sub.m and the temperature T.sub.m at which the turbidity of the cooling water K occurs in the measurement cell 12. The display 26 can be used in such cases to display the proportion of gas in the cooling water K. In addition to this the result of the comparison of the turbidity value TW with the threshold SW can also be displayed in the display 26. For detected gas in the cooling water K, i.e. for an occurrence of a turbidity in the measurement cell 12, a control signal 28 is also output which causes a degassing device, especially a vacuum degasser not shown in any greater detail here, disposed downstream of the device 2, to be activated. In this case, after the measurement in the measurement cell 12, the sub-quantity M of cooling water K is subsequently processed in the vacuum degasser.

(16) The device 2 for monitoring the gas content in the cooling water K in accordance with FIG. 2 differs from the device 2 in accordance with FIG. 1 essentially in that the arrangement in the second exemplary embodiment is suitable for a continuous measurement. For this the inlet valve 4 is replaced by a regulation valve 30. The regulation valve 30 is activated by the analysis and control unit 18 in order to regulate the inflow of cooling water K into the measurement cell 12. At a constant speed of the pump 24 the regulation valve 30 is closed slowly. In this case the pressure p.sub.m falls in the measurement cell 12 until gas bubbles form. The limit for the measurement of the gas content in the sub-quantity M lies in this case at around 50 mbar above the boiling pressure. It can thus be established from the occurrence of a turbidity in the measurement cell 12 in the measurement area whether gas is contained in the cooling water K. Through the further evaluation of the pressure p.sub.m and the temperature T.sub.m at the onset of the turbidity the gas content can also be measured.

(17) In accordance with FIG. 3 the cooling water K flows through a fixed choke valve 32 into the measurement cell 12. The pump speed is changed so that an ever lower pressure is set in the measurement cell 12 until turbidity occurs. The measurement begins if in particular a pressure difference of at least 100 mbar occurs via the choke valve 32.

(18) In all the figures shown the turbidity value TW is measured with the aid of a sub-quantity M of cooling water K, which is diverted through the ancillary line 16. As an alternative the device 2 for monitoring the gas content in the cooling water K can be integrated directly into the main line 14, so that the entire coolant flow is directed through the device 2 for measurement purposes.

(19) The measurement is started as a rule at a differential pressure of approximately 100 bar below the start pressure.

(20) In addition, in all versions shown, a flushing valve 34 with a large diameter is provided before the measurement cell 12, which is only shown in FIG. 3. If gas bubbles have attached themselves to the wall of the measurement cell 12, these are flushed away with the aid of the flushing valve 34, which allows a large throughflow. A falsification of the measurement results is thus avoided.

(21) The flushing out of the measurement cell 12 is explained in detail with reference to FIG. 4. The measurement cell 12 is supplemented by two connections which create a flow within the measurement cell 12, which winds itself in the shape of a screw around the central axis. This can be achieved if the connections for the flushing cycle are arranged radially, as shown in FIG. 4. To this end, as well as the flushing valve 34 already considered, two further valves 36, 38 are required, which switch over between flushing and measurement.

(22) For the measurement the flushing valve 34 or 36 is closed and the valve 38 opened. This produces an axial flow through the measurement cell 12.

(23) To cleanse the walls of bubbles adhering to them the flushing valves 34 and 36 are opened, valve 38 is closed and a rotary flow is produced. Through the rotation of the measurement cell content a higher flow speed on the wall is achieved than if the measurement cell 12 were to have an axial flow flowing through it with the same valve diameters. Bubbles are flushed away better by this. Small diameters can then be used for flushing valves 34, 36 with a radial throughflow than with an axial throughflow of the measurement cell 12. As an alternative the flushing valve 34 could be replaced by a choke valve or a regulation valve.