GB2037990A

GB2037990A - Fluid sensing

Info

Publication number: GB2037990A
Application number: GB7943907A
Authority: GB
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1978-12-21
Filing date: 1979-12-20
Publication date: 1980-07-16
Also published as: GB2037990B

Abstract

To compensate for variations in ambient temperature a system for giving a warning indication when liquid in a reservoir (2) reaches a predetermined level includes two reversed-biased zener diodes (D1 and D2) mounted at that level and having their anodes connected together. The cathodes of the diodes are connected to respective current sources (30 and 31), the current supplied to one diode (D1) being substantially more than that supplied to the other diode (D2) so that the one diode is thereby heated more than the other diode. When the liquid falls below the level of the diodes, heat dissipation becomes less thereby causing the difference between the temperatures and voltages across the two diodes to increase. <IMAGE>

Description

SPECIFICATION Fluid sensing This invention relates to methods and apparatus for detecting the presence of fluid or other flowable substance.

The invention is particularly, but not exclusively, concerned with apparatus for sensing when the level of fuel or lubricant (such as, for example, in an aircraft) reaches a predetermined level.

Previous arrangements have incorporated an electrical device, such a semiconductor diode, which responds to temperature changes by producing a corresponding change in current flow or voltage. This device is mounted in a liquid reservoir and current is supplied to the device to cause heating. The amount of heat dissipated by the device will usually be greater and hence its temperature will be lower, when it is immersed in liquid than when it is immersed in gas. By detecting the difference in current or voltage produced upon temperature change of the device it is therefore possible to determine when the device changes from being immersed in liquid to be immersed in gas.

By mounting a device at the bottom of the reservoir it is possible to determine when the reservoir is empty of liquid. Similarly, by mounting a device at the top of the reservoir, an indication can be provided when the reservoir is full.

The usefulness of such previous arrangements is, however, somewhat limited in that changes of the temperature of the liquid or gas in the reservoir may be sufficient to cause the device to respond in the same way as a transition from being immersed in gas to being immersed in liquid. Previous arrangements have been used where the difference in temperature between the gas and the liquid is always greater than any changes likely to be experienced in the temperature of the gas or the liquid alone. One such application has been in the detection of the level of cryogenic liquids (such as, for example, described in U.K. patent specification Nos.

1 099 409 and 1 297 077).

It is an object of the present invention to provide apparatus that can be used to overcome the aforementioned limitation and more particularly, that can be used for detecting the level of a liquid the temperature of which may vary over a wide range.

According to the present invention there is provided apparatus for detecting the presence of liquid or other flowable substance, the apparatus including first and second semi-conductor devices the performance of which is dependent upon temperature, wherein said first device is arranged to be supplied with current sufficient to cause heating of said device to a temperature above ambient temperature, and said second device is arranged to be supplied with current lower than said first current such that heating of said second device is substantially less than heating of said first device, and wherein said first and second devices are arranged to be mounted for immersion in said liquid or other flowable substance so that dissipation of heat from said first device is greater, and its temperature is consequently lower, when immersed than when not immersed, and such that the presence or absence of said liquid or other flowable substance can be detected by sensing change in the difference between the performances of said first and second devices.

The performance of the first device will change on a transition from being immersed in fluid to not being immersed in fluid whereas the performance of the second device will remain substantially the same. Changes in ambient temperature will, however, affect the two device in the same way, and by sensing the difference between the performances of the two devices compensation for the effects of changes in the ambient temperature is thereby achieved.

The first and semiconductor devices may be arranged to be mounted at substantially the same level relative to the surface of the flowable substance such that the devices are either both immersed or both not immersed in the flowable substance. The voltages across the first and second devices may vary in accordance with change of temperature of the devices, and the presence or absence of liquid or other flowable substance may be detected by sensing change in the difference between said voltages. The devices may be zener diodes and their anodes may be connected with one another.The apparatus may be arranged to provide an indication of the absence of fluid when the difference between the voltages across the zener diodes exceeds a value substantially midway between the difference in voltages across the zener diodes in the presence of said liquid or other flowable substance and the difference in voltages in the absence of said liquid or other flowable substance.

According to another aspect of the present invention there is provided a method of detecting the presence of absence of liquid or other flowable substance comprising the steps of supplying a first current to a first semiconductor device, the performance of which is dependent upon temperature, said first current being sufficient to cause heating of said first device to a temperature above ambient temperature; supplying a second current to a second semiconductor device, the performance of which is dependent upon temperature, said second current being lower than said first current such that heating of said second device is substantially less than said first device, said first and second devices being arranged to be mounted for immersion in said liquid or other flowable substance so that dissipation of heat from said first device is greater, and its temperature is consequently lower, when immersed than when not immersed; and sensing the difference between the performances of said first and second devices, such that the presence or absence of said liquid or other flowable substance can be detected by sensing change in said difference.A fuel-level warning system for an aircraft, in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a schematic diagram illustrating the system; Figure 2 illustrates the change in voltage across a zener diode supplied with a constant current, upon change in temperature of the diode; and Figure 3 illustrates the change in voltages across zener diodes supplied with different currents, upon change in ambient temperature.

With reference to Figure 1, the fuel level warning system includes a sensor unit 1 that is mounted in an aircraft fuel tank 2, a switching unit 3 that receives signals from the sensor unit, and a display unit 4 that receives signals from the switching unit to provide a warning indication when the fuel falls below a predetermined level.

The sensor unit 1, which may be in the form of an elongate probe is mounted vertically within the fuel tank 2 and is located near the bottom of the tank at the level at which it is required that a warning indication be provided. The sensor unit 1 includes a matched pair of silicon zener diodes D1 and D2 having their anodes connected together, both diodes being glass encapsulated and having positive temperature coefficients. The first zener diode D1 acts as the level sensor and is mounted at the lower end of the sensor unit 1 so as normally to be immersed in the fuel in the tank 2.

The second diode D2 is mounted close to the first diode D1 so as also to be normally immersed in fuel but is located so as not to be effected by any heating of the first diode. The two diodes D1 and D2 are arranged so that both will be exposed substantially simultaneously to the air or gas in the tank 2 above the fuel when the fuel falls below the predetermined level. The sensor unit 1 also includes a resistor 1 2 connected to the cathode of the second zener diode D2.

the first and second diodes D1 and D2 are reversed biased, current being supplied to the cathodes of the diodes via lines 20 and 21 respectively from two constant current sources 30 and 31 respectively in the switching unit 3. The two constant current sources 30 and 31 are supplied with a voltage from the aircraft's power supply unit 32. The switching unit 3 further includes a voltage comparator 33, which receives at its two inputs the voltages on lines 20 and 21.

The output of the comparator 33 is supplied to a resistor 34 connected in series with another resistor 35 between the comparator and a line 36 which extends from the junction of the two diodes D1 and D2 in the sensor unit 1. The switching unit 3 also includes a switching transistor 37 the base of which is connected to the junction of the potential divider formed by the resistors 34 and 35. The emitter of the transistor 37 is connected to the line 36 whilst its collector is connected to the line 36 whilst its collector is connected to the display unit 4 so that switching of the transistor completes the emitter-collector circuit to cause a warning indication on the display unit.

In operation, the first zener diode D1, acting as the level sensor, is supplied with a current of about 27mA to heat it to a temperature of between about 90 and 100 degrees Centigrade above the ambient temperature. The second diode D2 is supplied with a lower current of the order of 0.5mA producing a negligible power dissipation.

The two zener diodes D1 and D2 both have a substantially linear temperature coefficient of the form shown in Figure 2. Because the first diode D1 is supplied with greater current than the second diode D2, it will be at a higher temperature than Thwe second diode and will therefore have a larger voltage drop across it. The first diode D1 will, moreover, be at a higher temperature when it is exposed to the air than when it is immersed in fuel since the thermal conductivity of fuel is greater than that of air. The voltage drop across the two diodes Dl and D2 will therefore differ from one another, this difference between the two voltages being greater than the diodes are exposed to the air.

The change in voltage across the two diodes D1 and D2 upon change in ambient temperature is shown in Figure 3. It can be seen from Figure 3 that the voltage across the first diode D1 (as represented by line 41) when exposed to air, at a particular ambient temperature T1, is the same as that produced when the diode is immersed in fuel (as represented by line 42), at a higher ambient temperature T2. If only one zener diode were used it would, therefore, be difficult to determine whether or not it was immersed in fuel.

The second zener diode D2 of the present arrangement is used to provide compensation for the effects of changes in the ambient temperature and it is the change in the difference between the voltage across the two diodes D1 -and D2 that is used to provide an indication of the transition of the sensor unit 1 from being immersed in fuel to being immersed in air. Since the temperature coefficient of the two diodes D1 and D2 is linear, the voltage difference between them will be substantially the same at all temperatures while they remain immersed in fuel.It can be seen, however, that if the temperature coefficient of the diodes was not linear, the voltage difference between the two diodes would vary with ambient temperature and that difficulties would therefore be experienced in determining preciseiy when the fuel reaches the predetermined level.

In the present arrangement, the difference between the voltages across the two diodes Dl and D2 at which a warning indication will be provided is set at a fixed value VD by suitable selection of the value of the resistor 1 2 in the sensor unit 1.

This value VD is set between the two characteristics 41 and 42 of the diode Dl, in air and in fuel respectively, as represented by the broken line 40 in Figure 3. In this way, the warning indication is provided as the voltage difference between the two diodes D1 and D2 rise.s through VD, and is maintained whilst the voltage across diode D1 follows the characteristic line 41.

Because of the low dynamic impedance of D1 ,the voltage across it is substantially unaffected by small voltage perturbations or electrical interference.

The voltages across the two diodes D1 and D2 are compard by the voltage compatator 33. When the difference between the two voltages rises above the predetermined value VD, the comparator 33 produces a current at its input which raises the voltage at the base of the transistor 37, thereby switching it on. Current then flows in the emittercollector circuit of the transistor 37, forming the output of the switching unit 3, thereby causing a "low-fuel" warning on the display unit 4. The warning could take the form of a visual or audible warning. Alternatively, the output from the switching unit 3 could be used to effect automatic switching of fuel supply from another fuel tank.

In place of zener diodes, it would be possible to use other semiconductor diodes in a forwardbiased mode. These, however, have some disadvantages compared with reversed-biased zener diodes. For example, the temperature coefficient of a silicon diode is about 2 mV/OC which is lower than that of a 9.1 volt zener diode, being typically 6mV/ C. This would, therefore, result in a lower voltage change of the sensor unit 1 on the transition from being immersed in fuel to being exposed to air.

Another disadvantage of using standard silicon diodes, especially for fuel-level sensing, is that they require a higher current. With a 9.1 volt zener diodes, such as, for example, a Mullard BZY88C9V1 having a thermal resistance of 0.370 C/mV, only about 27mA current is required to raise the temperature of the sensing diode 900C above ambient temperature. Using a silicon diode with a forward voltage drop of about 0.6 volts, the current required to produce a similar temperature increase would be in the order of 500 mA. In aircraft having standard 28 volt d.c. power supplies, this relatively high current would be unacceptable where the sensor unit 1 is required to be mounted in the explosive air-fuel vapor mixture in a fuel tank.The highly linear temperature coefficient of zener diodes is also advantageous in that it enables accurate operation over the wide ambient temperature range likely to be experienced in aircraft fuel tanks, which may be from about -400C to +700C.

It will be appreciated that the system could be used for sensing the level of liquids other than aviation fuel. It could, for example, be used to provide an indication of the level of lubricating oil or hydrualic fluids in aircraft. The system need not necessarily be used only in aircraft, it could for example be used in land vehicles-or in static containers, such as, for example, are used in chemical engineering plants or liquid-fuel boiler systems.

The invention is not, moreover, limited to level sensing applications but could be used, for example, to indicate the presence of liquid in a pipe-line. The system is not, furthermore, confined to use with liquids but may be used with particulate materials and powders providing these materials have a thermal conductivity high enough to enable the gas-material transistion to be detected reliably. The system could also be used for detecting the interface between two liquids of different densities providing the thermal conductivities of the two liquids are sufficiently different.

By using several sensor units spaced apart one above the other in the reservoir, in the manner referred to earlier, it would be possible to provide an indication of the depth of fluid in the reservoir.

It would also be possible to provide an indication of the depth of fluid by using a movable sensor and moving the sensor until it becomes immersed in the fluid.

Claims

1. Apparatus for detecting the presence of liquid or other flowable substance, the apparatus including first and second semiconductor devices the performance of which is dependent upon temperature, wherein said first device is arranged to be supplied with current sufficient to cause heating of said device to a temperature above ambient temperature, and said second device is arranged to be supplied with current lower than said first current such that heating of said second device is substantially less than heating of said first device, and wherein said first and second devices are arranged to be mounted for immersion in said liquid or other flowable substance so that dissipation of heat from said first device is greater, and its temperature is consequently lower, when immersed than when not immersed, and such that the presence or absence of said liquid or other flowable substance can be detected by sensing change in the difference between the performances of said first and second devices.

2. Apparatus according to Claim 1, wherein said first and second semiconductor devices are arranged to be mounted at substantially the same level relative to the surface of said flowable substance such that said devices are either both immersed or both immersed in said flowable substance.

3. Apparatus according to Claim 1 or 2, wherein said first and second devices are arranged such that the voltages across them varies in accordance with change of temperature of the devices, and wherein the presence or absence of said liquid or other flowable substance is detected by sensing change in the difference between said voltages.

4. Apparatus according to Claim 3, wherein said first and second semiconductor devices are zener diodes.

5. Apparatus according to Claim 4, wherein the anodes of said zener diodes are connected with one another.

6 Apparatus according to Claim 4 or 5, wherein each said zener diode is connected with a source of substantially constant current such that said diodes are reversed-biased.

7. Apparatus according to Claim 6, including voltage comparator means arranged to provide an output in accordance with the difference between the voltage across said zener diodes.

8. Apparatus according to Claim 7, wherein said apparatus is arranged to provide an indication of the absence of said liquid or other flowable substance when the difference between the voltages across said zener diodes exceeds a value substantially midway between the difference in voltages across said zener diodes in the presence of said liquid or other flowance substance and the difference in voltages in the absence of said liquid or other flowable substance.

9. Apparatus according to Claim 7 or 8, including switching means wherein operation of said switching means is controlled in response to the output of said voltage comparator means.

10. Apparatus according to Claim 9, wherein said switching means includes a transistor.

11. Apparatus according to Claim 9 or 10, including a potential divider having one end connected with the output of said voltage comparator means, and wherein said switching means is controlled by a voltage across said potential divider.

12. Apparatus according to Claim 11 and 5, wherein the other end of said potential divider is connected with anodes of said zener diodes.

13. Apparatus according to any one of the preceding claims for providing an indication of the level of liquid or other flowable substance in a reservoir, including a plurality of sensor units, wherein each said sensor unit includes a said first and second semiconductor device, and wherein said sensor units are arranged to be located above one another in said reservoir such that the level of said liquid or other flowable substance can be determined by detecting which of said sensor units are immersed in said liquid or other flowable substance.

14. A liquid-level warning system substantially as hereinbefore described with reference to the accompanying drawings.

15. A method of detecting the presence or absence of liquid or other flowable substance comprising the steps of supplying a first current to a first semiconductor device, the performance of which is dependent upon temperature, said first current being sufficient to cause heating of said first device to a temperature above ambient temperature; supplying a second current to a second semi-conductor device, the performance of which is dependent upon temperature, said second current being lower than said first current such that heating of said second device is substantially less than said first device, said first and second devices being arranged to be mounted for immersion in said liquid or other flowable substance so that dissipation of heat from said first device is greater, and its temperature is consequently lower, when immersed than when not immersed; and sensing the difference between the performances of said first and second devices, such that the presence or absence of said liquid orother flowable substance can be detected by sensing change in said difference.

1 6. A method substantially as hereinbefore described with reference to the accbmpanying drawings.