FLOW SENSING DEVICE

Abstract: A fluid flow sensing device comprises first and second p-n junctions (T,D), one (T) of which is arranged to be dis­ posed in the fluid flow to be measured and the other of which is arranged to be disposed at a position such that its tem­ perature depends only on that of the fluid. An amplifier (20) monitors the voltage drop across the first p-n junction. A com­ parator (16) compares the measured voltage drop across the first p-n junction with a reference level (V REF2 ) to provide a feedback voltage (V c ) which is used to control the heating of the first p-n junction such as to maintain the temperature of that junction constant irrespective of variations in the fluid flow or the fluid temperature. A current is passed through the second p-n junction and the voltage drop across it is monito­ red and compared with the feedback voltage (V c ) to provide a signal which is a function of fluid flow alone.

Description: The present invention relates to a flow sensing device, and in particular, but not exclusively, to a device for measuring the flow of air into a domestic water heater. Modern domestic water heaters are provided with an electrically-operated fan to impel air into the heater and produce more efficient combustion of the fuel. It is a safety requirement that the heater should not be operable unless the fan is in operation, since combustion of the fuel without sufficient air may be dangerous. In an attempt to comply with this requirement, many heaters have a pressure-operated switch situated between the fan and the burner of the heater which, when the fan is working, causes the pressure operated switch to allow a valve to open controlling the input of the fuel. If the fan is not in operation, the pressure between the fan and the burner will be reduced, and the pressure-operated switch prevents the fuel control valve from opening. However, if the path of air through the heater is blocked, e.g. in the exhaust flue, the pressure-operated switch will allow fuel to flow, with potentially serious results. An alternative, electronic solution is to have a heated thermistor in the flow of the air. The flow of air across the thermistor tends to cool the thermistor which, because of a feedback system to an operational amplifier, is arranged to be heated up to the same temperature as previously. The voltage applied to the thermistor to keep it at a constant temperature is thus proportional to the flow of air past the thermistor. Compensation for the temperature of the air is obtained by a second, unheated, thermistor which detects the temperature of the air and whose voltage (which is due solely to air temperature) is subtracted from the voltage obtained from the first thermistor (which is due to the flow of air and the temperature of the air) to give a voltage which corresponds solely to the flow of air. However, the components used are relatively expensive and it is difficult to match the (non-linear) resistance/temperature characteristics of the two thermistors accurately, thus introducing temperature compensation errors. Furthermore, the thermistors themselves are relatively fragile and tend to require frequent replacement. It is also found that many thermistors are not sensitive to higher flow rates of air. Adaptions of the above approach are illustrated in United States Patent No 3 968 685 and United States Patent No 4 487 063. Regarding United States Patent No 3 968 685, a first self-heated, p-n junction is provided in association with a second p-n junction through which an electrical current is passed, the voltages being compared to compensate for ambient temperature variations. The approach taken by United States Patent No 4 487 063 is that a first p-n junction is associated with a heating element and its voltage compared to a reference voltage to give a feedback voltage irrespective of variations in the fluid temperature about the junction, a second p-n junction is provided through which an electrical current is passed. These Patents thus have in common p-n junctions used instead of thermistors whilst maintaining the same mode of operation with the inherent problems of temperature correction. It is an object of the present invention to provide a flow sensing device which overcomes or reduces the above problems. In accordance with the present invention there is provided a fluid flow sensor comprising first and second p-n junctions (T,D), one (T) of which is arranged, in use, to be disposed in the fluid flow to be measured, means (20) for monitoring the voltage drop across the first p-n junction (T), said first p-n junction being self-heated or being associated with means for heating it, means for comparing the measured voltage drop across the first p-n junction with a reference level to provide a feedback voltage (V c ) which is used to control the heating of the first p-n junction in such a manner as to maintain the temperature of that junction constant irrespective of variations in the fluid flow or the fluid temperature, means for passing a current through the second p-n junction (D), means for monitoring the voltage drop across the second p-n junction, and means for comparing the voltage drop across the second p-n junction with said feedback voltage (V c ) to provide a signal which is a function of fluid flow. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig 1 is a circuit diagram of one embodiment of a flow sensing device in accordance with the present invention; Fig 2 is a graph of base-emitter voltage against temperature for a transistor used in the device of Fig 1; Fig 3 is a graph of collector voltage against fluid flow for the transistor used in the device of Fig 1; and Fig 4 is a schematic diagram of a second embodiment of a flow sensing device. The flow sensing device of Fig 1 comprises an npn transistor T which is arranged to form a constant current device by the provision of a first reference voltage source 14, which produces a voltage and an emitter resistor R. Provided that V REF1 is much greater than the base emitter voltage V be , then a voltage substantially equal to V REF1 is established across the resistor R, resulting in a current I R in the transistor which is substantially constant, irrespective of variations in the collector load. The collector of the transistor T is connected to the output of an operational amplifier 16 whose inputs are connected respectively to a second reference voltage source 18, producing a voltage V REF2 , and the output of a second operational amplifier 20. Amplifier 20 is a differential amplifier which provides at its output a measure of the base-emitter voltage V be of the transistor T. The amplifier 16 compares this measure of the base-emitter voltage with the reference voltage V REF2 and controls the voltage on the collector of transistor T to maintain this comparison, and hence the base-emitter voltage, at a constant level. With a constant current being passed through the transistor T, its temperature varies with the voltage impressed on its collector as a result of the differing power to be dissipated. At the same time, a measure of the transistor junction temperature is obtained by measuring its base-emitter voltage. Thus, it will be seen that the transistor T is self-heated by the current passing through it and which, by virtue of voltage feedback arrangement described above, is maintained at a constant junction temperature (typically between 100°C - 180°C), the temperature being predetermined by fixing the reference voltage V REF2 of the amplifier 20. In use, the transistor T is placed in the airflow of a heater, between the fan and the burner, such that the flow of air attempts to lower the temperature of the transistor. However, because of the feedback heating system, the base-emitter junction of the transistor is maintained at a constant temperature by the automatic increase or decrease of the voltage applied to the transistor collector. As indicated diagrammatically in Fig.2, for a constant air flow over the transistor, the junction voltage (V be ) varies linearly with temperature. When, however, the temperature of the junction is maintained constant by means of the feedback arrangement described above, then a characteristic curve of the output voltage of comparator 16 against flow as shown in Fig.3 is obtained. For a given air temperature, therefore, the output of the comparator 16, and hence the heating voltage (V c ) applied to the transistor, is a function of the flow of air past the transistor T. As in the known devices, compensation must be made for the actual temperature of the air whose flow is being measured. This is achieved by means of a diode D which is unheated other than as a result of the temperature of the air itself. The diode D is supplied with a constant current I from a constant current source 22, the voltage on the diode D being fed into a conditioning amplifier 24. When a constant current is being passed through it, the voltage across the diode varies in a linear manner with temperature (in a manner similar to the curve of Fig.2, although not necessarily of the same slope). When the slopes have been matched, for example by varying the gain of amplifier 24, the output signal of the amplifier 24 (which is due only to the temperature of the air) is subtracted from the output signal of the amplifier 16 (which is due to the temperature of the air and to the flow of air) by a differential amplifier 26 to produce a signal V FLOW which is due only to the flow of air. It will be appreciated in this connection that the implementation of temperature compensation is much easier and more predictable in operation than in the case of the known devices since the voltage/ temperature characteristic of both the p-n junction (base-emitter junction) of the transistor 10 and the p-n junction of the diode D are linear when a constant current is passed therethrough. By compairing V FLOW with a reference level V REF3 in a comparator 28, a signal to open a fuel flow valve (not shown) will be given by comparator 28 if V FLOW is above V REF3 and a signal to close the valve will be given if V FLOW is below V REF3 . Alternatively, instead of incorporating the comparator 28, the voltage V FLOW may be used directly to modulate the flow of air, for example by varying the speed of the fan. The invention is not, of course, restricted to the use described above, between the fan and the burner of a heater, and can be used in many circumstances where a fluid flow is to be measured. Fig.4 shows an alternative embodiment of the flow sensor in which, instead of self-heating as in Fig.1, indirect heating of the first p-n junction is used. In this event, the first p-n junction need not be a transistor and can be second diode D₂. However, it is then necessary to provide a separate means of heating the diode D₂ and this is achieved by means of a heating device S which is arranged to be in thermal contact with the p-n junction of diode D₂, for example in or on a common, heat-conductive block 30. A constant current is maintained in the diode D₂ by means of a constant current source 32 and the voltage across the junction of D₂ is monitored by an amplifier 34 and compared with a reference voltage V REF . The output of amplifier 34 drives a current through the heating device S and also provides one input of a comparator 36. The second input of the comparator 36 is provided by a conditioning amplifier 38 which, depending on the voltage/power characteristic of the heating element S, may have a non-linear characteristic. This amplifier 38 monitors the voltage across a diode D₃ through which is passed a constant current from a constant-current source 40. As before, the diode D₃ is positioned so that its temperature varies only with the temperature of the air whose flow is to be measured. In operation, the temperature of the junction of diode D₂ drops as a result of air flow across it, and the comparison of the voltage drop across the junction with the reference V REF causes amplifier 34 to provide an increased voltage to the heater S which is heated such as to heat the diode D₂ to its original temperature and thereby restore the original balance. The principle of operation is thus exactly the same as that of the embodiment of Fig 1. Thus, the signal V FLOW can, for example, be compared with a further reference, as in the case of V REF3 of Fig 1, or can be used directly.

Claims: A fluid flow sensor comprising first and second p-n junctions (T,D), one (T) of which is arranged, in use, to be disposed in the fluid flow to be measured, means (20) for monitoring the voltage drop across the first p-n junction (T), said first p-n junction (T) being self-heated or being associated with means for heating it, means for comparing the measured voltage drop across the first p-n junction with a reference level to provide a feedback voltage (V c ) which is used to control the heating of the first p-n junction (T) in such a manner as to maintain the temperature of that junction (T) constant irrespective of variations in the fluid flow or the fluid temperature, means for passing a current through the second p-n junction (D), means for monitoring the voltage drop across the second p-n junction (D) and means for comparing the voltage drop across the second p-n junction (D) with said feedback voltage (V c ) to provide a signal which is a function of fluid flow. A fluid flow sensor as claimed in claim 1, wherein the first p-n junction (T) is formed by the base-emitter junction of a transistor, the latter junction being arranged to be heated by varying the collector voltage of the transistor whilst maintaining the collector emitter current substantially constant. A fluid flow sensor as claimed in claim 2, wherein the voltage of a constant voltage source (14) is applied to the base of the transistor and the base-emitter voltage of the transistor is monitored by an operational amplifier (20) and compared in a comparator (16) with a predetermined voltage reference to provide the feedback signal (V c ), which is returned to the collector of the transistor to result in the self-heating of the transistor to an extent sufficient to maintain the measured base emitter voltage of the transistor at a level set by said voltage reference. A fluid flow sensor as claimed in claim 2 or 3, wherein the second p-n junction is formed by a diode (D) through which a constant current is arranged to be passed from a constant current source (22). A fluid flow sensor as claimed in claim 1, wherein the first p-n junction is formed by a diode (D₂) through which a constant current is arranged to be passed from a constant current source (32) and wherein said associated means for heating it comprises a heating device (S) supplied with a voltage dependent upon the feedback voltage (V c ). A fluid flow sensor as claimed in claim 5, wherein the second p-n junction is formed by a second diode (D₃) through which a constant current is arranged to be passed from a further constant current source (40). A fuel/air fired water heater in which the combustion air is impelled into the heater by an electric motor driven fan and in which the flow of fuel to the heater is controlled by an electrically operated regulating valve, characterised in that the heater includes a fluid flow sensor as claimed in any one of claims 1 to 6 for sensing the flow of combustion air from the fan and in that the said signal that is a function of fluid flow, when indicative of insufficient flow of combustion air, cause the regulating valve to interrupt the flow of fuel to the heater.

Inventors: BODE, PETER ABIODUN AYOTUNDE, COLLINS, RODERICK JOHN
Applicants: IMI PACTROL LIMITED
Application year: 1986
Main classification: G01F1
IPC codes: G01F1