APPARATUS FOR DETERMINING HEAT-TRANSFER RATES AND THUS THE FLOW RATES OR THERMAL CONDUCTIVITIES OF FLUIDS

• US 3,803,913 A
• Filed: 01/04/1971
• Issued: 04/16/1974
• Est. Priority Date: 07/06/1970
• Status: Expired due to Term

• Current Assignee: Jay Tracer
• Original Assignee: Jay Tracer
• Inventors: Tracer, Jay
• Primary Examiner(s): Goldstein, Herbert
• Application Number:
  US05/103,367
• Time in Patent Office: 1,198 Days
• Field of Search: 73/189,204,362
• US Class Current: 73/204.14
• CPC Class Codes: G01F 1/68 by using thermal effects
• CPC Class Codes: G01F 1/684 Structural arrangements; Mounting of elements, e.g. in relation to fluid flow
• CPC Class Codes: G01F 1/698 Feedback or rebalancing circuits, e.g. self heated constant temperature flowmeters

Abstract

A thermistor bridge circuit and method in which a sensing thermistor generates a square wave having a varying duty cycle, such wave providing an accurate indication of the rate of heat transfer from the sensing thermistor to the ambient region, such rate of heat transfer varying with factors including the flow rate of fluid in the ambient region and the thermal conductivity of such fluid. In the preferred embodiment, an external control circuit is provided to maintain the frequency constant, and effects turn-on of the bridge whereas the bridge turns itself off. The width of each pulse is modulated in response to both (a) heat transfer and (b) the temperature of the sensing thermistor. The amplitude of each pulse is modulated in response to only the temperature of the sensing thermistor, and in such manner as to neutralize the effects of (b). The area of each pulse therefore varies only in accordance with heat transfer, and such area is indicated by a galvanometer. One of the two factors (voltage and current) representing power is maintained constant, so that the remaining one of the two factors will indicate the power which is supplied to the sensing thermistor in order to maintain it at a predetermined temperature differential above ambient. One embodiment incorporates a bridge which is self-oscillating, whereas the bridge in the above-indicated preferred embodiment is not self-oscillating since the control circuit is employed to turn on the bridge.

Claims

1. An instrument for indicating heat-transfer rates, which comprises:

a Wheatstone bridge, a sensing thermistor provided in one leg of said bridge between one terminal thereof and one adjacent terminal thereof, first, second and third resistors respectively provided in the remaining three legs of said bridge, the thermistor having a resistance value variable relative to the resistance values of the first, second and third resistances to provide the bridge with balanced and unbalanced conditions, means to form an input circuit including said one bridge terminal and the bridge terminal opposite thereto, said input circuit further including means providing at said sensing thermistor a voltage wave and a current wave, at least one of the waves characterized by changes between a fixed D. C. level and a datum level, means to form an output circuit including said one adjacent bridge terminal and the bridge terminal opposite thereto, said output circuit further including sensing and control means responsive to the condition of said bridge to change said one wave between said fixed D. C. level and said datum level, and an averaging meter coupled to the sensing thermistor and responsive to the duty cycle of the other of the voltage and current waves present at said thermistor and providing an output indicative thereof to indicate the power dissipated by the sensing thermistor and hence the rate of heat transfer between said sensing thermistor and the fluid medium to which it is exposed.

2. The invention as claimed in claim 1, in which said sensing and control means provides the other of the voltage and current waves with characteristics for heating said sensing thermistor to a particular temperature which is a fixed differential above the temperature of the medium to which said thermistor is exposed, and for maintaining the temperature of the thermistor at substantially the particular temperature.

3. The invention as claimed in claim 1, wherein;

said sensing thermistor self-heats in response to said current wave to provide a varying resistance in the one leg of the bridge during periods when said one wave is at said fixed D. C. level, and said sensing thermistor does not self-heat substantially when said one wave is at the datum level, and said bridge is responsive to the varying resistance of the sensing thermistor to approach the balanced condition.

4. The invention as claimed in claim 1, in which said sensing and control means comprises a transistor responsive to the balanced condition of said bridge to change the one wave from the D. C. level to the datum level, tHe transistor having a turn-on voltage tending to inhibit a rapid change between the fixed D. C. levels and the datum level, and in which means is provided to neutralize substantially the turn-on voltage of said transistor thereby to maintain said bridge close to the balanced condition and to effect a rapid change between the fixed D. C. level and the datum level.

5. The invention as claimed in claim 1, in which said one wave is the voltage wave, and in which said meter is a d'"'"''"'"'arsonval galvanometer.

6. The invention as claimed in claim 1, in which means are provided in said output circuit to generate a compensating voltage which in conjunction with said sensing and control means maintains said bridge close to the balanced condition at all times.

7. The invention as claimed in claim 6, in which said sensing and control means includes a transistor providing an emitter to base voltage drop from the terminal opposite the one adjacent terminal to the one adjacent terminal, and in which said means to generate a compensating voltage includes a diode providing an anode to cathode voltage drop from the one adjacent terminal to the terminal opposite to the one adjacent terminal, whereby the anode to cathode voltage drop opposes the emitter to base voltage drop to effect rapid changes in the one wave between the D. C. level and the datum level.

8. The invention as claimed in claim 1, in which the resistor present in said bridge between said one adjacent terminal thereof and said terminal thereof which is opposite said one terminal thereof is a compensating thermistor, the characteristics of said compensating thermistor being such that it will not heat substantially in response to passage of said voltage and current waves therethrough.

9. The invention as claimed in claim 8, in which said compensating thermistor is such that the readings of said meter will remain substantially the same despite variations in the temperature of the medium to which said sensing thermistor and compensating thermistor are exposed.

10. The invention as claimed in claim 1, in which said other of said voltage and current waves is square-wave in nature, and has a duty cycle which corresponds to the power applied to sensing thermistor and which varies in response to the rate of heat transfer between said sensing thermistor and said medium, and in which said sensing and control means and said first-named means are responsive to the condition of the bridge to maintains said sensing thermistor at a temperature which is a fixed differential above the temperature of the medium to which said thermistor is exposed.

11. The invention as claimed in claim 1, in which the meter is coupled to said one leg of said bridge and the invention further includes means coupled to said meter for zero-suppressing said meter in order to provide the said meter with a particular reading corresponding to a predetermined condition of the medium to which said sensing thermistor is exposed, said predetermined condition being one at which there is no relative movement between said sensing thermistor and said medium.

12. The invention claimed in claim 1 wherein the sensing and control means includes a transistor providing an emitter to base junction from the terminal opposite the one adjacent terminal to the one adjacent terminal, the transistor having a turn-on voltage which tends to vary the frequency of oscillations of the one wave, and wherein the output circuit includes means for altering the turn-on voltage of the transistor to control the frequency of oscillations of the one wave.

13. The invention recited in claim 12 wherein the altering means includes:

a diode providing an anode to cathode junction from the one adjacent terminal to the terminal opposite the one adjacent terminal;

a resistance coupled to the cathode of the diode to alter the voltage drop across the diode;

whereby the turn-on voltage of the transistor is altered to control the frequency of oscillations of the one wave.

14. The invention as claimed in claim 1 wherein:

said sensing and control means has regenerative characteristics for decreasing the time during which the one wave is changed from the datum level to the fixed D. C. level, and has degenerative characteristics for decreasing the time during which the one wave is changed from the fixed D. C. level to the datum level.

15. The invention recited in claim 1 wherein the datum level of the one wave has a magnitude of zero.

16. The invention recited in claim 1 wherein changes between the datum level and the fixed D. C. level occur at a substantially constant frequency.

17. The invention claimed in claim 16 wherein the sensing and control means includes a differential amplifier.

18. The invention recited in claim 17 wherein said second named means includes a unijunction oscillator.

19. An instrument for indicating the rate of flow of a medium, comprising:

a Wheatstone bridge having two input terminals and two output terminals, a sensing thermistor disposed in the medium and connected in said bridge between said one input terminal and the other of said output terminals, a first resistor connected in each bridge between said other output terminal and the other of said input terminals, a second resistor connected in said bridge between said other input terminal and the other of said input terminals, a third resistor connected in said bridge between said other input terminal and said one output terminal, means to form an input circuit including said input terminals, said input circuit further including voltage means exterior to said bridge and having characteristics for providing voltage at one of a fixed D. C. level and a datum level, means to form an output circuit including said output terminals, said output circuit further including sensing and control means responsive to the condition of said bridge to change between said D. C. level and said datum level the voltage impressed on said bridge, and an averaging ammeter associated with said thermistor and responsive to both the duty cycle and the magnitude of the current wave at said thermistor to provide an indication of power dissipated by said thermistor, the rate of heat transfer between said thermistor and the medium, and the rate of flow of the medium.

20. The invention as claimed in claim 19, in which said third resistor is a temperature-compensating thermistor disposed in said medium, the characteristics of said compensating thermistor being such that it does not heat substantially in response to said current wave.

21. The invention recited in claim 19 wherein said voltage means includes resistance means variable between limits corresponding to the fixed D. C. level and the datum levels.

22. The invention recited in claim 21 wherein said sensing and control means includes:

switching means responsive to an increasing voltage between the output terminals to provide a particular increasing current;

resistance means responsive to the particular increase in current to increase D. C. level impressed on the input terminals of the bridge;

the bridge responsive to the increased D. C. level on the input terminals to increase the voltage between the output terminals;

whereby the D. C. level impressed on the input terminals is regenerated thereby decreasing the time during which the voltage wave changes from the datum level to the fixed D. C. level.

23. The invention recited in claim 19 wherein the sensing and control means has regenerative and degenerative characteristics which decrease the time during which changes are made between the fixed D. C. level and the datum level.

24. Apparatus for indicating heat-transfer rates, and thus flow or other variables, which comprises:

a sensing thermistor adapted to be disposed in heat-transfer relationship to a medium, a power source of the two factors, voltage and current, determining electrical power, said source supplying one of said two factors at a constant amplitude, a control source for generating a plurality of first control signals at a predetermined fixed frequency, first means responsive to the first control signals from said control source to cause said power source to apply said one factor to said thermistor, whereby the other of said two factors is also applied by said power source to said thermistor, differential amplifier means responsive to the temperature of said thermistor to terminate application of said one factor by said first means to said thermistor as soon as power from said power source has caused self-heating of said thermistor to a predetermined constant temperature differential above the temperature of said medium, whereby application of said other factor is also terminated, said application and subsequent termination of said other factor causing a pulse of said other factor to be applied to said thermistor, and second means responsive to the area of said pulse of said other factor and providing an output indicative thereof to indicate the rate of heat transfer from said thermistor to said medium.

25. The invention as claimed in claim 24, in which said one factor is voltage and said other factor is current.

26. The invention as claimed in claim 24, in which said control signal is characterized by a plurality of control spikes and said fixed frequency is such that each pulse of said other factor is initiated in response to each control spike from said control source, and is terminated prior to the next subsequent control spike from said control source.

27. The invention as claimed in claim 24, in which said second means is a galvanometer coupled to the sensing thermistor.

28. The invention claimed in claim 24 wherein the differential amplifier includes:

a first amplifier responsive to the temperature of said thermistor to provide a second control signal, and the first means responsive to the second control signal to maintain the application of the said one factor to said thermistor after the termination of each of said first control signals; and

a second amplifier responsive to the temperature of said thermistor to provide a third control signal, the first amplifier responsive to the third control signal to terminate the second control signal and the first means responsive to the termination of the seconc control signal to teminate application of said one factor to said thermistor.

29. The invention claimed in claim 24 in which said control source includes a unijunction transistor.

30. The invention claimed in claim 24 wherein said pulse of said other factor is characterized by changes between a first D. C. level and a zero level.