CA1103052A - Thermistor device for identifying an unknown fluid - Google Patents
Thermistor device for identifying an unknown fluidInfo
- Publication number
- CA1103052A CA1103052A CA293,360A CA293360A CA1103052A CA 1103052 A CA1103052 A CA 1103052A CA 293360 A CA293360 A CA 293360A CA 1103052 A CA1103052 A CA 1103052A
- Authority
- CA
- Canada
- Prior art keywords
- monitoring device
- detector
- resistance
- monitoring
- bridge
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
- G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
- G01N27/14—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of an electrically-heated body in dependence upon change of temperature
- G01N27/18—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of an electrically-heated body in dependence upon change of temperature caused by changes in the thermal conductivity of a surrounding material to be tested
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
- G01N33/18—Water
- G01N33/1826—Water organic contamination in water
- G01N33/1833—Oil in water
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A20/00—Water conservation; Efficient water supply; Efficient water use
- Y02A20/20—Controlling water pollution; Waste water treatment
Abstract
Abstract of the Disclosure Detecting conditions related to the heat absorptive characteristics of a surrounding medium by a heated monitoring device exposed for heat trans-ter to the medium, and connected to a restorative energizing electronic circuit that supplies boating power. The restorative circuit responds to differences between the electrical characteristics (e.g. resistance) of the monitoring device containing the monitoring elements and a reference value to vary the power flow through the device and thus to restore the resistance Or the device toward the reverence value. With two or more elements in the device, the elements are heated in unison by the single energizing circuit each element is disposed in a different relationship to the conditions to be discriminated and differences between the individual element electrical characteristics, reflecting differing ? heat l?? the medium provide indications of the various condi? ? ? ? ? is used, the power flow through the overall device c? ? ? ? of conditions of the surrounding medium. Features of the ? ?ying two elements include positioning one element for convect with water and the other element above the oil/water interface to detect a layer of oil if present. Unique circuit arrange-ments are shown that achieve effective thermistor action with an economt of parts and energizing power.
Description
~`3~)5~
This inventi'on relates to detectors which respond to a heat absorptive characteristi'c condi't;on of a surround;ng medium and to detectors for discriminating at least one condition from a s-et of different conditions that are related to the heat a~sorpti've properties of the medium. The conditions may relate to thermal conductivity, temperature, etc., for purposes such as- detect;ng oil spills on water or determining the identity of a fluid.
The purpose of the invention i-s to provide detectors that are simple, durable, and can be made at low cost. Another purpose is to enable the various factors affecting fieat transfer ;nto a medium to be discrimina-ted from one another, to el;m;nate spur;ous conditions, such as temperature changes, to achieve devïces that are extremely sensitive. A further object is to enable fluids that are dïfficult to d;scriminate, from water for example, to be reliably detected.
According to the invention, there is provided apparatus for detecting the presence of foreign li'qu;d comprising a monitoring device exposed for heat transfer contact with a liquid when immersed therein, said monitoring device bei'ng capable of diss;pating electric power and having an electrical characteri'stic tHat changes as a function of temperature, a restorative electrical circuit connected to said monitoring device to supply electrical power to the element to be dissipated thereby and responsive to changes in said cfiaracteristic of the device to adjust said supply of electrical power to counteract said changes, output circuitry responsive to said changes to provide an i'ndication of changes in the rate of heat loss from the device. The invention embodies an electrical sensing device having at least one monitoring element having a temperature dependent electrical characteristic, e.g. a thermistor, exposed to a surrounding heat absorptive medium and connected to a restorative energizing electronic circuit supplying heating power to the device to restore its electrical characteristic, e.g. resistance, toward a predetermined ~r ~ 1 ~
reference value. Maintaining the resi.stance of the sensing device near the reference value ass.ures that the resistance, and hence temperatures, of the individua.l thermistors remain within their designed operating ranges, eliminating the pos-sibility of burning out one or more thermistors or igniting a volatile fluid, if present; because the restorative circuit, powered preferably by-a single, direct current source, energizes the device as a whole, any thermistor in the leg is affected as the direct current flow through the device is controlled in response to changing conditions of the surrounding medium. Also when more than one thermistor is located in a single sensing device, noise inherent in any active electronic -la-- ~.
~3~
circuit influences the whole set of thermistors equally, allowing the effects of such noise to be eliminated.
In embodiments of the invention in which the sensing device comprises a single thermistor, the power required to maintain the thermistor resistance equal to a reference resistance is a measure of the thermal loss of the thermistor to the medium. An output signal representing this power is therefore a measure of the heat absorptive characteristics of the medium, and may be used, for example, to identify an unknown fluid or to distinguish between an unknown fluid and water.
In embodiments employing multiple thermistors, similarly useful information is derived from the differential change in resistance between the various thermistors in the device, made possible by the common means of applying power to the device.
In preferred embodiments of the invention for discrimination among a number of conditions affecting heat flow into a medium, the sensing device comprises an electrical network that includes a set of thermistors corresponding to the number of conditions to be discriminated. Each of the thermistors is disposed in a different heat transfer relationship with the medium and means are provided to derive an output signal, dependent upon the resistance values of the individu~1 thermistors, indicatin~ the condition to be sensed. A detector, therefore, embodying the invention provides information about the rate of heat loss to a surrounding medium due to a single condition of interest, eliminating the effects of other conditions, e.g., temperature fluctuations.
In a specific preferred embodiment an electrical network of thermistors, i.e. the device, comprises one leg of a resistance comparing bridge. Whenever the resistance of the network differs from a reference resistance in another leg of the bridge, the restorative circuit varies the power flow through the network, bringing the bridge into balance.
In preferred embodiments the electrical network itself comprises ~3~
an ancillary bridge including two thermistors and two resistors of equal value comprising a voltage divider, allowing the resistances of the two thermistors to be compared. Any difference in resistance between the therm-istors produces a voltage across the ancillary bridge which serves as an output signal and indicates the condition of interest. The two thermistors may be connected in series or parallel within the ancillary bridge for this purpose.
In another preferred embodiment the sensing device or the electrical network of multiple thermistors is connected in series with a temperature-independent reference resistor in an electronic energizing circuit. Because of the series connection, the same current flows through the reference resistor and the network. Connected across the reference resistor and across the network are the inputs to separate operational amplifier means whose outputs are measures of the voltage across the reference resistor and the network and are also measures of the respective resistance values. A further operational amplifier means is disposed in the circuit so that its input is the difference between the outputs of the operational amplifier means whose inputs are connected across the network and reference resistor. The output of this further operational amplifier means is proportion~l to the difference in resistance between the series network and the reference resistor, and is used to regulate the power flow through the network in a manner tending to maintain the resistance of the network equal to the resistance value of the reference resistor.
Another preferred embodiment of the invention is a detector for sensing a difference between an unknown fluid, e.g., oil, and water. In this embodiment, the electrical network whose resistance is maintained relative to the predetermined value includes two thermistors, one a sensor and the other a reference, incorporated into a probe for positioning the sensing thermistor at a level for heat transfer contact with the fluid, if present, and the reference thermistor at a level for heat transfer contact with water only.
~3~
When the water surface is contaminated by a fluid whose viscosity is lowerthan that of water, such as high petroleum distillates, the heating effect of the sensing thermistor upon the unknown fluid is less than when water alone is present, thereby allowing reduced conveotion heat loss to improve the accuracy of detection of the unknown fluid.
In another embodiment, a first thermistor in a sensing device net-work serves as a sensor for a condition to be sensed and a second thermistor in the network constitutes a temperature compensator, exposed to the same temperature as the first thermistor but not exposed to the condition to be sensed. Here again the resistance of the overall network is compared and maintained with reference to the predetermined value. In an embodiment used as a fluid immersion or level detector, the reference thermistor is exposed constantly to air or liquid and the sensing thermistor is exposed for contact with liquid upon immersion.
Preferred embodiments of the invention for these various applications feature a resistance comparator signalling the difference between the present value of the resistance of the sensing device and a temperature independent reference, and a power regulator responsive to the comparator to vary the power flow through the sensing device to reduce the resistance di~ference. In one such em~odiment the resistance comparator preferably comprise6 at least one reference resistor and a transistor means connected so that the signal on an output lead of the transistor means represents the resistance difference.
The power regulator preferably comprise6 a transistor means operative as a series regulator connected between a single, direct current energy source and the sensing device. The output lead of the resistance comparator is connected to the effective base of the power regulator transistor means, thereby cont-rolling the flow of power through the sensing device.
In a specific preferred embodiment the thermistor sensing device and a plurality of resistors comprise a bridge connected to a resistance comparator transistor means to signal the degree of unbalance of the bridge ~ - , 3~
attributable to the difference between the present resistance value of the sensing device and the predetermined reference value. Specifically, the bridge comprises a first resistor having a resistance value equal to the predetermined reference value, and a pair of series-connected resistors of equal value comprising a divider. The divider forms one path from the power regulator to ground and the first resistor, connected in series with the sensing device forms a second path from the power regulator to the ground.
The resistance comparator transistor means has its base effectively connected `
to the mid-point of the divider, and its emitter effectively connected between the first resistor and sensing device. Its effective collector is connected to control the power regulator.
In a still further embodiment of the invention herein, the restor-ative circuit consists of a constant current circuit having two parallel branches, one branch containing the sensing device and the other connected through the collector and emitter of a transistor means, the base of the transistor means connected in a feedback relation to respond to the voltage change across the sensing device and thereby driving the resistance of the sensing device toward the reference resistance.
In a still further embodiment of the invention herein, the therm-istor circuit consistE; of a constant current circuit having two parallel branches, one branch containing a thermistor, and the other connected through the collector and emitter of a transistor means, the base of the transistor means connected in a feedback relation to respond to the voltage change across the thermistor.
Embodiments of the invention will now be described, by way of example, in conjunction with the accompanying drawings, in which:
Figure 1 is a schematic diagram of the circuitry of a preferred embodiment according to the invention employing a series connection of two thermistors;
Figure 2 is a schematic diagram, similar to Figure 1, of an ' .
.
3~5~
embodiment of the invention employing a parallel connection of two thermistors;
Figure 3 is a schematic diagram of circuitry of an embodiment of the invention employing operational amplifiersj Figure 3a is a schematic diagram of circuitry of an embodiment of the invention employing one operational amplifier;
Figure 4 is a diagrammatic view of an oil detection unit having two thermistors and employing the circuitry of Figure 1 or Figure 2;
Figure 5 is a schematic diagram of an embodiment of the invention employing a constant current circuit and a negative temperature coefficient thermistor, and Figure 6 is a schematic diagram of circuitry combining two of the thermistor circuits of Figure 1 in a summing arrangement using single therm-istors in the sensing legs.
Referring to Figure 1, transistor Ql' a five-watt silicon transis-tor with ~ over 100, operates as a standard series regulator. Transistor Q2' a standard NP~ entertainment transistor with ~ over 100, serves to modify transistor Ql's base current in response to the current flow between the base and emitter of transistor Q2 caused in turn by a resistance imbalance in the resi~tance comparing bridge circuit shown. Two arms of this bridge are formed by 1000 ~ resistors Rl and R2; the other arms are formed by a 190 ~ reference resistor R3 and an ancillary bridge comprising the series connections of the two thermistors Tl and T2 in parallel with series-connected resistors R4 and R5. Thermistors Tl and T2, both Fenwal GD31SM2, have negative terrlperature coefficients. Resistors R4 and R5 are each 1000 ~ .
The base and emitter of transistor Q2 are connected to the midpoints of the resistance comparing bridge circuit. The diode D is a standard milliwatt device which compensates for the forward potential between the base and emitter of transistor Q2. At 25 C ambient temperature conditions, the thermistors Tl and T2 each have a resistance of 1000 ~ .
~3Q~:
In operation, whenever the resistance comparing bridge of Figure 1 is unbalancedJ a potential difference develops between points A and B. When the resistance of the ancillary bridge is higher than the reference resistor, transistor Q2 is turned off allowing transistor Ql to be fully on. Power, therefore, flows through the ancillary bridge to ground causing heating of the thermistors and an attendent decrease in their resistance values. Power will continue to flow until the resistances of thermistors Tl and T2 change, thereby reducing the potential difference between A and B and bringing the bridge into balance. Transistor Q2 then begins to turn on, robbing transistor Ql of some of its base current, thereby turning it partly off. As transistor Ql progressively tuIns off the current flow through the ancillary bridge is reduced. Thus, this restorative electronic circuit attempts to maintain the resistance of the ancillary bridge, consisting of the series-connected thermistors and resistors R4 and R5, equal to the resistance of the reference resistor R3 in the face of changing conditions in the surrounding medium.
Whenever thermistors Tl and T2 are exposed to a surrounding medium such that the rate of heat loss from each of the thermistors is the same, the ancillary bridge will be balanced; the voltage as measured by volbneter V will be zero. lf, for example, the temperature of thc surrounding medium were to change, the resistance values of thermistors Tl and T2 will change.
The resistance comparing bridge will then become unbalanced and the restor-ative electronic circuit will adjust the power flow through the ancillary bridge to restore balance. The output voltmeter V will continue to read zeroJ however, since the temperature of thermistor Tl remains equal to the temperature of thermistor T2, even though the flow of power through the thermistors will have changed. Temperature compensation has been achieved very simply by putting the two thermistors in the same arm of the main bridge circuit. When, however, the temperatures of thermistors Tl and T2 differ, as, for example, when one thermistor is exposed to a still fluid and the other to the same fluid in motion, the ancillary bridge becomes unbalanced ~l~3~
and a voltage will register on voltmeter V indicating the velocity of the fluid in motion.
In the similar embodiment shown in Figure 2, thermistors Tl and T2 are connected in parallel within the ancillary bridge having Tl, T2, R4 and R5. Its operation is similar to the embodiment of Figure 1. Only when the temperatures of thermistors Tl and T2 differ will voltmeter V register.
Figure 3 is an embodiment of the invention embodying operational amplifiers. Operational amplifier 10 is connected across reference resistor Rl and serves as a gain of one inverter amplifier, with its output propor-tional to the voltage across resistor Rl. Operational amplifier 11 is connec-ted across the ancillary bridge having arms Tl, T2, Rg and Rlo and serves as a gain of one amplifier with its output proportional to the voltage across the ancillar~ bridge. The two outputs are electrically subtracted and the difference serves as the input to operational amplifier 12, which serves as a gain of ten summing amplifier, with its output proportional to the difference in the voltages across the ancillary bridge and reference resistor Rl. Be-cause the current flow through the ancillary bridge equals that through reference resistor Rl, the output of operational amplifier 12 is proportional to the difference in resistance between the ancillary bridge and reference resistor Rl. The output signal from operational amplifier 12 is connected to the bace of transistor Q thereby controlling the flow of power through the ancillary bridge and controlling its resistance. Voltmeter 16 is connected across the ancillary bridge to indicate the difference in resistance between thermistors Tl and T2.
In this embodiment the frequency compensation and power supply connections to the operational amplifiers (routine to the art) are not shown.
The follcwing components are used:
Thermistors Tl and T2 Fenwal GD25SM2 series regulator Q 2N1038 30operational ampliiers 10, 11, 12 709 resistances:
\
~3~;2 1 33 K ,~
R2 2.7K
R3 2.7K
R4 2.7K
R5 2.7K
Rg 1000 10Rlo 1000 rl Figure 3a shows an embodiment of the invention similar to Figure 1 in which transistor Q2 f Figure 1 has been replaced by operational amplifier 12 whose output is a measure of the main bridge unbalance. This output controls the power flow through the sensing leg via transistor Ql Figure 4 shows the invention embodied as an oil detection unit employing the circuit of Figure 1 or 2. The oil detection system includes a buoyant, tubular housing 50 designed to float on water 52 to be monitored.
Housing 50 has upper and lower recesses 54, 56. The buoyancy of housing 50 is such that recess 54 is disposed at the sur~ace of the water and recess 56 i9 submerged. Reference thermistor 20 is disposed in recess 56 so that it remains under water. Sensing thermistor 22 is disposed in recess 54 at the air-liquid interface so that it is exposed to oil should a film of oil 24 exist on the monitored surface. The oil detection unit may be self-contained and include batteries 30 (which function as ballast), the electronic circuit of Figures 1 or 2, 32 and an output indicator 34 on its upper surface. In another embodiment, the unit may be connected by flexible cable (not shown) to a remote power supply and to remote output indicator circuitry.
Figure 5 is an embodiment of the invention employing a constant current circuit which attempts to maintain a constant current flow through 3 collector resistor Rl. There are two parallel paths for current to flow ~ 3~
from the energy source to ground, one through resistor Rl and transistor Q
and the other through thermistor T and R2. Thermistor T is connected between the base and collector of transistor Q, enabling transistor Q to control the flow of current through thermistor T. The component values are chosen so that the circuit keeps current through T constant thereby tending to restore a predetermined resistance value, however imperfectly. If conditions change in the surrounding medium such that thermistor T cools, for example, its resistance increases causing the potential at the base of transistor Q to rise, progressively turning transistor Q off, thereby allowing more current to flow through the thermistor T. This increased power through thermistor T
heats it, causing its resistance to decrease. Voltmeter V measuring the voltage across the thermistor is an indication of the heat transfer from the thermistor to the surrounding medium. For a positive temperature coefficient thermistor, the positions in the circuit of thermistor T and voltmeter V are exchanged with that of resistor Rl.
Referring to Figure 6, two of the circuits of Figure 1 are shown connected in a summing arrangement using single thermistors in the sensing legs. One of the thermistor circuits acts as a reference. Whenever the power ~low through thermistor Tl diff`ers f`rom the power flow through thermis-tor T2, an~loutput signal develops on leads ~ and ~. If`, for example, thermistor T2, the reference, is exposed to water, and thermistor Tl is exposed to water with a small amount of alcohol added, the power required to maintain the two thermistors at a constant, preselected temperature will differ because of the diff`erent heat transfer characteristics of the two fluids. An output signal will therefore develop on leads ~1 and ~2 from which the presence of an adulterant can be inferred. For certain applications the reference circuit need not be another thermistor circuit; it may consist of a simple resistance circuit to serve as a reference.
While the preferred devices used to perform the thermistor functions of this invention are those semiconductor units sold as "thermlstors," it will 3~
be understood that certain ~eatures of the invention can be obtained using other devices, or combinations whose effect upon the circuit varies with temperature in a single valued relation. For instance, a temperature sensitive diode or transistor may be employed in certain instances, provided its temp-erature characteristic corresponds to the needs of the particular application involved. Also, a metallic filament, such as tungsten, whose resistance is temperature dependent, may be employed.
This inventi'on relates to detectors which respond to a heat absorptive characteristi'c condi't;on of a surround;ng medium and to detectors for discriminating at least one condition from a s-et of different conditions that are related to the heat a~sorpti've properties of the medium. The conditions may relate to thermal conductivity, temperature, etc., for purposes such as- detect;ng oil spills on water or determining the identity of a fluid.
The purpose of the invention i-s to provide detectors that are simple, durable, and can be made at low cost. Another purpose is to enable the various factors affecting fieat transfer ;nto a medium to be discrimina-ted from one another, to el;m;nate spur;ous conditions, such as temperature changes, to achieve devïces that are extremely sensitive. A further object is to enable fluids that are dïfficult to d;scriminate, from water for example, to be reliably detected.
According to the invention, there is provided apparatus for detecting the presence of foreign li'qu;d comprising a monitoring device exposed for heat transfer contact with a liquid when immersed therein, said monitoring device bei'ng capable of diss;pating electric power and having an electrical characteri'stic tHat changes as a function of temperature, a restorative electrical circuit connected to said monitoring device to supply electrical power to the element to be dissipated thereby and responsive to changes in said cfiaracteristic of the device to adjust said supply of electrical power to counteract said changes, output circuitry responsive to said changes to provide an i'ndication of changes in the rate of heat loss from the device. The invention embodies an electrical sensing device having at least one monitoring element having a temperature dependent electrical characteristic, e.g. a thermistor, exposed to a surrounding heat absorptive medium and connected to a restorative energizing electronic circuit supplying heating power to the device to restore its electrical characteristic, e.g. resistance, toward a predetermined ~r ~ 1 ~
reference value. Maintaining the resi.stance of the sensing device near the reference value ass.ures that the resistance, and hence temperatures, of the individua.l thermistors remain within their designed operating ranges, eliminating the pos-sibility of burning out one or more thermistors or igniting a volatile fluid, if present; because the restorative circuit, powered preferably by-a single, direct current source, energizes the device as a whole, any thermistor in the leg is affected as the direct current flow through the device is controlled in response to changing conditions of the surrounding medium. Also when more than one thermistor is located in a single sensing device, noise inherent in any active electronic -la-- ~.
~3~
circuit influences the whole set of thermistors equally, allowing the effects of such noise to be eliminated.
In embodiments of the invention in which the sensing device comprises a single thermistor, the power required to maintain the thermistor resistance equal to a reference resistance is a measure of the thermal loss of the thermistor to the medium. An output signal representing this power is therefore a measure of the heat absorptive characteristics of the medium, and may be used, for example, to identify an unknown fluid or to distinguish between an unknown fluid and water.
In embodiments employing multiple thermistors, similarly useful information is derived from the differential change in resistance between the various thermistors in the device, made possible by the common means of applying power to the device.
In preferred embodiments of the invention for discrimination among a number of conditions affecting heat flow into a medium, the sensing device comprises an electrical network that includes a set of thermistors corresponding to the number of conditions to be discriminated. Each of the thermistors is disposed in a different heat transfer relationship with the medium and means are provided to derive an output signal, dependent upon the resistance values of the individu~1 thermistors, indicatin~ the condition to be sensed. A detector, therefore, embodying the invention provides information about the rate of heat loss to a surrounding medium due to a single condition of interest, eliminating the effects of other conditions, e.g., temperature fluctuations.
In a specific preferred embodiment an electrical network of thermistors, i.e. the device, comprises one leg of a resistance comparing bridge. Whenever the resistance of the network differs from a reference resistance in another leg of the bridge, the restorative circuit varies the power flow through the network, bringing the bridge into balance.
In preferred embodiments the electrical network itself comprises ~3~
an ancillary bridge including two thermistors and two resistors of equal value comprising a voltage divider, allowing the resistances of the two thermistors to be compared. Any difference in resistance between the therm-istors produces a voltage across the ancillary bridge which serves as an output signal and indicates the condition of interest. The two thermistors may be connected in series or parallel within the ancillary bridge for this purpose.
In another preferred embodiment the sensing device or the electrical network of multiple thermistors is connected in series with a temperature-independent reference resistor in an electronic energizing circuit. Because of the series connection, the same current flows through the reference resistor and the network. Connected across the reference resistor and across the network are the inputs to separate operational amplifier means whose outputs are measures of the voltage across the reference resistor and the network and are also measures of the respective resistance values. A further operational amplifier means is disposed in the circuit so that its input is the difference between the outputs of the operational amplifier means whose inputs are connected across the network and reference resistor. The output of this further operational amplifier means is proportion~l to the difference in resistance between the series network and the reference resistor, and is used to regulate the power flow through the network in a manner tending to maintain the resistance of the network equal to the resistance value of the reference resistor.
Another preferred embodiment of the invention is a detector for sensing a difference between an unknown fluid, e.g., oil, and water. In this embodiment, the electrical network whose resistance is maintained relative to the predetermined value includes two thermistors, one a sensor and the other a reference, incorporated into a probe for positioning the sensing thermistor at a level for heat transfer contact with the fluid, if present, and the reference thermistor at a level for heat transfer contact with water only.
~3~
When the water surface is contaminated by a fluid whose viscosity is lowerthan that of water, such as high petroleum distillates, the heating effect of the sensing thermistor upon the unknown fluid is less than when water alone is present, thereby allowing reduced conveotion heat loss to improve the accuracy of detection of the unknown fluid.
In another embodiment, a first thermistor in a sensing device net-work serves as a sensor for a condition to be sensed and a second thermistor in the network constitutes a temperature compensator, exposed to the same temperature as the first thermistor but not exposed to the condition to be sensed. Here again the resistance of the overall network is compared and maintained with reference to the predetermined value. In an embodiment used as a fluid immersion or level detector, the reference thermistor is exposed constantly to air or liquid and the sensing thermistor is exposed for contact with liquid upon immersion.
Preferred embodiments of the invention for these various applications feature a resistance comparator signalling the difference between the present value of the resistance of the sensing device and a temperature independent reference, and a power regulator responsive to the comparator to vary the power flow through the sensing device to reduce the resistance di~ference. In one such em~odiment the resistance comparator preferably comprise6 at least one reference resistor and a transistor means connected so that the signal on an output lead of the transistor means represents the resistance difference.
The power regulator preferably comprise6 a transistor means operative as a series regulator connected between a single, direct current energy source and the sensing device. The output lead of the resistance comparator is connected to the effective base of the power regulator transistor means, thereby cont-rolling the flow of power through the sensing device.
In a specific preferred embodiment the thermistor sensing device and a plurality of resistors comprise a bridge connected to a resistance comparator transistor means to signal the degree of unbalance of the bridge ~ - , 3~
attributable to the difference between the present resistance value of the sensing device and the predetermined reference value. Specifically, the bridge comprises a first resistor having a resistance value equal to the predetermined reference value, and a pair of series-connected resistors of equal value comprising a divider. The divider forms one path from the power regulator to ground and the first resistor, connected in series with the sensing device forms a second path from the power regulator to the ground.
The resistance comparator transistor means has its base effectively connected `
to the mid-point of the divider, and its emitter effectively connected between the first resistor and sensing device. Its effective collector is connected to control the power regulator.
In a still further embodiment of the invention herein, the restor-ative circuit consists of a constant current circuit having two parallel branches, one branch containing the sensing device and the other connected through the collector and emitter of a transistor means, the base of the transistor means connected in a feedback relation to respond to the voltage change across the sensing device and thereby driving the resistance of the sensing device toward the reference resistance.
In a still further embodiment of the invention herein, the therm-istor circuit consistE; of a constant current circuit having two parallel branches, one branch containing a thermistor, and the other connected through the collector and emitter of a transistor means, the base of the transistor means connected in a feedback relation to respond to the voltage change across the thermistor.
Embodiments of the invention will now be described, by way of example, in conjunction with the accompanying drawings, in which:
Figure 1 is a schematic diagram of the circuitry of a preferred embodiment according to the invention employing a series connection of two thermistors;
Figure 2 is a schematic diagram, similar to Figure 1, of an ' .
.
3~5~
embodiment of the invention employing a parallel connection of two thermistors;
Figure 3 is a schematic diagram of circuitry of an embodiment of the invention employing operational amplifiersj Figure 3a is a schematic diagram of circuitry of an embodiment of the invention employing one operational amplifier;
Figure 4 is a diagrammatic view of an oil detection unit having two thermistors and employing the circuitry of Figure 1 or Figure 2;
Figure 5 is a schematic diagram of an embodiment of the invention employing a constant current circuit and a negative temperature coefficient thermistor, and Figure 6 is a schematic diagram of circuitry combining two of the thermistor circuits of Figure 1 in a summing arrangement using single therm-istors in the sensing legs.
Referring to Figure 1, transistor Ql' a five-watt silicon transis-tor with ~ over 100, operates as a standard series regulator. Transistor Q2' a standard NP~ entertainment transistor with ~ over 100, serves to modify transistor Ql's base current in response to the current flow between the base and emitter of transistor Q2 caused in turn by a resistance imbalance in the resi~tance comparing bridge circuit shown. Two arms of this bridge are formed by 1000 ~ resistors Rl and R2; the other arms are formed by a 190 ~ reference resistor R3 and an ancillary bridge comprising the series connections of the two thermistors Tl and T2 in parallel with series-connected resistors R4 and R5. Thermistors Tl and T2, both Fenwal GD31SM2, have negative terrlperature coefficients. Resistors R4 and R5 are each 1000 ~ .
The base and emitter of transistor Q2 are connected to the midpoints of the resistance comparing bridge circuit. The diode D is a standard milliwatt device which compensates for the forward potential between the base and emitter of transistor Q2. At 25 C ambient temperature conditions, the thermistors Tl and T2 each have a resistance of 1000 ~ .
~3Q~:
In operation, whenever the resistance comparing bridge of Figure 1 is unbalancedJ a potential difference develops between points A and B. When the resistance of the ancillary bridge is higher than the reference resistor, transistor Q2 is turned off allowing transistor Ql to be fully on. Power, therefore, flows through the ancillary bridge to ground causing heating of the thermistors and an attendent decrease in their resistance values. Power will continue to flow until the resistances of thermistors Tl and T2 change, thereby reducing the potential difference between A and B and bringing the bridge into balance. Transistor Q2 then begins to turn on, robbing transistor Ql of some of its base current, thereby turning it partly off. As transistor Ql progressively tuIns off the current flow through the ancillary bridge is reduced. Thus, this restorative electronic circuit attempts to maintain the resistance of the ancillary bridge, consisting of the series-connected thermistors and resistors R4 and R5, equal to the resistance of the reference resistor R3 in the face of changing conditions in the surrounding medium.
Whenever thermistors Tl and T2 are exposed to a surrounding medium such that the rate of heat loss from each of the thermistors is the same, the ancillary bridge will be balanced; the voltage as measured by volbneter V will be zero. lf, for example, the temperature of thc surrounding medium were to change, the resistance values of thermistors Tl and T2 will change.
The resistance comparing bridge will then become unbalanced and the restor-ative electronic circuit will adjust the power flow through the ancillary bridge to restore balance. The output voltmeter V will continue to read zeroJ however, since the temperature of thermistor Tl remains equal to the temperature of thermistor T2, even though the flow of power through the thermistors will have changed. Temperature compensation has been achieved very simply by putting the two thermistors in the same arm of the main bridge circuit. When, however, the temperatures of thermistors Tl and T2 differ, as, for example, when one thermistor is exposed to a still fluid and the other to the same fluid in motion, the ancillary bridge becomes unbalanced ~l~3~
and a voltage will register on voltmeter V indicating the velocity of the fluid in motion.
In the similar embodiment shown in Figure 2, thermistors Tl and T2 are connected in parallel within the ancillary bridge having Tl, T2, R4 and R5. Its operation is similar to the embodiment of Figure 1. Only when the temperatures of thermistors Tl and T2 differ will voltmeter V register.
Figure 3 is an embodiment of the invention embodying operational amplifiers. Operational amplifier 10 is connected across reference resistor Rl and serves as a gain of one inverter amplifier, with its output propor-tional to the voltage across resistor Rl. Operational amplifier 11 is connec-ted across the ancillary bridge having arms Tl, T2, Rg and Rlo and serves as a gain of one amplifier with its output proportional to the voltage across the ancillar~ bridge. The two outputs are electrically subtracted and the difference serves as the input to operational amplifier 12, which serves as a gain of ten summing amplifier, with its output proportional to the difference in the voltages across the ancillary bridge and reference resistor Rl. Be-cause the current flow through the ancillary bridge equals that through reference resistor Rl, the output of operational amplifier 12 is proportional to the difference in resistance between the ancillary bridge and reference resistor Rl. The output signal from operational amplifier 12 is connected to the bace of transistor Q thereby controlling the flow of power through the ancillary bridge and controlling its resistance. Voltmeter 16 is connected across the ancillary bridge to indicate the difference in resistance between thermistors Tl and T2.
In this embodiment the frequency compensation and power supply connections to the operational amplifiers (routine to the art) are not shown.
The follcwing components are used:
Thermistors Tl and T2 Fenwal GD25SM2 series regulator Q 2N1038 30operational ampliiers 10, 11, 12 709 resistances:
\
~3~;2 1 33 K ,~
R2 2.7K
R3 2.7K
R4 2.7K
R5 2.7K
Rg 1000 10Rlo 1000 rl Figure 3a shows an embodiment of the invention similar to Figure 1 in which transistor Q2 f Figure 1 has been replaced by operational amplifier 12 whose output is a measure of the main bridge unbalance. This output controls the power flow through the sensing leg via transistor Ql Figure 4 shows the invention embodied as an oil detection unit employing the circuit of Figure 1 or 2. The oil detection system includes a buoyant, tubular housing 50 designed to float on water 52 to be monitored.
Housing 50 has upper and lower recesses 54, 56. The buoyancy of housing 50 is such that recess 54 is disposed at the sur~ace of the water and recess 56 i9 submerged. Reference thermistor 20 is disposed in recess 56 so that it remains under water. Sensing thermistor 22 is disposed in recess 54 at the air-liquid interface so that it is exposed to oil should a film of oil 24 exist on the monitored surface. The oil detection unit may be self-contained and include batteries 30 (which function as ballast), the electronic circuit of Figures 1 or 2, 32 and an output indicator 34 on its upper surface. In another embodiment, the unit may be connected by flexible cable (not shown) to a remote power supply and to remote output indicator circuitry.
Figure 5 is an embodiment of the invention employing a constant current circuit which attempts to maintain a constant current flow through 3 collector resistor Rl. There are two parallel paths for current to flow ~ 3~
from the energy source to ground, one through resistor Rl and transistor Q
and the other through thermistor T and R2. Thermistor T is connected between the base and collector of transistor Q, enabling transistor Q to control the flow of current through thermistor T. The component values are chosen so that the circuit keeps current through T constant thereby tending to restore a predetermined resistance value, however imperfectly. If conditions change in the surrounding medium such that thermistor T cools, for example, its resistance increases causing the potential at the base of transistor Q to rise, progressively turning transistor Q off, thereby allowing more current to flow through the thermistor T. This increased power through thermistor T
heats it, causing its resistance to decrease. Voltmeter V measuring the voltage across the thermistor is an indication of the heat transfer from the thermistor to the surrounding medium. For a positive temperature coefficient thermistor, the positions in the circuit of thermistor T and voltmeter V are exchanged with that of resistor Rl.
Referring to Figure 6, two of the circuits of Figure 1 are shown connected in a summing arrangement using single thermistors in the sensing legs. One of the thermistor circuits acts as a reference. Whenever the power ~low through thermistor Tl diff`ers f`rom the power flow through thermis-tor T2, an~loutput signal develops on leads ~ and ~. If`, for example, thermistor T2, the reference, is exposed to water, and thermistor Tl is exposed to water with a small amount of alcohol added, the power required to maintain the two thermistors at a constant, preselected temperature will differ because of the diff`erent heat transfer characteristics of the two fluids. An output signal will therefore develop on leads ~1 and ~2 from which the presence of an adulterant can be inferred. For certain applications the reference circuit need not be another thermistor circuit; it may consist of a simple resistance circuit to serve as a reference.
While the preferred devices used to perform the thermistor functions of this invention are those semiconductor units sold as "thermlstors," it will 3~
be understood that certain ~eatures of the invention can be obtained using other devices, or combinations whose effect upon the circuit varies with temperature in a single valued relation. For instance, a temperature sensitive diode or transistor may be employed in certain instances, provided its temp-erature characteristic corresponds to the needs of the particular application involved. Also, a metallic filament, such as tungsten, whose resistance is temperature dependent, may be employed.
Claims (21)
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for detecting the presence of foreign liquid comprising a monitoring device exposed for heat transfer contact with a liquid when immersed therein, said monitoring device being capable of dissipating electric power and having an electrical characteristic that changes as a function of temperature, a restorative electrical circuit connected to said monitoring device to supply electrical power to the element to be dissipated thereby and responsive to changes in said characteristic of the device to adjust said supply of electrical power to counteract said changes, output circuitry responsive to said changes to provide an indication of changes in the rate of heat loss from the device.
2. The detector of claim 1 wherein said monitoring device comprises a single monitoring element and said output circuitry is responsive to power flow conditions through said element.
3. The detector of claim 1 wherein said monitoring device comprises an electrical network that includes a set of monitoring elements, and said restorative circuit is responsive to change in an electrical characteristic of said overall network to vary the flow of power to said network in a manner to restore the actual electrical characteristic of said overall network, and means to derive an output signal dependent upon electrical characteristic values of individual monitoring elements of said monitoring device.
4. The detector of claim 1, 2 or 3 wherein said monitoring device includes at least one thermistor monitoring element.
5. The detector of claim 1, 2 or 3 wherein said monitoring device includes at least one monitoring element of temperature-sensitive metal such as tungsten.
6. The detector of claim 1 wherein said electrical characteristic is resistance and said restorative circuit includes a resistance-comparator signalling the difference between the present resistance value of said monitoring device and the resistance value of a reference means, and a power regulator responsive to said comparator to vary power flow through said monitoring device to reduce said difference.
7. The detector of claim 6 wherein said power regulator comprises a transistor means operative as a series regulator, connected between an energy source and said monitoring device.
8. The detector of claim 7 wherein said resistance comparator comprises at least one reference resistor and means connected so that the signal on an output lead of said means represents said difference, the effective base of said power regulator transistor means connected to said comparator output lead.
9. The detector of claim 6 wherein said resistance comparator com-prises first, second, and third operational amplifier means, the first connected so that its output represents the resistance value of said reference, the second connected so that its output represents the resistance value of said monitoring device and said third operational amplifier means having as inputs said outputs of said first and second operational amplifier means, and connected so that the output of said third operational amplifier means represents said difference between the present resistance value of said monitoring device and said reference.
10. The detector of claim 1 wherein said electrical characteristic is electrical resistance, said restorative circuit includes a resistance comparator which comprises at least one reference resistor and a trans-istor means connected so that the signal on an output lead of said transistor means represents the difference between the present actual value of the resistance of said monitoring device and the resistance value of said reference resistor.
11. The detector of claim 10 wherein said monitoring device and a plurality of resistors comprise a bridge network, said transistor means connected to represent the degree of unbalance of said bridge network attributable to the difference of the present resistance value of said monitoring device from said reference value.
12. The detector of claim 17 wherein said bridge network comprises a first resistor having a resistance value equal to said reference value, and a pair of series connected resistors of equal value comprising a divider, said divider forming one path from said regulator to ground and said first resistor and said monitoring device connected in series forming a second path from said regulator to ground, said transistor means having its base effectively connected to the midpoint of said divider and its effective emitter connected between said first resistor and said monitoring device and its- effective collector connected to control said power regulator.
13. The detector of claim 12 wherein means are provided to produce said signal as the voltage across said bridge, said voltage being inter-dependent with the current through said monitoring device.
14. The detector of claim 3 wherein said electrical characteristic is electrical resistance and said network comprises one leg of a resistance-comparing bridge, said network connected to be driven toward balance in said bridge by said restorative circuit.
15. The detector of claim 14 wherein said network comprises an ancillary bridge including two monitoring elements and two resistors of equal value comprising a divider, and means to derive a signal from said ancillary bridge dependent upon the difference in resistance of said monitoring elements.
16. The detector of claim 15 wherein said monitoring elements are connected in series within said ancillary bridge, said signal being the voltage across said ancillary bridge, between points respectively between said two monitoring elements and said two resistors.
17. The detector of claim 16 wherein said monitoring elements are connected in parallel within said ancillary bridge, said signal being the voltage across said ancillary bridge, between two points in the respective legs of said ancillary bridge, each point lying between one of said monitoring elements and its respective resistor.
18. The detector of claim 1 wherein said monitoring device is incorporated in a probe for positioning a monitoring element of said monitoring device at the surface boundary of a liquid that may be contaminated by an immiscible liquid.
19. The detector of claim 18 wherein said probe comprises a float adapted to freely float upon the surface of said liquid.
20. The detector of claim 18 wherein said liquid to be sensed is floating on water, characterized in that said monitoring device comprises a monitoring element having an electrical resistance decreasing with increasing temperature, whereby the heating effect of said monitoring element, when said floating liquid has lower viscosity than water, is less than when water is present, thereby enabling reduced convention losses in the presence of high oil distillates and the like to enable detection thereof.
21. The detector of claim 1, 2 or 3, wherein a first monitoring element serves as a sensor for a condition of said liquid to be sensed and a second monitoring element serves as a temperature compensator that is exposed to the same temperature as said first monitoring element and is not exposed to said liquid to be sensed.
Applications Claiming Priority (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/752,199 US4116045A (en) | 1976-12-20 | 1976-12-20 | Oil detector |
| US752,199 | 1976-12-20 | ||
| US78534777A | 1977-04-07 | 1977-04-07 | |
| US785,347 | 1977-04-07 | ||
| US825,670 | 1977-08-18 | ||
| US05/825,670 US4159638A (en) | 1976-12-20 | 1977-08-18 | Thermistor detector circuit and discriminating network for heat absorptive media |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1103052A true CA1103052A (en) | 1981-06-16 |
Family
ID=27419425
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA293,360A Expired CA1103052A (en) | 1976-12-20 | 1977-12-19 | Thermistor device for identifying an unknown fluid |
Country Status (10)
| Country | Link |
|---|---|
| JP (1) | JPS5928256B2 (en) |
| AU (1) | AU513588B2 (en) |
| CA (1) | CA1103052A (en) |
| DE (1) | DE2756859A1 (en) |
| DK (1) | DK566377A (en) |
| FR (1) | FR2374639A1 (en) |
| GB (1) | GB1596648A (en) |
| IT (1) | IT1091381B (en) |
| NO (1) | NO148689C (en) |
| SE (1) | SE433007B (en) |
Families Citing this family (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4419888A (en) * | 1979-06-14 | 1983-12-13 | Kabushikikaisha Shibaura Denshi Seisakusho | Humidity measuring method |
| DE3639435A1 (en) * | 1986-11-18 | 1988-05-26 | Hoelter Heinz | Volume flow monitoring system for filtering devices |
| FR2611907B1 (en) * | 1987-03-05 | 1989-06-16 | Tolectromed Sarl | MEDICAL FLUID IDENTIFIER |
| JPH053947Y2 (en) * | 1988-02-02 | 1993-01-29 | ||
| JPH02290205A (en) * | 1989-02-23 | 1990-11-30 | Kurita Water Ind Ltd | Coagulating apparatus |
| DE69109236T2 (en) * | 1990-11-09 | 1996-01-11 | Hewlett Packard Co | Methods and systems for identifying liquids and determining flow. |
| US5265459A (en) * | 1991-08-22 | 1993-11-30 | The Perkin Elmer Corporation | Single-element thermal conductivity detector |
| IT1312311B1 (en) * | 1999-05-07 | 2002-04-15 | Thermoquest Italia Spa | DEVICE FOR THE MEASUREMENT OF THE THERMAL CONDUCTIVITY OF A FLUID |
| US7003418B2 (en) | 2003-08-28 | 2006-02-21 | Honeywell International Inc. | Methods and systems for temperature compensation of physical property sensors |
| DE102018130890A1 (en) * | 2018-12-04 | 2020-06-04 | Bcs Automotive Interface Solutions Gmbh | Liquid sensor, manufacturing method for a liquid sensor and measuring method for determining a liquid contact |
Family Cites Families (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1249566B (en) * | 1967-09-07 | |||
| US2650496A (en) * | 1948-05-17 | 1953-09-01 | Phillips Petroleum Co | Fluid flowmeter with heated resistance bridge circuit |
| US3429178A (en) * | 1965-01-07 | 1969-02-25 | Enoch J Durbin | Measuring system |
| US3719936A (en) * | 1971-06-01 | 1973-03-06 | Durham Ass Inc | Oil spillage detection system |
| GB1372695A (en) * | 1971-12-07 | 1974-11-06 | Pye Ltd | Thermal conductivity detector apparatus |
| US3864959A (en) * | 1971-12-16 | 1975-02-11 | Pye Ltd | Thermal conductivity detector apparatus |
| US3780565A (en) * | 1972-10-19 | 1973-12-25 | Gen Motors Corp | Fluid vaporization tester |
| AT326753B (en) * | 1972-11-16 | 1975-12-29 | Danfoss As | MEASUREMENT TRANSDUCER WITH A COMPENSATION BRIDGE CIRCLE |
| US3913379A (en) * | 1973-10-18 | 1975-10-21 | Tibor Rusz | Dynamic gas analyzer |
| AT350299B (en) * | 1975-06-26 | 1979-05-25 | Uher Ag | CIRCUIT WITH LINEAR TEMPERATURE-DEPENDENT TOTAL RESISTANCE |
-
1977
- 1977-12-07 NO NO774182A patent/NO148689C/en unknown
- 1977-12-14 AU AU31522/77A patent/AU513588B2/en not_active Expired
- 1977-12-14 GB GB52032/77A patent/GB1596648A/en not_active Expired
- 1977-12-19 IT IT69842/77A patent/IT1091381B/en active
- 1977-12-19 DK DK566377A patent/DK566377A/en not_active Application Discontinuation
- 1977-12-19 CA CA293,360A patent/CA1103052A/en not_active Expired
- 1977-12-19 SE SE7714459A patent/SE433007B/en unknown
- 1977-12-20 JP JP52153494A patent/JPS5928256B2/en not_active Expired
- 1977-12-20 FR FR7738513A patent/FR2374639A1/en active Granted
- 1977-12-20 DE DE19772756859 patent/DE2756859A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| AU3152277A (en) | 1979-06-21 |
| SE433007B (en) | 1984-04-30 |
| NO774182L (en) | 1978-06-21 |
| DE2756859A1 (en) | 1978-07-06 |
| FR2374639A1 (en) | 1978-07-13 |
| AU513588B2 (en) | 1980-12-11 |
| FR2374639B1 (en) | 1984-10-26 |
| GB1596648A (en) | 1981-08-26 |
| JPS5928256B2 (en) | 1984-07-11 |
| DK566377A (en) | 1978-06-21 |
| DE2756859C2 (en) | 1991-04-25 |
| JPS5383795A (en) | 1978-07-24 |
| SE7714459L (en) | 1978-06-21 |
| NO148689B (en) | 1983-08-15 |
| NO148689C (en) | 1983-11-30 |
| IT1091381B (en) | 1985-07-06 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4159638A (en) | Thermistor detector circuit and discriminating network for heat absorptive media | |
| US2947938A (en) | Electrothermal measuring apparatus and method for the calibration thereof | |
| US4255968A (en) | Flow indicator | |
| EP0364982B1 (en) | Frost and dew sensor | |
| US7607823B2 (en) | Leak detector comprising a self-heated thermistor control circuit | |
| US4609913A (en) | Fluid level sensor | |
| US4487063A (en) | Solid state mass air flow sensor | |
| CA1103052A (en) | Thermistor device for identifying an unknown fluid | |
| KR0184673B1 (en) | Thermal type flowmeter | |
| JPS589057A (en) | Detector for leakage of fluid | |
| US3580074A (en) | Temperature-compensated liquid quantity gage | |
| US3216249A (en) | Differential pressure responsive signal circuit | |
| Ferreira et al. | Hot-wire anemometer with temperature compensation using only one sensor | |
| US4781065A (en) | Solid-state anemometers and temperature gauges | |
| US4845984A (en) | Temperature compensation for a thermal mass flow meter | |
| US4563098A (en) | Gradient compensated temperature probe and gradient compensation method | |
| US4220041A (en) | Alien liquid detector and control | |
| US5159318A (en) | Universal single point liquid level sensor | |
| US6132083A (en) | Real-time measuring method | |
| US2982908A (en) | Sensing apparatus | |
| EP0164885A1 (en) | Fluid flow sensor | |
| US5421202A (en) | Liquid sensor having thermistors | |
| US4596140A (en) | Constant overheat anemometer with sensor lead wire impedance compensation | |
| AU736573B2 (en) | Liquid level sensor | |
| US2738678A (en) | Liquid manometers |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |