CA1036690A - Apparatus for sensing variations in the heat exchange properties of a medium - Google Patents
Apparatus for sensing variations in the heat exchange properties of a mediumInfo
- Publication number
- CA1036690A CA1036690A CA229,140A CA229140A CA1036690A CA 1036690 A CA1036690 A CA 1036690A CA 229140 A CA229140 A CA 229140A CA 1036690 A CA1036690 A CA 1036690A
- Authority
- CA
- Canada
- Prior art keywords
- switching devices
- switching
- medium
- heat exchange
- exchange properties
- 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
- 230000001172 regenerating effect Effects 0.000 claims abstract description 16
- 238000012806 monitoring device Methods 0.000 claims abstract description 12
- 230000008878 coupling Effects 0.000 claims abstract description 9
- 238000010168 coupling process Methods 0.000 claims abstract description 9
- 238000005859 coupling reaction Methods 0.000 claims abstract description 9
- 230000001419 dependent effect Effects 0.000 claims abstract description 9
- 230000004044 response Effects 0.000 claims abstract description 6
- 230000001747 exhibiting effect Effects 0.000 claims abstract description 4
- 230000001939 inductive effect Effects 0.000 claims abstract description 3
- 230000009471 action Effects 0.000 claims description 3
- 239000003570 air Substances 0.000 description 12
- 239000007788 liquid Substances 0.000 description 10
- 239000012530 fluid Substances 0.000 description 9
- 230000008859 change Effects 0.000 description 5
- 230000009467 reduction Effects 0.000 description 4
- 238000001514 detection method Methods 0.000 description 3
- 230000002457 bidirectional effect Effects 0.000 description 2
- 238000010438 heat treatment Methods 0.000 description 2
- 229910021542 Vanadium(IV) oxide Inorganic materials 0.000 description 1
- XHCLAFWTIXFWPH-UHFFFAOYSA-N [O-2].[O-2].[O-2].[O-2].[O-2].[V+5].[V+5] Chemical compound [O-2].[O-2].[O-2].[O-2].[O-2].[V+5].[V+5] XHCLAFWTIXFWPH-UHFFFAOYSA-N 0.000 description 1
- 239000012080 ambient air Substances 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 239000004020 conductor Substances 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000011521 glass Substances 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 238000000034 method Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- 230000006641 stabilisation Effects 0.000 description 1
- 238000011105 stabilization Methods 0.000 description 1
- 229910000314 transition metal oxide Inorganic materials 0.000 description 1
- GRUMUEUJTSXQOI-UHFFFAOYSA-N vanadium dioxide Chemical compound O=[V]=O GRUMUEUJTSXQOI-UHFFFAOYSA-N 0.000 description 1
- 229910001935 vanadium oxide Inorganic materials 0.000 description 1
Landscapes
- Control Of Temperature (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE:
An apparatus for sensing variations in the heat exchange properties of a medium is disclosed. The apparatus includes two temperature sensitive switching devices having substantially identical electrical characteristics and exhibiting regenerative switching at a power level which is dependent on the temperature sensed by the switching devices. Both switching devices are exposed to the same ambient temperature in a surrounding medium but only one of them is exposed to variations in the heat exchange properties of the sur-rounding medium. Means are provided for applying D.C. power to both switching devices at a level such as to maintain them below their switching point under preset conditions of the medium. A coupling device is interconnected between the two switching devices for inducing regenerative switching in one of the switching devices in response to unbalance in the heat dissipated by the swit-ching devices due to variations in the heat exchange properties of the sur-rounding medium from the preset conditions. A monitoring device is connected to both switching devices for indicating which one of the switching devices has switched as an indication that the heat exchange properties of the medium have changed.
An apparatus for sensing variations in the heat exchange properties of a medium is disclosed. The apparatus includes two temperature sensitive switching devices having substantially identical electrical characteristics and exhibiting regenerative switching at a power level which is dependent on the temperature sensed by the switching devices. Both switching devices are exposed to the same ambient temperature in a surrounding medium but only one of them is exposed to variations in the heat exchange properties of the sur-rounding medium. Means are provided for applying D.C. power to both switching devices at a level such as to maintain them below their switching point under preset conditions of the medium. A coupling device is interconnected between the two switching devices for inducing regenerative switching in one of the switching devices in response to unbalance in the heat dissipated by the swit-ching devices due to variations in the heat exchange properties of the sur-rounding medium from the preset conditions. A monitoring device is connected to both switching devices for indicating which one of the switching devices has switched as an indication that the heat exchange properties of the medium have changed.
Description
103~ 90 This invention relates to an apparatus for sensing variations in the heat exchange properties of a medium, and more particularly to an apparatus for sensing variations in the flow of a fluid medium such as air or liquid, or for sensing the presence or absence of a liquid as an indication of the level of the liquid in a reservoir.
Various devices have been used in the prior art for sensing varia-tions i~ the flow of a fluid medium or for sensing liquid level For sensing fluid flow, for example, various mechanical impact devices have been used.
However, they present substantial interference with the fluid flow.
Generally, the mechanical devices whether used for sensing fluid flow or for fluid level detection have poor response characteristics and cannot sense relatively small variations in the heat exchange properties of the surrounding medium. They are also susceptible to nuisance operations and subject to wear.
Negative and positive temperature coefficient devices have also been used for sensing variations in the heat exchange properties of a medium. Such devices are normally placed in contact with the medium and the temperature sensed by the devices varies in accordance with the rate at which the medium ; is drawing heat away from them. Negative and positive temperature coefficient devices can thus be used for measuring fluid flow or for sensing fluid levels. The above prior art arrangements are, however, very sensitive to ambient temperature and power supply variations-and means are required for providing adequate compensation. Ambient temperature compensation may be effected by providing a bridge circuit including a second temperature sensitive device which is exposed to ambient temperature but not in contact with the surrounding medium, as disclosed in Canadian Patent No. 5~2,o40 : issued September ~, 1959. However, bridge circuits generally have a very small output which cannot be used directly to control an alarm device and : ~
complicated circuitry must be used to amplify the output of the bridge It is therefore the object of the present invention to provide an ~i 30 apparatus for sensing variations in the heat exchange properties of a medium which is relatively immune to temperature and power supply variations.
It is also an object of the present invention to provide an 103~690 apparatus for sensing variations in the heat exchange properties of a medium which exhibits a rapid and significant change in its output upon a predeter-mined change in the heat exhange properties of the medium, thus providing an adequate signal for directly operating a simple monitoring device.
The apparatus, in accordance with the invention, comprises a first and a second temperature sensitive switching devices having substantially identica~l electrical characteristics and exhibiting regenerative switching at a power level which is dependent on the temperature sensed by the switching devices. Both switching devices are~exposed to the same ambient temperature in a surrounding medium but only one of them is exposed to variations in the heat exchange properties of the surrounding medium. Means are provided for applying D.C. power to both switching devices at a level such as to maintain them below their switching point under preset conditions of the medium. A
coupling device is interconnected between the first and second switching devi-ces for rapidly inducing regenerative switching in one of the switching devices in response to unbalance of the heat dissipated by the switching devices due to variations in the heat exchange properties of the surrounding medium from the preset conditions. A monitoring device is connected to the first and second . switching devices for indicating which one of the switching devices has switched.
In one embodiment of the invention, a ~esistor is connected in series with each temperature sensitive switching device and the amplitude of the D.C. source and the value of the resistors are selected so as to maintain both switching devices below their switching point under preset conditions of the medium.
In one embodiment of the invention, the coupling device intercon-nected between the switching devices to induce regenerative switching is a transistor having its base and emitter connected respectively between the first and second switching devices. A pilot light is connected to the collector of the transistor and acts as a monitoring device to indicate variations in the heat exchange properties of the surrounding medium. In another embodiment of the invention, a triac connected in series with a pilot ; - 2 -light is used as a monitoring device. In such a case, the gate of the triac is connected to the collector of the transistor.
The invention will now be disclosed, by way of examples, with reference to preferred embodiments illustrated in the accompanying drawings in which:
Figure 1 illustrates a first embodiment of the invention;
' Figure 2 illustrates the temperature dependent current-voltage characteristics of a vanadium oxide device suitable for use as a temperature sensitive switching device; and Figure 3 illustrates a second embodiment of an apparatus in accor-dance with the invention.
Referring to Figure 1, there is shown a first embodiment of an ; apparatus for sensing variations in the heat exchange properties of a medium.
Such apparatus is for sensing air flow and, more particularly for detecting ; failure of an air flow source such as a fan. The apparatus comprises a first - temperature sensitive switching device Sl connected in series with a resistor Rl across a suitable D.C. source V~, a second temperature sensitive switching device S2 having substantially the same electrical characteristic as the first switching device Sl connected in series with a resistor R2 across the same D.C. source, a coupling device in the form of a transistor Q intercon-necting the switching devices Sl and S2 and a monitoring device in the form of a pilot light L connected to transistor Q. As illustrated in Figure 1, the base of transistor Q is connected to the connecting point of resistor R
':4 with temperature sensitive switching device Sl, the emitter of transistor Q
is connected to the connecting point of resistor R2 with temperature sensitive switching device S2, and the collector of transistor Q is connected to the D.C.
source V~ through pilot light L.
The temperature sensitive switching device must show regenerative switching at a value of voltage which is dependent on temperature. Examples 3 of suitable switching devices are the amorphous glass devices, transition metal oxide devices, and certain classes of negative temperature switching devices. In most cases, elevated temperatures correspond to a reduction in ~03~690 the voltage threshold necessary to cause switching. The expression "regene-rative switching devices" as used herein means devices exhibiting avalance behaviour due to their very high negative temperature coefficient of resis-tance. As commonly known, most thermal avalanche behaviour is due to a current crowding effect in which current crowding leads to local heating in the device which in turn leads to further current crowding. On completion of this process, most of the device current flows through a very narrow filament in the device. Because of the relatively low resistance of the conducting material, total device resistance becomes quite low. Such devices therefore behave like switches and go f~om a high resistance to a low resistance state in a very short period of time. The basic switching mechanism of the tempera-ture dependent regenerative switching devices is usually thermal and occurs in response to both Joule heating and the ambient thermal energy. In most cases, switching action is observed when the sum of all energies yields a given temperature at the switching element. In other words, whatever the source of heat, the switching device always switches at an essentially fixed temperature.
Figure 2 of the drawings shows the temperature dependent current-voltage characteristics of a vanadium dioxide device which is well suited for the present invention. It will be seen that the current-voltage characteris-tics of such temperature dependent switching devices follow curves ToJ T25, T30, etc. depending on whether the temperature sensed by the switching device is 0, 25, 30, etc.
In order to detect a change in the heat exchange properties of a medium, both temperature sensitive switching devices must be exposed to the same ambient temperature in the medium being monitored. However, one switching device onlg is exposed to variations in the heat exchange properties of the medium. In a device such as an air flow sensor, as shown on Figure 1, both switching devices may be exposed to a stream of moving air and switching device Sl placed in a heat sink so as to render it non-responsive to reduction ~0 or loss of air flow. Under predetermined ambient temperature, with the fanblowing at a preset speed, the amplitude of the D.C. source and the value of resistors Rl and R2 are set so that not enough current flows through the -~ :
. . ~
-, , . .~ .
.' ~Q36~9U . .
temperature sensitive switching devices Sl and S2 to make them switch. It is assumed that the load line determined by the parameters of the circuit is as illustrated in Figure 2 and that the operating point is VoIo. It is also assumed that the relative values of the circuit parameters are such that there is not enough voltage across the base-emitter of the transistor Q to render the transistor conductive. However, upon a substantial reduction or loss of'air flow, the temperature sensitive switching device S2 will heat up whereas the temperature sensitive switching device Sl will not warm up appreciably because it is placed in a heat sink. Consequently, as illustra-ted by the load line of Figure 2, the voltage appearing across the switching device S2 will be reduced so that the new operating point will be say VlIl.
A current will therefore start to flow from source V~, resistor Rl, base-emitter of transistor Q and temperature sensitive switching device S2. This current will further heat the temperature sensitive switching device S2 and the operating point will move to say V2I2. This will turn the transistor Q
more conductive and so on thus causing rapid regenerative switching of tempe-rature sensitive switching device S2 and stabilization thereof at a point which we assume is V.I. The full conduction of transistor Q will energize .~ 1 1 ., pilot light L to indicate substantial reduction or loss of air flow.
Although the above apparatus has been disclosed with reference to detection of air flow) it is to be understood that it may also be used for detection of a flow of liquid or even for liquid level sensing. In the later case, temperature sensitive switching devices Sl and S2 are normally placed in contact with the liquid medium when the level of the liquid in a reservoir is at a predetermined hight. However, if this level falls, the temperature sensitive switching devices will be out of the liquid and exposed to the ambient air The rate of heat removal from the switching device which is exposed to the variations in the heat exchange properties of the surrounding medium will certainly change because air does not have the same heat exchange properties as liquid and switching device S2 will heat up and finally switch as explained previously.
Figure ~ illustrates an apparatus very similar to the one illustra-':
--~036690 ted in Figure 1 and the elements corresponding to the ones of Figure 1 have been identified by tbe same reference characters. In this embodiment, the -- monitoring device consists of a triac TR connected in series with a pilot light across an A.C. source. The triac is fired by a gate circuit including -; resistor R3 which is connected across resistor R4 located in the collector circuit of transistor Q.
- ~ The above disclosed apparatus is binary in operation and is inten-ded primarily for application where simple go no-go sensing is required.
The basic advantages of such apparatus are:
1. It is inherently regenerative switching and thus exhibits rapid - switching. As soon as a small unbalance in the heat dissipated by the temperature sensitive switching devices is sensed, an avalanche type regene-rative switching action is generated so as to quickly reduce the voltage across one of the temperature sensitive switching devices and provide a posi-tive indication of the change in the heat exchange properties of the medium being monitored.
Various devices have been used in the prior art for sensing varia-tions i~ the flow of a fluid medium or for sensing liquid level For sensing fluid flow, for example, various mechanical impact devices have been used.
However, they present substantial interference with the fluid flow.
Generally, the mechanical devices whether used for sensing fluid flow or for fluid level detection have poor response characteristics and cannot sense relatively small variations in the heat exchange properties of the surrounding medium. They are also susceptible to nuisance operations and subject to wear.
Negative and positive temperature coefficient devices have also been used for sensing variations in the heat exchange properties of a medium. Such devices are normally placed in contact with the medium and the temperature sensed by the devices varies in accordance with the rate at which the medium ; is drawing heat away from them. Negative and positive temperature coefficient devices can thus be used for measuring fluid flow or for sensing fluid levels. The above prior art arrangements are, however, very sensitive to ambient temperature and power supply variations-and means are required for providing adequate compensation. Ambient temperature compensation may be effected by providing a bridge circuit including a second temperature sensitive device which is exposed to ambient temperature but not in contact with the surrounding medium, as disclosed in Canadian Patent No. 5~2,o40 : issued September ~, 1959. However, bridge circuits generally have a very small output which cannot be used directly to control an alarm device and : ~
complicated circuitry must be used to amplify the output of the bridge It is therefore the object of the present invention to provide an ~i 30 apparatus for sensing variations in the heat exchange properties of a medium which is relatively immune to temperature and power supply variations.
It is also an object of the present invention to provide an 103~690 apparatus for sensing variations in the heat exchange properties of a medium which exhibits a rapid and significant change in its output upon a predeter-mined change in the heat exhange properties of the medium, thus providing an adequate signal for directly operating a simple monitoring device.
The apparatus, in accordance with the invention, comprises a first and a second temperature sensitive switching devices having substantially identica~l electrical characteristics and exhibiting regenerative switching at a power level which is dependent on the temperature sensed by the switching devices. Both switching devices are~exposed to the same ambient temperature in a surrounding medium but only one of them is exposed to variations in the heat exchange properties of the surrounding medium. Means are provided for applying D.C. power to both switching devices at a level such as to maintain them below their switching point under preset conditions of the medium. A
coupling device is interconnected between the first and second switching devi-ces for rapidly inducing regenerative switching in one of the switching devices in response to unbalance of the heat dissipated by the switching devices due to variations in the heat exchange properties of the surrounding medium from the preset conditions. A monitoring device is connected to the first and second . switching devices for indicating which one of the switching devices has switched.
In one embodiment of the invention, a ~esistor is connected in series with each temperature sensitive switching device and the amplitude of the D.C. source and the value of the resistors are selected so as to maintain both switching devices below their switching point under preset conditions of the medium.
In one embodiment of the invention, the coupling device intercon-nected between the switching devices to induce regenerative switching is a transistor having its base and emitter connected respectively between the first and second switching devices. A pilot light is connected to the collector of the transistor and acts as a monitoring device to indicate variations in the heat exchange properties of the surrounding medium. In another embodiment of the invention, a triac connected in series with a pilot ; - 2 -light is used as a monitoring device. In such a case, the gate of the triac is connected to the collector of the transistor.
The invention will now be disclosed, by way of examples, with reference to preferred embodiments illustrated in the accompanying drawings in which:
Figure 1 illustrates a first embodiment of the invention;
' Figure 2 illustrates the temperature dependent current-voltage characteristics of a vanadium oxide device suitable for use as a temperature sensitive switching device; and Figure 3 illustrates a second embodiment of an apparatus in accor-dance with the invention.
Referring to Figure 1, there is shown a first embodiment of an ; apparatus for sensing variations in the heat exchange properties of a medium.
Such apparatus is for sensing air flow and, more particularly for detecting ; failure of an air flow source such as a fan. The apparatus comprises a first - temperature sensitive switching device Sl connected in series with a resistor Rl across a suitable D.C. source V~, a second temperature sensitive switching device S2 having substantially the same electrical characteristic as the first switching device Sl connected in series with a resistor R2 across the same D.C. source, a coupling device in the form of a transistor Q intercon-necting the switching devices Sl and S2 and a monitoring device in the form of a pilot light L connected to transistor Q. As illustrated in Figure 1, the base of transistor Q is connected to the connecting point of resistor R
':4 with temperature sensitive switching device Sl, the emitter of transistor Q
is connected to the connecting point of resistor R2 with temperature sensitive switching device S2, and the collector of transistor Q is connected to the D.C.
source V~ through pilot light L.
The temperature sensitive switching device must show regenerative switching at a value of voltage which is dependent on temperature. Examples 3 of suitable switching devices are the amorphous glass devices, transition metal oxide devices, and certain classes of negative temperature switching devices. In most cases, elevated temperatures correspond to a reduction in ~03~690 the voltage threshold necessary to cause switching. The expression "regene-rative switching devices" as used herein means devices exhibiting avalance behaviour due to their very high negative temperature coefficient of resis-tance. As commonly known, most thermal avalanche behaviour is due to a current crowding effect in which current crowding leads to local heating in the device which in turn leads to further current crowding. On completion of this process, most of the device current flows through a very narrow filament in the device. Because of the relatively low resistance of the conducting material, total device resistance becomes quite low. Such devices therefore behave like switches and go f~om a high resistance to a low resistance state in a very short period of time. The basic switching mechanism of the tempera-ture dependent regenerative switching devices is usually thermal and occurs in response to both Joule heating and the ambient thermal energy. In most cases, switching action is observed when the sum of all energies yields a given temperature at the switching element. In other words, whatever the source of heat, the switching device always switches at an essentially fixed temperature.
Figure 2 of the drawings shows the temperature dependent current-voltage characteristics of a vanadium dioxide device which is well suited for the present invention. It will be seen that the current-voltage characteris-tics of such temperature dependent switching devices follow curves ToJ T25, T30, etc. depending on whether the temperature sensed by the switching device is 0, 25, 30, etc.
In order to detect a change in the heat exchange properties of a medium, both temperature sensitive switching devices must be exposed to the same ambient temperature in the medium being monitored. However, one switching device onlg is exposed to variations in the heat exchange properties of the medium. In a device such as an air flow sensor, as shown on Figure 1, both switching devices may be exposed to a stream of moving air and switching device Sl placed in a heat sink so as to render it non-responsive to reduction ~0 or loss of air flow. Under predetermined ambient temperature, with the fanblowing at a preset speed, the amplitude of the D.C. source and the value of resistors Rl and R2 are set so that not enough current flows through the -~ :
. . ~
-, , . .~ .
.' ~Q36~9U . .
temperature sensitive switching devices Sl and S2 to make them switch. It is assumed that the load line determined by the parameters of the circuit is as illustrated in Figure 2 and that the operating point is VoIo. It is also assumed that the relative values of the circuit parameters are such that there is not enough voltage across the base-emitter of the transistor Q to render the transistor conductive. However, upon a substantial reduction or loss of'air flow, the temperature sensitive switching device S2 will heat up whereas the temperature sensitive switching device Sl will not warm up appreciably because it is placed in a heat sink. Consequently, as illustra-ted by the load line of Figure 2, the voltage appearing across the switching device S2 will be reduced so that the new operating point will be say VlIl.
A current will therefore start to flow from source V~, resistor Rl, base-emitter of transistor Q and temperature sensitive switching device S2. This current will further heat the temperature sensitive switching device S2 and the operating point will move to say V2I2. This will turn the transistor Q
more conductive and so on thus causing rapid regenerative switching of tempe-rature sensitive switching device S2 and stabilization thereof at a point which we assume is V.I. The full conduction of transistor Q will energize .~ 1 1 ., pilot light L to indicate substantial reduction or loss of air flow.
Although the above apparatus has been disclosed with reference to detection of air flow) it is to be understood that it may also be used for detection of a flow of liquid or even for liquid level sensing. In the later case, temperature sensitive switching devices Sl and S2 are normally placed in contact with the liquid medium when the level of the liquid in a reservoir is at a predetermined hight. However, if this level falls, the temperature sensitive switching devices will be out of the liquid and exposed to the ambient air The rate of heat removal from the switching device which is exposed to the variations in the heat exchange properties of the surrounding medium will certainly change because air does not have the same heat exchange properties as liquid and switching device S2 will heat up and finally switch as explained previously.
Figure ~ illustrates an apparatus very similar to the one illustra-':
--~036690 ted in Figure 1 and the elements corresponding to the ones of Figure 1 have been identified by tbe same reference characters. In this embodiment, the -- monitoring device consists of a triac TR connected in series with a pilot light across an A.C. source. The triac is fired by a gate circuit including -; resistor R3 which is connected across resistor R4 located in the collector circuit of transistor Q.
- ~ The above disclosed apparatus is binary in operation and is inten-ded primarily for application where simple go no-go sensing is required.
The basic advantages of such apparatus are:
1. It is inherently regenerative switching and thus exhibits rapid - switching. As soon as a small unbalance in the heat dissipated by the temperature sensitive switching devices is sensed, an avalanche type regene-rative switching action is generated so as to quickly reduce the voltage across one of the temperature sensitive switching devices and provide a posi-tive indication of the change in the heat exchange properties of the medium being monitored.
2. It is insensitive to ambient temperature and supply voltage variations. Indeed, both temperature sensitive devices are exposed to ambient temperature and power supply variations so that such variations cancelled each other. The above disclosed apparatus is also made of small switching devices which present very little interference with the air flow, have relatively low power consumption, work at low operating temperatureJ have a high sensitivity to air flow and are also very inexpensive.
Although the above invention has been disclosed we reference to preferred embodiments thereof, it is to be understood that various modifica-tions may be made to such apparatus within the scope of the claims and the spirit of the invention. For example, the coupling device may be any bidirec-tional or unidirectional conducting element and is not limited to a transistor ;as disclosed in the present embodiment. When using a bidirectional conducting element, regenerative switching may be induced in any one of the switching devices in response to unbalance in the heat dissipated by the switching devices. Consequently, using the circuit of Figure 1, an increase in fluid .
103ti690 flow could be sensed Indeed, such would cool switching device S2 and raise the potential across switching device S2. This would in turn cause a current flow from source V+ through resistor R2, the coupling resistor, and switching device Sl. Such an additional current would heat switching device Sl and cause its regenerative switching.
Any type of temperature sensitive switching devices which exhibit regeneràtive switching at a power level which is dependent on the temperature sensed may also be used. Furthermore, any type of monitoring devices can be utilized for sensing regenerative switching of any one of the switching devices
Although the above invention has been disclosed we reference to preferred embodiments thereof, it is to be understood that various modifica-tions may be made to such apparatus within the scope of the claims and the spirit of the invention. For example, the coupling device may be any bidirec-tional or unidirectional conducting element and is not limited to a transistor ;as disclosed in the present embodiment. When using a bidirectional conducting element, regenerative switching may be induced in any one of the switching devices in response to unbalance in the heat dissipated by the switching devices. Consequently, using the circuit of Figure 1, an increase in fluid .
103ti690 flow could be sensed Indeed, such would cool switching device S2 and raise the potential across switching device S2. This would in turn cause a current flow from source V+ through resistor R2, the coupling resistor, and switching device Sl. Such an additional current would heat switching device Sl and cause its regenerative switching.
Any type of temperature sensitive switching devices which exhibit regeneràtive switching at a power level which is dependent on the temperature sensed may also be used. Furthermore, any type of monitoring devices can be utilized for sensing regenerative switching of any one of the switching devices
Claims (5)
1. Apparatus for sensing variations in the heat exchange properties of a medium comprising:
a) first and second temperature sensitive switching devices having substantially identical electrical characteristics and exhibiting regenerative switching at a power level which is dependent on the temperature sensed by said switching devices, both said switching devices being exposed to the same ambient temperature in a surrounding medium but only one of them being exposed to variations in the heat exchange properties of the surrounding medium;
b) means for applying D.C. power to both said switching devices at a level such as to maintain them below their switching point under present conditions of the medium;
c) a coupling device interconnecting said first and second switching devices for rapidly inducing regenerative switching action in one of the switching devices in response to unbalance in the heat dissipated by the switching devices due to variations in the heat exchange properties of the surrounding medium from said preset conditions; and d) a monitoring device connected to said coupling device, for indicating that one of said switching devices has switched as an indication that the heat exchange properties of the medium have changed.
a) first and second temperature sensitive switching devices having substantially identical electrical characteristics and exhibiting regenerative switching at a power level which is dependent on the temperature sensed by said switching devices, both said switching devices being exposed to the same ambient temperature in a surrounding medium but only one of them being exposed to variations in the heat exchange properties of the surrounding medium;
b) means for applying D.C. power to both said switching devices at a level such as to maintain them below their switching point under present conditions of the medium;
c) a coupling device interconnecting said first and second switching devices for rapidly inducing regenerative switching action in one of the switching devices in response to unbalance in the heat dissipated by the switching devices due to variations in the heat exchange properties of the surrounding medium from said preset conditions; and d) a monitoring device connected to said coupling device, for indicating that one of said switching devices has switched as an indication that the heat exchange properties of the medium have changed.
2. Apparatus as defined in claim 1, wherein each said switching device is connected in series with a respective resistor, and wherein the amplitude of the D.C. power and the value of each resistor are selected so as to maintain the switching devices below their switching point under said preset conditions.
3. Apparatus as defined in claim 1, wherein said coupling means is a transistor having its base and emitter connected respectively to said first and second switching devices and its collector connected to the monitoring device.
4. Apparatus as defined in claim 3, wherein said monitoring device is a pilot lamp connected to the collector of said transistor.
5. Apparatus as defined in claim 3, wherein said monitoring device is a triac connected in series with a pilot light across an A.C. source, said triac having its gate connected to the collector of said transistor.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA229,140A CA1036690A (en) | 1975-06-02 | 1975-06-02 | Apparatus for sensing variations in the heat exchange properties of a medium |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA229,140A CA1036690A (en) | 1975-06-02 | 1975-06-02 | Apparatus for sensing variations in the heat exchange properties of a medium |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1036690A true CA1036690A (en) | 1978-08-15 |
Family
ID=4103324
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA229,140A Expired CA1036690A (en) | 1975-06-02 | 1975-06-02 | Apparatus for sensing variations in the heat exchange properties of a medium |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1036690A (en) |
-
1975
- 1975-06-02 CA CA229,140A patent/CA1036690A/en not_active Expired
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