EP0364298A2

EP0364298A2 - Heat sensing apparatus and method

Info

Publication number: EP0364298A2
Application number: EP89310543A
Authority: EP
Inventors: Joseph Ralph Beatty; Thomas Marion Kiec
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-10-13
Filing date: 1989-10-13
Publication date: 1990-04-18
Also published as: JPH02236428A; EP0364298A3; AU4290889A; CA2000611A1

Abstract

A temperature sensing apparatus directly measuring the increases and decreases in temperature comprising one or more conductors intermittently secured to a support structure which expands and contracts due to variation in temperature, whereby the conductor or conductors is caused to expand and contract equally the resulting variation in temperature resistance being used to trigger an electric circuit.

Description

The present invention relates generally to a temperature sensing apparatus for the detection of temperature increases and decreases.
Fire loses have been, and continue to be, a major problem. This dramatic loss due to fire has prompted corporations, insurance companies and individuals to spend large sums of money annually in an effort to detect and to prevent fires and their associates loses. Residential fires in the United States are estimated to be the single greatest monetary loss at approximately several billion U.S. dollars annually. One out of four households will be affected by fire in the cooking area. Total fire losses in the United States provide for approximately one percent of the gross national product. Property loss alone is estimated to cost approximately 4.5 billion dollars annually. There are approximately 2.3 million fires reported yearly.
In recent years, fire detection systems have become more cost effective and reliable, primarily due to the use of semiconductor technology and the listing of codes and performance specifications. However, the sensors associated with the fire detection systems presently used are based upon smoke, flame or fire-gas detection. Heat sensors are limited due to the secondary nature of what they detect, e.g., differential expansion, insulation melt point and melting of a conducted medium. Typical heat sensing devices operate only when the detector itself, not the surrounding air, reaches the preset temperature. The difference between the ambient temperature and the detector temperature can be identified as the thermal lag which results from heat being transferred from the surrounding air to the detector for bringing the detector temperature to its activated state. Two primary factors should be considered when using a heat activated sensor. When a fixed temperature or rate-of-rise detector is activated, the surrounding air is always hotter than the detector. The thermal lag or delay is proportional to the speed at which the temperature is rising in the ambient surroundings.
Two types of rate-of-rise detectors, pneumatic and thermoelectric, are in common use. The pneumatic-type detector operates on the principle that an increase in temperature causes an increase in the pressure of a confined gas. The pressure actuates a switch which in turn sends an electric signal to a control point for activating an alarm or the like. Thermoelectric detectors measure a small change in electrical current which is initiated when heat is applied to the junction of two dissimilar metals. Typically, the line-type thermoelectric detector consists of two pairs of wires which are enclosed in a sheath used to protect the wires from physical damage. One wire of each pair has a high coefficient of heat resistance. The wire with the same coefficient of heat resistance (one from each pair) are insulated against heat. The other wires are open to temperature effects in the protected space. The wires are connected to a device that measures the resistance of the wires. An increase in temperature in the protected space shows up as an unbalance in the resistance of the wires. A high enough rate of unbalancing causes the alarm to be activated.
Combined fixed-temperature and rate-of-rise devices can be activated when temperature rises at, or faster than, a preset rate. If the temperature rises slowly but continuously, the rate-of-rise device may not be activated. Then, the fixed temperature device will eventually initiate the alarm. The primary advantage of the combined detector is the additional protection provided. The fixed-temperature device responds to a slow increase in temperature that may not activate the rate-of-rise device, which resets itself, but the fixed-temperature device does not.
Generally, the sensors associated with the currently used fire detection system are typically based upon antiquated technology. Existing smoke detectors either can not function in many homes and industrial environments or are ineffective. The main operational difficulty results from false alarms. Industrial environments, improper placement and improper utilization result in ineffective, inoperable or false alarms. Thus, much prior work has been focused on the residential use of smoke detectors. Two main objections to the use of smoke detectors are indistinguishable "friendly smoke" from cooking and critical response awareness time. Smoke detectors have been responsible for the response and the warning of a fire in process. Noxious gases of combustion are sensed by particulate emission or by photoelectric differential. However, the detection is operational only as a function of a fire already in progress. No doubt these devices are useful and save lives, however, there is a great amount of property damage and loss which is without protection. Industrial environments, e.g., coal mining, waste disposal, etc., have exceptionally difficult problems with utilizing currently available sensors for detecting fires. Thus, much work has been initiated to evacuate signals received from outmoded sensors. Even though the state of the art of signal-sensing devices has increased dramatically, the same heat sensors are utilized by commercial and residential users as has been utilized for many years.
Work has been initiated to maximize the warning time for evacuation as well as to reduce property damage. The United States National Fire Protection Agency strongly recommends the use of both heat sensors and smoke detectors in a supporting, complementary role.
JP-A-53-144699 discloses a fire detecting sheet having layered patterns printed on conductive ink on a biaxially oriented plastic film. The sheet has recesses positioned in a checkered form such that, as the film is heated to soften, it shrinks making holes and thus cutting the conductive path and causing an alarm to sound. US-A-4520352 describes a heat sensor having a continuous wire secured to a sheet or sandwiched between two or more sheets of heat migrating plastic such that, as the plastic is exposed to heat, it shrinks breaking the wire or the wire is melted causing an alarm to be activated.
US-A-4408904 described a plurality of meltable segments which are spaced apart one from another along the length of a conductor and in positions separating one from the other. Each segment is identically fabricated from electrically conductive material having a preselected melting point below the melting points of the conductors. Electrical insulators that are mounted along the conductors confine molten material that flows from the respective segments, so as to position the molten material in bridging contact between the conductors at specific locations along the length. Thus, temperature profiles associated with related components are attainable by monitoring changes in resistance between terminals at opposite ends of the conductors.
US-A-4388267 also describes a temperature profile detector which utilizes a pair of elongated electrical conductors that are spaced one from the other. When one conductor melts and bridges the space between the respective conductors, a change in resistance occurs in the conductor. The change in resistance can be related to a change in temperature. Thus, a temperature profile can be created from the change in conductance of the melted conductor.
Fire detectors can be classified in two categories: (1) heat activated sensors, and (2) smoke activated sensors. Both heat activated and smoke activated sensors have been extensively developed within their own technology. Nonetheless, most of the sensors which are currently used in the marketplace have been used for twenty years or more. Advances in electronics utilizing low cost semi-conductors have made the currently used electronic detector systems more affordable and has offered more sophisticated and versatile alarm mechanisms. However, the sensor technology associated with the alarm system technology has not progressed at as fast a rate as the technology associated with the electronics.
Both categories of sensors, heat activated and smoke activated, depend on an advanced fire condition of either high heat or high smoke concentration, respectively. Also, it is a necessity in most cases that the high concentration of heat and/or smoke be in the immediate proximity of the sensor for activation. Thus, the surrounding air must be either hot or saturated with particulates which are the basis for smoke, or electrically charged as are many fine smoke particulates.
According to the present invention there is provided a temperature sensing apparatus comprising a support structure having elastic characteristics and thermal expansion characteristics sufficient for repeated expansions and contractions due to changes in temperature, and an electrical conductor operatively associated with said structure for expanding and contracting in unison with said structure whereby the expansion and contraction of said conductor causes a change in its electrical resistance whereby the temperature can be sensed by detecting its electrical resistance and a pair of leads connected to spaced locations of said electrical conductor.
Most substances increase in length and/or volume when heated. The addition of heat energy to any object, composed as it is of bound atoms and molecules, tends to increase the thermal agitation. This energy of motion is in direct competition with the energy that binds the material together to retain its original shape. Simplistically viewed, the component particles, atoms and molecules, are farther apart at a higher temperature than at a lower temperature. This "distance between particles" corresponds to an overall expansion of the object. The thermal expansion property is approximately defined in terms of the linear coefficient of thermal expansion. As would be expected, the physical properties of the expanding material tend to change with an increase in volume or length. Of special importance is the fact that the density or mass per unit volume reduces with an increase in volume. Likewise, the number of atoms per unit volume reduces as the volume increases. These physical characteristics have tended to discourage prior experimenters from utilizing materials that expand based upon thermal activity. For example, US-A-4520352 and 4388267 and JP-A-53-144369, all demonstrate the fact that as the change in length or volume increases, the density and number of atoms per unit volume are sufficiently reduced to cause the heated material to separate. Specifically, reference is made to the requirement for the "breaking of the foil" in US-A-4520352 and to the requirement that "one conductor will melt and bridge the space between the two conductors" in US-A-4388267. In JP-A-53-144369, it is clearly stated that as the film is heated it makes holes thus cutting the conductive path and causing a buzzer to sound.
If the thermal expansion is sufficiently interconnected with the properties of electrical conductivity, a unique and innovative fire detection sensor is possible. Elasticity is generally defined as the experimental observation that the force associated with the elasticity tends to restore the original shape and size of the elastic materials. The elastic force tending to restore the original size and shape to a material is proportional to the displacement of the elastic material based upon Hooke's law. Obviously, such behaviour is limited to a range of forces and displacements that do not permanently deform the elastic material. Hooke's law is typically defined as the relationship between stress, the force per unit area, and strain, the fraction of elongation. The stress is proportional to the strain based upon Young's modulus of elasticity, which depends only on the material in question and not on its shape.
Of primary concern in the present invention is to cooperate the well proven physical phenomena of thermal expansion with that of elasticity to provide a heat sensing apparatus which can readily detect a temperature profile including increases and decreases.
In an effort more accurately to measure temperature, more accurately to measure changes in the rate of temperature rise and to reduce the alarm response time, the present invention utilizes reactions of heat dissipation, coefficient of thermal conductivity, dielectric strength, specific resistance and thermoplastic polymer chemistry.
Thermoplastic polymers are long chain polymers that become soft when heated. They are comprised of linear branch chained polymer with little or not cross linking. when cast into thin films, end products are light in weight, have excellent chemical resistance to corrosion, and are durable. Due to their chemical structure, thermoplastic polymers also have good properties of electrical resistance, dielectric strength and specific thermal resistance. The chemical properties of thermoplastic polymers can be formulated to respond to changes in heat at very small changes in temperature or rates of change of temperature. Such responsiveness or reaction has chemical and physical influence on the film support structure.
Resistance opposes the flow of electrons. The amount of opposition to a current flow a material has depends on the amount of available free electrons the material contains and what type of molecular obstacles the electrons encounter chemically as they attempt to travel through the substance under the influence of a potential difference. Electrons collide with atoms and reactive chemical sights in the conductive and nonconductive materials. As the thermoplastic resins react chemically and physically to heat, effects on the dielectric of the given material will be present.
In the present invention a conductor is applied to a support structure in such a manner that the conductor adheres to the support structure and responds to physical changes in the support structure induced by temperature. A potential applied across the conductor has a specific profile of measurable resistance that changes at a constant rate as temperature increases or decreases. Electronics are available to measure specific resistance points or the rate of change profiles.
In order that the present invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawings in which:-

Fig. 1 is a cut-away view of one embodiment of a single conductor sensor associated with a temperature sensing apparatus of the present invention;
Fig. 2 is a schematic view of one embodiment of the heat sensing apparatus of the present invention;
Fig. 3 is a perspective, cut-away view of a preferred embodiment of a multiple conductor sensor associated with the heat sensing apparatus of the present invention;
Fig. 4 is a graph of resistance versus temperature for a heat sensing apparatus encompassed by the present invention;
Fig. 5 is a graph of resistance versus time and temperature for a heat sensor as associated with a heat sensing apparatus of the present invention, and;
Fig. 6 is a schematic view of a second embodiment of heat sensing apparatus according to the invention.

As shown in Fig. 1, a single conductor sensor 100 associated with the heat sensing apparatus of the present invention. The single conductor sensor 100 has as its primary components a first support structure 110, an optional support structure 130, and a conductor 120. If the second support structure 130 is provided it may be laminated to extend over the first support structure 110 and over the conductor 120. The conductor 120 is fixedly and intimately secured at least to the first support structure 110. The intimate securing provides that the conductor 120 acquires the expansion and contraction characteristic of the first support structure 110. The conductor 120 is illustrated "sandwiched" between the first support structure 110 and the optional support structure 130. The conductor 120 has associated therewith leads 124, each affixed to the conductor 120 with a contact 122 at spaced locations.
It should be realized that the process of securing the conductor 120 to the support structure 110 can be accomplished using different techniques depending on the materials used for the support structure and the conductor as well as the use and result to be achieved. For example, the conductor 120 can be intimately secured to the support structure 110 by metallizing, laminating, pressure sensitive adhesion, thermal curing, thermal plastic lamination, ultraviolet curing, printing and electrodeposition techniques.
In one embodiment, the first support structure 110 and the optional support structure 130 are each a plastic substrate, for example, make of polyethylene, polypropylene, polyester, nylon, polycarbonate, blended plastics and various fire retardant plastics. The first support structure 110 and the optional support structure 130 can vary in thickness from, for example, approximately 0.5 mils to approximately 15 mils. Also, the first support structure 110 and the optional support structure 130 may differ in thickness or composition depending on the result desired.
The conductor 120 which is affixed between the first support structure 110 and the optional support structure 130 can comprise various conductive media. For example, the conductor 120 has been found to adequately function with vacuum metallized aluminum, printed conductive ink of silver, nickel, copper or gold, a conductive adhesive, a thin gold or aluminum foil, or most commercially available conductive coatings such as lead, nickel, copper, gold and conductive pigments.
Of primary importance in the present invention is that the conductor 120 "take on" the physical properties associated with the first support structure 110, and possibly properties of the optional support structure 130. Thus as the first and the optional support structures 110 and 130 expand and contract with increasing and decreasing temperature, the conductor 120 expands and contracts while maintaining its conductive properties. However, as the conductor 120 expands and contracts the resistivity/conductivity associated with the conductor 120 changes. The changes in resistivity/conductivity can be readily monitored by an electronic monitoring device, i.e., an electric circuit, which in turn can be calibrated to very accurately monitor the temperature associated with the thermal expansion of the support structures 110 and 130.
Fig. 2 is a circuit diagram of an alarm 814 incorporating a sensor 700 according to the invention. The alarm is powered by a battery 816 and comprises a voltage comparator 804 controlling the conduction of an NPW transistor 810 which in turn controls an alarm buzzer 812.
The comparator 804 compares the voltages at the centre taps a, b of two voltage divides comprising, respectively, resistor 806 and the conductor of sensor 700, and variable resistor 802 and fixed resistor 808. When the voltage at a exceeds that at b, the output of the comparator 804 changes from low to high, turning on the driver transistor 810 and hence sounding the buzzer 812. The threshold temperature at which the alarm trips can be set by adjusting variable resistor 802, which varies the voltage at b.
It will be appreciated that although the above described alarm operates as an overtemperature alarm, it could equally well be used as an undertemperature alarm (e.g. for monitoring refrigeration apparatus) simply by reversing the connections of the inverting and non-inverting inputs of comparator 804.
Fig. 3 is perspective cutaway view of the multiple conductor sensor 400 associated with the heat sensing apparatus 10 as illustrated in Fig. 6 to be described later. The sensor 400 has associated therewith first, second and third conductors 410, 420, 430, laminated together with the interposition of insulating support structure layers (not shown) and connected electrically in series. The first conductor 410 is connected adjacent its right end to a first lead 411 by a first contact 412, and at its left end to a second lead 413 by a contact (not shown). The second conductor 420 is connected adjacent its right end to a lead 424 by a contact 422 and adjacent its left end to lead 423 by a contact (now shown). Similarly, third conductor 430 is connected to spaced leads 424, 442 by contacts 422, 440 respectively.
The multiple conductor sensor 400, can thus act as a resistance ladder with the lead 442 connected to one side of the potential to be divided and the lead 411 connected to the other side. Two intermediate potentials can thus be detected at leads 413, 423 and 424, 434 respectively, these being determined by the resistance of the conductors 430, 420, 410 respectively. In this way, temperature sensing apparatus of Fig. 3 can provide a continuous measurement of increases and decreases in temperature and sense different pre-set temperatures determined by the conductors 430, 420, 410, these temperatures being of increasing value. The sensor is a flexible, positional, temperature sensor that acts as a transducer for an electronic monitoring device and reflects changes in resistivity and conductivity which occur based upon the expansion or contraction of the support structure of the sensor as the temperature increases or the temperature decreases in the vicinity of the sensor.
Fig. 7 illustrates an example of the heat sensing apparatus of the invention which can be used to evaluate differing temperature characteristics.
The circuit illustrated in Fig. 6 includes three sub-circuits A, B and C consisting of resistors 212, 222, 232 and transistors 214, 224, 234, resistors 212, 222 being variable resistors. A battery 240 is connected via a switch 250 and a variable resistor 260 across the leads 411, 422 of the sensor 400. The bases of each of the transistors 214, 224, 234 are connected between the switch 250 and the variable resistor 260. The collectors of transistors 214, 224 are connected via light emitting diodes 216, 226 to the other side of the battery and the collector of transistor 234 is connected to the three resistors 212, 222, 232. Resistor 212 is connected to lead 411 of the first conductor 410 of the sensor, resistor 222 is connected to leads 413, 423 of the first and second conductors 410, 420 respectively and resistor 232 is connected to leads 424, 434 of conductors 420, 430 respectively. In this way the sensor 400 acts as a voltage ladder and when the temperature sensor reaches a first value determined by conductor 410 transistor 214 will transmit and LED 216 will illuminate. When a second temperature sensed by conductor 420 is reached, transistor 224 will conduct and LED 226 will illuminate and finally when the temperature is reached which is sensed by conductor 430, transistor 234 will conduct and alarm 300 will sound.
One subcircuit of a modified control circuit could be a simple wire connector adapted for use without utilizing the change in resistance characteristics. The wire connector subcircuit can be used as a monitoring device against which another subcircuit can be compared or evaluated.
Fig. 4 is a graph of resistance versus temperature for a heat sensor as illustrated in Fig. 3. The abscissa reflects increasing temperatures and the ordinate reflects increasing resistance. It is readily observable that the resistance increases at a very proportional rate with respect to temperature. The directly proportional relationship between resistance and temperature for sensors which practice the present invention is readily adapted for use in an alarm system.
Fig. 5 is a graph of resistance versus time and temperature for a single heat sensor as illustrated in Fig. 4. In Fig. 5, the ordinate indicates increasing resistance in ohms with displacement from the origin. The abscissa, with displacement form the origin, reflects increasing time as well as increasing and decreasing temperature with respect to the time throughout an experiment. Particularly of interest for the heat sensing apparatus 10 of the present invention is the trimodal curve 600. The trimodal curve 600 comprises the first peak 610, the second peak 620 and the third peak 630. Of particular interest is the fact that each peak 610, 620 and 630 is directly associated with a heating period as reflected by increasing time. The temperature increased to 193°C at the beginning or base of each peak. The increase in temperature was provided in a step temperature function. Thus, the temperature was increased immediately to 193°C whereby the first peak began to rise. After a period of time, the temperature was reduced to ambient room temperature and the first peak 610 immediately dropped. After a waiting period, the temperature was again increased in the vicinity of the heat sensing apparatus. The temperature increased immediately to 193°C resulting in the increase of the second peak 620. After a period of time, the temperature was reduced to ambient room temperature and the second peak 620 immediately dropped. Again, there was a waiting period and a step function of heat was applied with the same results. Also of interest in Fig. 5 is the fact that the heat was applied to the heat sensing apparatus for consecutively shorter periods of time. Thus, as would be expected, the peaks are successfully reduced at their full width at half maximum value.
By varying the chemistry, thickness and combination of the plastic support structure, the coefficient of thermal expansion can be adjusted to effect the conductor and to cause a change in the resistance/conductivity. As the support structure expands, the conductor expands and the resistance/conductivity changes. In a preferred embodiment, the support structure and the conductor expand equal amounts. The changes in resistance/conductivity can be measured and correlated to a temperature increase or a temperature decrease. The support structure material and the conductor material are selected based upon the specific situation in which the heat sensing apparatus is to be used. Characteristics which should be considered when selecting conductor materials or support structure materials are the linear coefficient of thermal expansion of the materials, the elasticity of the materials, Young's modulus of the materials, the bulk modulus of the materials, Poisson's ratio for the materials and the compressibility of the materials. Additionally, other physical parameters and characteristics of the materials may be relevant based upon the use to which the invention is applied and the result desired.
In an alternate embodiment, referring to Fig. 1, the first support structure and the second support structure may differ in material to alter the overall coefficient of thermal expansion of the combination and ultimately the resistance/conductivity profile of the conductor 120. By using different material for the first support structure than used in the second support structure, an expansion bias can be created. The expansion bias can be used to alter the expansion of constriction characteristics of the conductor 120. Such physical changes in the support structures 110 and 130 can significantly alter the electrical properties of the sensor 100 and exhibit a change in the resistance/conductivity profiles as illustrated in Figs. 4 and 5.
A thermoplastic film is used by casting the thermoplastic into a film having a thickness 0.25 to 25.0 mils. A metal conductor is physically associated with the thermoplastic support structure. The metal conductor can be deposited by vacuum deposition, ion sputtering, chemical vapour plating and glow discharge as well as other similar techniques.
In a vacuum deposited conductor, a thin single or multi-layered coating is applied to the support structure by deposition of the coating metal from its vapour phase. Most metals and even some conductive non-metals, e.g., silicon oxide, can be vapour deposited. Vacuum evaporated films, or vacuum metalized films, of aluminum, silver and gold are most common. Such vacuum deposited films are applied by vaporizing the metal in a high vacuum and then allowing the vaporized metal to condense on the film to be coated. Vacuum-metalized films are extremely thin ranging from 0.002 to 0.3 mils.
In additional to vacuum evaporation, vapour deposited films can be produced by ions sputtering, chemical vapour plating and a glow discharge process. In ion sputtering, a high voltage dispelled to a target of the coating material in an ionized gas media causes ions (atoms) to be dislodged and then to condense as a sputtered coating on the base or support structure. In chemical-vapour plating, a film is deposited when a metal bearing gas thermally decomposes on contact with a heated surface of the base or support structure. In the glow-discharge process, applicable only to polymer films, a gas discharge deposits and polymerizes the plastic film on the base or support material. Also, conductive inks can be used. Conductive inks contain a conductive pigment of silver, indium, tin, nickel, iron and the like which provide electrical characteristics.
Temperature affects the resistance of a conductor. As the temperature of a material increases, the atoms in the material increase their activity which causes the flow of electrons to undergo more collisions and hence encourage and create more obstacles to the flow of electrons.
For most materials, the hotter the material, the more resistance the material will have if used as a conductor. As the temperature increases, the atoms making up these materials vibrate more and collide with electrons as they try to travel through the material. With such materials, the resistance increases as the temperature increases. Due to the thickness of the support structure, heat transfer is almost immediate. The thermoplastic support structure begins to react, transferring heat energy through its length in the plastic. This phenomena is called heat dissipation. The metal coating itself reacts with the thermoplastic responding to the heat transfer in changing electrical resistance. Due to the nature of the metal coating, the material will remain in ultimate contact with the support film and continue to transfer electrons until a separation of the film by melting is initiated. The support film cannot respond so quickly as to interfere with the conductivity of the metal. However, at the molecular interface, there is a transfer of electrons and the dielectric properties of both the metal and the thermoplastic support film must be considered. Certain plastics have been compounded so changes in temperature hardly effect their physical properties or their electrical properties. These materials are said to have a zero or near zero temperature coefficient.

Claims

1. A temperature sensing apparatus comprising a support structure having elastic characteristics and thermal expansion characteristics sufficient for repeated expansions and contractions due to changes in temperature, and an electrical conductor operatively associated with said structure for expanding and contracting in unison with said structure whereby the expansion and contraction of said conductor causes a change in its electrical resistance whereby the temperature can be sensed by detecting its electrical resistance and a pair of leads connected to spaced locations of said electrical conductor.

2. Apparatus according to claim 1, wherein said support structure comprises a plastics member.

3. Apparatus according to claim 2, wherein said plastics member comprises polyethylene, polypropylene, polyester, nylon, polycarbonate, blended plastics or a fire retardant plastics

4. Apparatus according to any preceding claim, wherein said conductor comprises a metallic medium.

5. Apparatus according to claim 4, wherein said metallic medium comprises aluminum, silver, gold, lead, nickel, copper or a conductive pigment.

6. Apparatus according to any preceding claim, wherein said conductor is fixedly secured to said support structure by metallization, lamination, pressure sensitization, thermal curing, thermal plastic lamination, ultraviolet curing, a printing technique or electrodeposition.

7. Apparatus according to any one of claims 1 to 5, wherein said conductor is fixedly secured to said support structure by an adhesive.

8. Apparatus according to any preceding claim wherein the support includes a plurality of electrical conductors.

9. Apparatus according to any preceding claim and further comprising means for monitoring the electrical resistance of the or each conductor.

10. An alarm system comprising a temperature sensing apparatus according to any one preceding claim and an alarm device connected to said leads to be operative when the temperature as sensed by the apparatus passes through a threshold valve.