WO1992014128A1

WO1992014128A1 - Temperature sensor

Info

Publication number: WO1992014128A1
Application number: PCT/GB1992/000205
Authority: WO
Inventors: Laurence Welsh; George Gansbuehler; Narinder Kehal; Richard Copley; Kevin Cooke; Mark Peapell
Original assignee: Raychem Limited
Priority date: 1991-02-04
Filing date: 1992-02-04
Publication date: 1992-08-20
Also published as: GB9102385D0

Abstract

A sensor for detecting a change in temperature comprises an element at least part of which expands in response to a change in temperature causing a change primarily in the surface electrical resistance of the element. In a preferred embodiment a component of the element expands and/or forms in response to the change in temperature, bursting through the surface of the element to form a conductive network or bridge on the surface of the element. Preferably, the surface electrical resistance decreases in response to a temperature increase. The expanded component that has burst through the surface of the element may form a conductive bridge after the temperature change between first and second conductive members that are wrapped around the element.

Description

DESCRIPTION

TEMPERATURE SENSOR

This invention relates to devices and methods for detecting changes in temperature, and particularly to devices and methods for detecting increases in temperature. Preferred devices according to the invention are suitable for use as fire sensors.

A number of methods and devices have been proposed for detecting changes in variables, including changes in temperatures. For example, EP-A- 0133748 (MP0869COM EPC) describes methods and devices for monitoring the occurrence of, and specifically for locating the position along an elongate path, of an "event", such as a leak of water, insufficient or excessive pressure, or too high or too low a temperature. In the simplest aspect according to EP-A-0133748 (MP0869COM EPC), upon occurrence of the event at least one electrical connection is made between a source member and a locating member of known impedance characteristics, the connection or connections being effective at a first point at which the event takes place (or whose location is defined by some other characteristic of the event). A current of known size is then driven through the electrical connection(s) and down the locating member to a second point whose location is known. The voltage drop between the first and second points is then measured and the location of the first point can be determined.

The entire disclosure of EP-A-0133748 (MP0869COM EPC) is incorporated herein by reference.

Also, WO86/07483 (MP1072 EPC) describes another elongate sensor, which incorporates a swelling member which, in the preferred embodiment, swells in the presence of an organic liquid. Preferably the swelling member is itself conductive, or acts as a conductive member, to form a conductive bridge between two elongate conductive wires, or sensor wires, in the presence of the organic liquid. GB-A-1445206 describes a device for protecting an electric motor against high temperature damage. The device comprises a thermally sensitive elastomeric component which expands in response to an increase in temperature, and which contains a plurality of electrically conductive particles. The particles render the device conductive at normal temperatures but non- conductive at abnormally high temperatures

We have discovered that a new sensor can be made by providing an element at least part of which expands in response to a change (usually an increase) in temperature, whereby this expansion causes a change (usually a decrease) primarily in the surface resistance of the element.

A first aspect of the invention provides a sensor suitable for detecting a change in temperature comprising

(i) a first member that is electrically conductive at least after the change in temperature, and

(ii) an element, having a surface, wherein

(a) in response to a change in temperature, at least part of the element expands causing a change primarily in the surface resistance (as hereinbefore defined) of the said element, and

(b) at least after expansion, the said surface of the element is in electrical contact with the first member.

As used herein, surface resistance of the element means surface electrical resistance, and is defined by measurements made on 1.5 mm thick, 2.5 cm by 4.5 cm strips, cut from 9cm by 9cm plaques. Surface resistance is measured using two concave head, spring loaded test probes supplied by R.S. Components Ltd (Stock No. 434-778). The probes are rigidly held on a support and positioned on the plaque such that the centre to centre spacing of the probes is 0.9 cm. For high resistances (20 x 10⁶ ohms or higher) resistance measurements are made using a solid state electrometer (eg Keithley Instruments Model 602). For lower resistances (less than 20 x 10⁶ ohms) a multimeter is used eg a Keithley 175 Autoranging multimeter. Measurements are made in a Faraday cage, and to account for any surface variations, several, preferably 5 or more, measurements are made and an average surface resistance calculated.

Preferably the element comprises a component which expands in response to the change in temperature (or forms such a component in response to the change in temperature). The expansion (or formation) of the component preferably causes that component to burst through the surface of the element, thereby forming an electrically conductive network on the element's surface, ie resulting in the said change (decrease) primarily in the surface resistance.

In one embodiment, for example, the element comprises a composition comprising a polymer matrix and a conductive filler, and the conductive filler preferably expands in response to a change in temperature. This will usually cause some expansion of the composition and the element as a whole, and typically the expansion of the filler causes it to burst through onto the surface of the element. At the surface, it is unrestrained by surrounding polymer, and therefore may expand along the surface of the element. Thus the surface resistance of the element as measured as hereinbefore defined will be lower than the bulk resistance of the element, as measured for example by inserting probes into the body of the element.

In a second embodiment where a component of the element bursts through the surface of the element, the element comprises (at least after the change in temperature) a metal or alloy, and the surface of the element is coated with an insulating layer. In response to the change in temperature the metal or alloy forms and/or melts and bursts through the insulting layer to form an electrically conductive network on the element's surface.

The invention also envisages that not only one component, but the whole of the composition of the element may expand in response to a change in temperature. As a result in addition to the primary change in surface resistance of the element, there may also be a smaller change in its bulk resistance.

At least part of the element expands in response to a change, preferably an increase in temperature, but it is also envisaged that the part of the element may expand in response to a decrease in temperature. Preferably the change in ohm of the surface resistance of the element (as measured as hereinbefore defined) is at least an order of magnitude, preferably at least two orders of magnitude. Preferably the change is a decrease, but it is also envisaged that the element may increase in surface resistance on expansion.

Preferably the resistance change occurs when the temperature is changed by 10° C to 50° C in the range 0° C to 400° C.

At least after expansion of the said at least part of the element, the said surface of the element is in electrical contact with the first member. Thus, for example for an element which decreases in surface resistance substantially on expansion, the sensor may be arranged such that on expansion of said at least part of the element, caused by an increase in temperature, an electrical current can flow between the first member and the said surface of the element, since at least after expansion the element is, itself, capable of conducting an electrical current. The sensor can, for example, act as a switch, changing from an "off state" in which no electrical current is passed, to an "on state" in which electrical current is passed, when the temperature rises. The sensor may be arranged eg to activate an alarm in the "on state". Similarly if the said surface of the element increases in surface resistance on expansion, and is in electrical contact with the first member both before and after expansion it may change from a state in which it does conduct electricity to a state in which it does not.

At least the said surface of the element may be directly connected into an electrical circuit, or it may, at least after expansion be in electrical contact with a second member which like the first member is conductive at least after the change in temperature. In one embodiment the sensor comprises a second member which is similar to, but spaced from the first member (so that it is not in direct electrical contact with it), and the said surface of the element, at least after expansion, is in electrical contact with both the first and the second members. In this embodiment the said surface of the element provides a bridge between the first and second members. Preferably this bridge changes from a non- electrically-conductive bridge to an electrically conductive bridge, or vice versa, on expansion of the filler or composition or both.

The first and, if present also the second, member(s) may be any suitable shape and design. For example they may be discrete members eg suitable for use as a point switch, or they may be elongate members, eg elongate wires, suitable for use as elongate sensors, for example as fire detectors in long line structures, such as tunnels or in underground train systems.

When first and second members are present, and they are both elongate, they may be arranged in one of the configurations described in EP-A-0133748 (MP0869COM EPC) referred to above. In this configuration the first and second members according to this invention could provide the so called source member and locating member of EP-A-0133748, and a change in temperature causing the expansion of part of the element, could constitute an event connecting the source and locating members. One or both of the first and second members in this case preferably comprises a conductive core, eg. a copper wire coated with a conductive polymeric jacket. These first and second members may be helically wrapped around an elongate core so that they are separated from each other, until the change in temperature causes the expansion of part of the element to bridge the two members.

In the above embodiments, and in other embodiments the first (and if present, the second) member(s) are electrically conductive at normal and at elevated temperatures. In other embodiments the first member may be similar to or identical to the said element of the invention. Thus it may only show a change in surface resistance, eg. become electrically conductive, after the change (eg increase) in temperature. Thus in another embodiment the first member and element may each comprise elongate members extending parallel to each other or twisted together the surface of each of which becomes electrically conductive only after the change in temperature. In one embodiment according to the invention the element comprises a polymer matrix containing intercalated graphite as a conductive filler.

Intercalated compounds are known. They are produced by the insertion of an atomic or molecular species into a host lattice Many common solid materials possess the property of acting as a host lattice for such insertion. A common material capable of intercalation is graphite. The intercalation process for graphite involves a chemical treatment to insert water or other molecules between the planes which are present in a graphite structure. On heating the interplanar water or other molecules vaporise, forcing the graphite planes apart. It is believed that this causes the formation of so-called graphite "worms". The expansion results in a considerable increase in volume, and a consequent decrease in density, and this property has led to the widespread use of intercalated graphite as a fire retardant. For example, intercalated graphite may be introduced into polyurethane foam to increase its fire retardancy. The expanded intercalated graphite, which is also referred to as "exfoliated graphite", is also known for use in compressed foil form as a sealing product. A number of these applications are referred to in "Exfoliated Graphite" by J. Lancaster, Engineering, May 1989.

We have discovered that the expansion properties of intercalated graphite can usefully be employed in a temperature sensor, eg a fire sensor, by forming an element by incorporating an appropriate amount of graphite in a suitable polymer matrix so that when the graphite expands on exfoliation, there is a significant decrease primarily in the surface resistance of the element.

The exfoliation of the graphite may also cause a consequential expansion of the overall polymer/graphite composition. However, preferably the main effect is the exfoliated graphite bursting through the surface of the polymer to provide a conductive network on the surface of the element, and thereby achieving a substantial decrease in the surface resistance.

Exfoliation changes the graphite flake structure into the "worm-like" structure referred to above. It is this worm-like graphite which bursts through the surface of the polymer matrix to provide the conductive network on the surface of the composition. As an example a 1mm diameter graphite flake can, on heating, form a worm-like material over 0.5cm long.

In a preferred sensor according to the invention, incorporating exfoliated graphite, the element has a surface resistance, as measured as hereinbefore defined between two probes held 0.9 cm apart, greater than 10¹⁰ ohm, which decreases to at most 10⁸ ohm, preferably 10⁶ ohm when the graphite is in its exfoliated state, i.e. the surface (and the bulk) composition is effectively insulating in the unexpanded state and at least the surface is effectively conducting when the graphite is in its exfoliated state.

The exfoliation process is a continuous process, not a step-like process, the exfoliation and expansion increasing as the temperature increases. Exfoliation usually starts to occur at a certain temperature, the exact value of which depends on the nature of the graphite. As an example, for Callotek 531/92, supplied by Technik (UK) Ltd, (Callotek is a trademark), exfoliation typically starts at about 200°C, but continues at a rate which decreases with increasing temperature so that at about 450° C most of the expansion has occurred. The temperature at which exfoliation begins to occur will be referred to herein as the "exfoliation-start" temperature. Preferably the composition is arranged such that a change in surface resistance (as hereinbefore defined) of at least 10⁷ preferably at least 10⁴ ohm is achieved when the temperature is increased from room temperature to the exfoliation-start temperature.

The change in surface resistance of the element incorporating exfoliated graphite depends inter alia on the amount of graphite added, and on the constraints on the expansion of the surrounding polymer matrix.

Considering first the amount of graphite to be added. If too much graphite is included in the polymer matrix it will form a continuous conductive network even at room temperature, and therefore exfoliation will not result in a significant change in surface or bulk resistance. On the other hand, it too little graphite is included in the polymer matrix, a continuous conductive path will not be produced, either at the surface or in the bulk, when the temperature changes, even though the individual graphite flakes may expand. Preferably the composition has a high bulk and surface resistance initially, and at least a surface resistance, and optionally also a bulk resistance which decreases significantly on exfoliation. We have found that to achieve this graphite concentrations of less than 50% by weight are preferred, especially 5-40% by weight, more especially 10-30% by weight. All weight percentages are based on the combined weight of the polymer and the graphite. However, we have also found that the most rapid response to a temperature change is achieved by increasing the amount of graphite. We have found that for acceptable response times the amount of graphite is preferably at least 5 wt%, more especially at least 10 wt%. Where particularly rapid response is desired it may also be preferable to have graphite loadings of at least 20 wt%, preferably at least 30 wt%, more preferably at least 40 wt% and even at least 50 or 60 wt%.

At loadings greater than 50 wt% especially greater than 60 wt%, the surface resistance (as hereinbefore defined) before the temperature change, eg at room temperature is low, eg for 60 wt% of graphite it is of the order of 10² ohm, and there is a risk of conduction of electrical current even before the temperature change, caused by fortuitous connection between the intercalated graphite flakes in localised areas.

For all loadings of graphite, where the surface of the composition is uncovered there is a risk of water of other ionic conductors or extraneous conductive parts contacting the surface and providing a conductive path between adjacent spaced graphite particles, ie a short circuit. Therefore in a preferred embodiment a thin layer of an insulating film is provided between the composition and the first conductive member, and if a second conductive member is present, afso between the second conductive member and the composition. This thin film is sufficient to insulate the composition from the conductive member before the temperature change, but is disrupted when the temperature changes and the graphite exfoliates, allowing electrical contact between the composition and the conductive member(s). Thus the composition is preferably arranged to allow the graphite to "burst through" the insulating film on exfoliation. The intercalated graphite is desirably incorporated into the polymer by a process which does not prematurely cause expansion or exfoliation of the graphite. Therefore the graphite is desirably incorporated into the matrix polymer by a process which is below, preferably well below the exfoliation-start temperature (as hereinbefore defined). For example the graphite can be incorporated into the polymer matrix via a non heat process eg solution casting, latex binders, liquid prepolymers. If a melt processing technique is to be used this provides a first criteria for polymer selection because the polymer must be melt processable below the exfoliation-start temperature. For this reason, if a melt processing technique is to be used the polymer preferably has a melting point (as measured by Differential Scanning Calorimetry) less than the exfoliation-start temperature, especially at least 10°C, more preferably at least 30°C below the exfoliation-start temperature of the graphite. For certain intercalated graphites incorporated in a matrix polymer, the melting point of the polymer is preferably less than 200°C.

Also where melt processing is to be used a polymer with a low melt viscosity is desirable, so that the graphite can be incorporated with minimal shear heat generated. Also, whatever the processing route, (non-heat or melt- process) the viscosity of the polymer is preferably low enough that at the expansion temperature of the graphite, the graphite particles are free to expand. If this is not the case the polymer matrix will constrain the particles and reduce the resistivity change. We have found by experiment that preferably the polymer matrix has a viscosity defined by a complex Youngs modulus E^* at the exfoliation-start temperature of less than 10⁸ Pa, especially less than 10⁷ Pa, when measured by subjecting a beam of material to a sinusoidal force, applied at its mid point, at a frequency of 1 Hz. A particularly preferred polymer for use as the matrix polymer is a high melt flow index (ie. a low melt viscosity) ethylene-vinyl acetate copolymer, eg Elvax 4310 from Dupont (Elvax and Dupont are trademarks).

Where a material is used which has a low melting point for easy melt- processability, there is a possibility that when the temperature increases (eg, in a fire) in addition to the graphite exfoliating, the polymer may drip. Therefore, preferably, in such instances, the polymer is cross-linked to an appropriate cross-link density. Depending on the polymer, and the presence or absence or prorads or antirads, the amount of radiation to achieve the desired cross-link density will vary. As an example we have found that electron beam irradiation of Elvax 4310 to a dose of 5 MRads produces a preferred cross-link density. We have also found that this level of cross-linking does not inhibit expansion of the graphite.

Exfoliation of graphite is usually achieved by the inclusion of water molecules in the graphite structure. However other molecules could be used. The vaporisation temperature of the molecules and hence the exfoliation temperature of the graphite will depend on the type of molecule used. Therefore the included molecule can be appropriately selected to tailor the temperature sensed.

Where an element comprising exfoliated graphite in a matrix polymer used according to the invention, the polymer preferably comprises an ethylene vinyl acetate copolymer.

in one embodiment using exfoliated graphite according to the present invention a tubular sleeve shaped element of a composition comprising a polymer matrix incorporating intercalated graphite is melt processed and extruded. Then two sensor wires, providing the first and second members of the invention are helically wrapped around the tubular sleeve so that they are separated from each other on the surface of the sleeve. Preferably they are held in place by an insulating braid. An optional outer insulating layer may also be included to prevent short-circuiting by extraneous conductive parts or fluids. At room temperature no electrical current can flow between the sensor wires. On heating at any point along the length of the sleeve the graphite exfoliates, expanding the composition at that point and the graphite bursting through the surface to form conductive networks thereby decreasing the surface resistance to an extent such that electrical current can pass between the sensor wires at that point. A third reference wire may be incorporated within the sleeve. This may be connected to the sensor wires, in the manner described in EP-A- 0133748.(MP0869COM EPC) so that the position of the point of current passage between the wires, ie the point of temperature change, can be located. Other wire configurations, including a four wire configuration with a fourth reference wire inside the sleeve (for interrogation of the circuit to check continuity) may also be used. Four wire configurations are also described in EP-A-0133748.

The sensor wires forming the first and second members of the invention preferably comprise "a metal core and an elongate jacket which electrically surrounds the core and which is composed of a conductive polymer. The term "electrically surrounds" is used herein to mean that all electrical paths to the core (intermediate the ends thereof) pass through the jacket. Normally the conductive polymer will completely surround the core, being applied for example by a melt-extrusion process; however it is also possible to make use of a jacket which has alternate insulating sections and conductive sections.

The term "conductive polymer" is used herein to denote a composition which comprises a polymer component (e.g. a thermoplastic or an elastomer or a mixture of two or more such polymers) and, dispersed in the polymeric component, a paniculate conductive filler (e.g. carbon black, graphite, a metal powder or two or more of these). Conductive polymers are well known and are described, together with a variety of uses for them, in for example U.S. Patents Nos.2,952,761 , 2,978,665, 3,243,753, 3,351 ,882, 3,571 ,777, 3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861 ,029, 4,017,715, 4,072,848, 4,117,312, 4,177,446, 4,188,276, 4,237,441 ,4,242,573, 4,246,468, 4,250,400, 4,255,698, 4,271 ,350, 4,272,471 , 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,315,237, 4,317,027, 4,318,881 , 4,330,704, 4,361 ,799, 4,398,084, 4,459,473 and ; J. Applied Polymer Science ISL 813-815 (1975), Klason and Kubat; Polymer Engineering and Science Ifi, 649-653 (1978), Narkis et al; German OLS 2,634,999; 2,755,077; 2,746,602; 2,755,076; 2,821 ,799; European application No. 38,718; 38,715, 38,718, 38,713, 38,714, 38,716, 63,440, 68,688, 67,679, 74,281 , 92,406; and UK Application No. 2,076,106 A.

In another embodiment according to the invention embodying the principle of the invention, but not involving the use of exfoliated graphite, the element of the invention comprises a metal or alloy, at least after the change in temperature, and the element is coated with an insulating layer. In response to a change in temperature the metal may melt and/or an alloy may form, the molten material bursting through the insulating layer to make electrical contact with the first member. This embodiment has been described briefly above.

In a preferred form according to the above embodiment the element comprises a first metal wire which is coated with a second metal layer, for example copper wire coated with a layer of aluminium eg vacuum deposited aluminium. In response to an increase in temperature a eutectic alloy forms between the two metal materials eg between the copper and the aluminium. This molten eutectic composition bursts through the insulating layer on the element. The insulating layer on the element may be, for example, a ceramic layer, such as a silica layer or an alumina layer. We have found that the formation of a eutectic in this way causes a spontaneous and sudden burst- through of the insulating layer at a certain elevated temperature. By appropriate choice of metal components, other eutectic alloys can be formed at different elevated temperatures. Thus the shaφ triggering temperature, ie shaφ sensing temperature, achieved by this embodiment can be adjusted by appropriate choice of alloy components.

The element according to this invention is arranged to be in contact with another member (the first member) so that after burst through of the metal/alloy an electrical contact is made. The first member may be conductive at normal and elevated temperatures. However as a preferred embodiment the first member is similar to, or identical to the said element, ie it also comprises coaxial metal wires, wherein the metals form a eutectic and burst through surrounding insulation at elevated temperatures. In a preferred embodiment two or more similar, coaxial, dual metal, wires extend parallel to each other, or are twisted together. Insulation may extend around each individual coaxial metal wire, a single insulation may surround a bundle of such wires, or insulation may extend around only one or some of the wires.

Embodiments of the invention will now be described, by way of example, the reference to the accompanying drawings, wherein:

Figure 1 is a side elevation of a sensor according to the invention; Figure 2 is a cross-sectional view of the sensor of Figure 1 ;

Figure 3 is a schematic representation of the electrical circuit in which the sensor of Figures 1 and 2 can be incorporated; and

Figures 4 and 5 are cross sectional views of another sensor according to the invention, before and after an increase in temperature.

Referring now to the drawings, Figures 1 and 2 show a tubular element 2 comprising a polymer matrix containing intercalated graphite. The composition has a bulk and surface resistance (as hereinbefore defined) of 1000 ohm at room temperature and is effectively insulating. Two elongate conductive members or sensors 4 and 6 are helically wrapped around the element 2. Each of sensors 4 and 6 comprises a conductive polymer jacket 7 (see Fig 2). Each turn of sensor wire 4 is spaced about 0.5cm from the adjacent turn of sensor wire 6, on the surface of the element 2. The sensor wires 4 and 6 are held in place by a glass fibre braid 8 which also provides abrasion resistance. Optionally an outer insulating layer (not shown in drawings) may also be included. A third conductive wire 10 extends within the sleeve. This may form a reference wire, as described below with reference to Figure 3. The composition of element 2 is insulating at room temperature. But if the temperature at any point along the element reaches the exfoliation-start temperature of the graphite, or higher, the graphite therein exfoliates, expanding the composition, bursting through the surface and causing a significant decrease in the surface resistance, thereby forming a conductive bridge to allow an electrical current to flow between sensor wires 4 and 6.

Figure 3 is a schematic drawing showing a circuit in which devices according to the invention can be incoφorated. The circuit comprises a source wire 12, a locating wire 14 and a return wire 16. Wires 12 and 14 which correspond to sensor wires 4 and 6, are initially separated from each other but can be connected by a change in temperature which results in a conductive bridge (represented in Figure 3 by event E) being provided, by the exfoliated carbon containing composition, between the sensor wires 4 and 6. When connection is made, a test circuit is formed comprising wires 12, 14 and event E. A constant current is driven through that circuit. Locating wire 14 (corresponding to sensor wire 6) has a known impedance which is constant or varies in a known way along its length, and together with a return wire 16 (corresponding to reference wire 10 in Figures 1 and 2) forms a reference circuit, in which a voltage measuring device is included. Provided the impedance values of all elements in the reference circuit are known, and given the voltage measurement of the reference circuit and the known current following through locating wire 14 it is possible to determine the position of event E. Thus the sensor of Figures 1 to 3 can be used to detect and locate a change in temperature, eg due to a fire.

Figures 4 and 5 shows a second embodiment according to the invention. Figure 4 shows a twisted pair 20 of multi-stranded conductors 22. Each conductor comprises a copper wire 24 coated with a thin layer of aluminium 26 (which is preferably vacuum deposited) at least on that part of the surface facing the other one of the conductor pair 22. Each of the multi-stranded conductors 22 is coated with a ceramic insulating layer 28 eg silica. Instead of vacuum deposition of aluminium, aluminium clad wire that is partially anodized may be used.

Figure 5 shows the configuration of Figure 4 after an increase in temperature. Eutectic balls 30 have formed between the copper and aluminium, and these molten balls have burst through the insulating ceramic layer 28 where they have cooled and solidified. Thus contact is formed by the eutectic balls 30 between the conductors 22 of the twisted pair 20. Formation and burst-through of the eutectic occurs at a shaφ triggering temperature.

Examples

The following two examples describe manufacture of sensors according to the invention. The sensors described are similar to that described with reference to Figures 1-3, but in both examples include two reference wires in place of reference wire 10, and in example 2 include an insulating over jacket. Example 1

The following formulation was extruded over a twisted pair of reference wires.

using a Baughan 32mm extruder using the following temperature profiles:

BARREL DIE

1 2 3 4 1 2 3

70°C 80°C 90°C 85°C 65°C 65°C 65°C

The extruder speed was 10 φm.

(Elvax, Callotek, Santicier and Irganox are trademarks of Dupont, Technik(UK),

Monsanto, and Ciba Geigy, respectively)

The antioxidant was dissolved in warm plasticiser. This solution was then added to the polymer pellets contained in a 5 gallon drum and stirred to produce "wet" pellets. The graphite was then added and stirred. The graphite stuck to the individual pellets thus producing an excellent dispersion of the graphite under low shear conditions.

The thickness of the graphite/polymer composite covering the twisted pair wires was between 1 and 2 mm giving an overall diameter of about 6mm. This cable was then cross-linked in an electron beam to a dose of 5Mrads.

This cable was then overbraided with two conductive polymer coated wires and 10 glass fibre yarns to give a very open braid where the spacing of the two conductive wires was 0.5cm. The two conductive polymer wires were connected to an ohmmeter which indicated on overload condition ie the resistance being measured was greater than 20M.ohms. When the cable was heated to over 200°C, within 20 seconds the ohmmeter registered the reading of less than lOOOohms.

This cable was then connected to a circuit as described in EP-A-0133748 using the central conductors to form a 4 wire circuit. When heat was was applied to the cable, the module alarmed and accurately displayed the location of the event along the 50 metre cable.

Example 2

In order to prevent any other conductive path (besides the expanded graphite) triggering the sensor, eg H2O, metal etc, a second sensor was prepared which triggered only when the graphite expanded.

This time the twisted pair was jacketed with the same compound in an identical manner and again beamed to 5MRad. However, the conductive polymer wires were embedded into the compound by wrapping whilst the compound was warmed. This negated the need for a braid to hold the wires onto the graphite/polymer surface and produced a low profile surface.

This construction was then jacketed with a thin layer (approximately 0.1 mm thick) of the same conductive polymer formulation, thus isolating the two sensor wires from accidental contact with H2O or metallic parts.

Finally, the cable was completely over-jacketed with a close braid of glass fibre yarn to provide an abrasion resistant layer.

Claims

1. A sensor suitable for detecting a change in temperature comprising

(ii) an element, having a surface, wherein

(a) in response to the change in temperature, at least part of the element expands causing a change primarily in the surface resistance (as hereinbefore defined) of the said element, and

2. A sensor according to claim 1 , wherein only the surface resistance of the element changes.

3. A sensor according to claim 1 wherein the whole of the composition changes in bulk resistance on the said expansion.

4. A sensor according to any of claims 1 to 3, wherein the change in surface resistance of the element is caused by a conductive component of the element expanding and/or forming in response to the temperature change, and bursting through the surface of the element thereby forming a conductive network on the said surface of the element.

5. A sensor according to any of claims 1 to 4, wherein the element comprises a composition comprising a polymer matrix and a conductive filler.

6. A sensor according to claim 5, wherein the conductive filler expands in response to the change, preferably an increase in temperature.

7. A sensor according to any preceding claim, wherein at least part of the element begins to expand when the temperature has been increased to at least 100°C, preferably to at least 180°C, more preferably to at least 200°C.

8. A sensor according to any preceding claim , wherein the said change in resistance is a decrease.

9. A sensor according to claim 8, wherein the decrease in resistance is a decrease in surface resistance (as hereinbefore defined) of at least 10² ohm when the temperature increases from room temperature to 80°C or higher.

10. A sensor according to any preceding claim, comprising a second member that is conductive at least after the change in temperature, which second member is spaced from the first member, and wherein the said surface of the element at least after expansion, is in electrical contact with both the first and the second members.

11. A sensor according to any preceding claim, wherein the sensor is an elongate sensor, and wherein the first member, and if present, preferably also the second member are elongate.

12. A sensor according to any preceding claim , wherein the element comprises a composition comprising a polymer matrix containing an intercalated graphite, and the expansion of at least part of the element is preferably caused by exfoliation of the intercalated graphite.

13. A sensor according to claim 12 wherein the amount of graphite is at least 1 wt%, preferably at least 5 wt% , more preferably at least 10 wt% based on the combined weight of the graphite and the polymer.

14. A sensor according to claims 12 or 13, wherein the amount of graphite is at most 50 wt%, preferably at most 40 wt% based on the combined weight of the graphite and the polymer.

15. A sensor according to any of claims 12 to 14, wherein an insulating film separates the conductive member(s) from the composition before exfoliation of the graphite, the film being disrupted on exfoliation so that the exfoliated graphite in the said portion of the composition is in electrical contact with the first and, if present, the second member.

16. A sensor according to any of claims 12 to 16 wherein the polymer has a melting point (as defined by differential scanning calorimetry) which is less than the exfoliation temperature of the graphite.

17. A sensor according to any of claims 12 to 16, wherein the composition is made from a latex comprising the said polymer.

18 A sensor according to any preceding claim, wherein the said element is in the form of an elongate sleeve or core, and first and second conductive members are present which are both elongate and helically wrapped around the sleeve or core.

19. A sensor according to claim 18 wherein each of the elongate conductive members comprises a metal core and an elongate jacket surrounding the metal core which jacket comprises a conductive polymer.

20. A sensor according to any of claims 1 to 11 , wherein at least after the change in temperature the element comprises a metal or alloy, the element is coated with an electrically insulating layer, and in response to an increase in temperature the metal or alloy bursts through the insulating layer to make electrical contact with the first member.

21. A sensor according to claim 20, wherein before the change in temperature the said element comprises a first metal wire, coated with a layer of second metal, and in response to the increase in temperature, a eutectic alloy is formed between the said first and second metallic materials which bursts through the insulating layer.

22. A sensor according to claim 21 , wherein the first metal wire comprises copper wire, and this is coated with an aluminium layer which is preferably vacuum deposited.

23. A sensor according to any of claims 20 to 22, where the first member is similar or identical to the said element.

24. A sensor according to claim 23 comprising two or more stranded wires coated with insulation extending parallel to, or twisted with, each other.