Patent Publication Number: US-2021164849-A1

Title: Temperature sensor and indicator

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to, and the benefit of, U.S. provisional patent application No. 62/702,166 filed 23 Jul. 2018, the entirety of which is incorporated by reference herein for all purposes. 
    
    
     TECHNICAL FIELD 
     Some embodiments relate to apparatus for sensing increases in temperature above a predetermined threshold. Some embodiments relate to apparatus for providing a visual indication that temperature has increased above a predetermined threshold. Some embodiments relate to apparatus for sensing changes in temperature within electrical equipment above a predetermined threshold and providing a visual indication that a temperature change above the predetermined threshold has occurred within the piece of electrical equipment. Some embodiments relate to apparatus for sensing changes in temperature within electrical equipment above two distinct predetermined thresholds and providing a visual indication that only the lower temperature threshold or both the lower temperature threshold and the higher temperature threshold have been exceeded. 
     BACKGROUND 
     Electrical equipment is a common feature of modern society. Electrical power distribution grids use a variety of electrical equipment, such as transformers, capacitors, reactors and voltage regulators. 
     The life expectancy of electrical equipment such as transformers may be decreased as the operating temperature of the piece of electrical equipment is increased. For example, for some electrical equipment such as transformers, the life expectancy of the equipment may be reduced by as much as one-half for every approximately 5° C. to 10° C. increase in continuous operating temperature that the equipment experiences. 
     If a piece of electrical equipment is regularly or consistently operating at an elevated temperature, the piece of electrical equipment may fail prematurely (i.e. before the predicted lifespan of the electrical equipment has elapsed). It can be prudent to replace such a piece of electrical equipment with a piece of electrical equipment having a larger load capacity if it is regularly or consistently operating at a temperature higher than the desired operating temperature. 
     As an example, transformer loss-of-life is a function of both time and temperature, so the longer that a transformer is operating at an over-loaded temperature, the more the expected lifetime of the transformer is reduced. A brief over-load will not have a significant impact on the expected lifetime unless it is at very extreme temperatures; however, frequent over-loading will have a significant impact on the expected lifetime of the transformer. Therefore, if a transformer is slightly over-loaded, utilities will monitor further to determine if this is a regular occurrence or a chance event. If they find it to be a regular occurrence, they may replace the transformer with a larger version designed to handle higher loads. If the transformer is heavily over-loaded, it is a sign that a significant loss-of-life may have already occurred, and that the transformer is likely somewhat over-loaded on a regular basis. 
     Some utilities have developed practices for optimizing the lifetime of their equipment and the effort required to maintain it. Such practices may involve categorizing over-loaded equipment based on its operating temperature relative to a reference temperature and performing different actions based on such categorization. For example, if a transformer is designed to operate at a reference temperature of 90° C., a transformer may be categorized as ‘over-loaded’ if it is operating at 110° C. and ‘extremely over-loaded’ if it is operating at 120° C. A piece of equipment that is ‘over-loaded’ m ay be monitored more closely for a period of time, while equipment that is ‘extremely over-loaded’ may be replaced immediately. 
     There is a need to provide apparatus capable of sensing and indicating changes in temperature within electrical equipment to assist in determining if the electrical equipment is operating in an ‘over-loaded’ or ‘extremely over-loaded’ state. There is also a need to provide such apparatus wherein the indication of a change in temperature within the electrical equipment to determine whether the electrical equipment is operating in an ‘over-loaded’ or ‘extremely over-loaded’ state can be easily assessed visually from the exterior of the electrical equipment. 
     The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings. 
     SUMMARY 
     In some aspects, a temperature sensor is provided. The temperature sensor has a heat-sensitive element contained within a chamber, the heat-sensitive element being selected to change configuration from a first configuration to a second configuration at a predetermined temperature, and a biased member biased toward the chamber, the biased member being initially prevented from entering the chamber when the heat-sensitive element is in the first configuration and movable into the chamber when the heat-sensitive element is in the second configuration. In some aspects, the heat-sensitive element is a material having a melting temperature corresponding to the predetermined temperature and the change in configuration is a phase change from a solid state to a liquid state, and the chamber has a portion that is formed from a selectively permeable element to retain the heat-sensitive element within the chamber when the heat-sensitive element is in a solid state, but to allow the heat-sensitive element to exit the chamber when the heat-sensitive element is in a liquid state. In some aspects, the selectively permeable element is a selectively permeable membrane. In some aspects, the selectively permeable membrane interposes the biased member and the heat-sensitive element. 
     In some aspects, an indicator is operatively engaged with the temperature sensor to provide a visual or other perceptible indication that the predetermined temperature has been exceeded. In some aspects, the indicator has a release mechanism that is operatively engaged with the biased member, and which is releasable upon movement of the biased member into the chamber. 
     In some aspects, a temperature sensor and indicator is provided having a shell with a generally axially extending bore with one or more angled retaining surfaces formed therein and a temperature sensor as described above. An indicator is positioned for sliding movement within the bore of the shell and has a proximal portion with at least one resilient activator foot having angled release surfaces, the angled release surfaces of each one of the at least one resilient activator foot being initially retained in contact with the corresponding at least one angled retaining surface on the shell by contact of a distal end of the biased member of the temperature sensor with the at least one resilient activator foot, and a surface for contacting the biasing element of the temperature sensor so that the biasing element applies a distal biasing force against the indicator, and a mechanism for providing an indication that the indicator has been released. 
     In some aspects, first and second temperature sensor and indicators are provided together, the first temperature sensor and indicator being configured to release at a low temperature and the second temperature sensor and indicator being configured to release at a high temperature. In some aspects, the first and second temperature sensor and indicators are provided in a single housing and can be independently removed from the housing and replaced with a different temperature sensor and indicator, for example that is configured to activate at a different predetermined temperature threshold. 
     In some aspects, a method of sensing an increase in temperature above a predetermined temperature threshold is provided. A heat-sensitive element is provided within a chamber, the heat-sensitive element being selected to change from a first configuration to a second configuration at the predetermined temperature threshold. A biased member is biased towards the chamber, and is initially prevented from entering the chamber when the heat-sensitive element is in the first configuration. After the temperature rises above the predetermined temperature threshold, the biased member is permitted to enter the chamber when the heat-sensitive element is in the second configuration. In some aspects, the heat-sensitive element has a first configuration that is solid and a second configuration that is liquid, and the step of permitting the biased member to enter the chamber when the heat-sensitive element is in the second configuration involves allowing the heat-sensitive element to flow out of the chamber through a selectively permeable element or membrane. 
     In some aspects, an indication that the temperature has exceeded a predetermined temperature threshold is provided. After an increase in temperature above a predetermined temperature threshold is sensed as described above, the biased member is allowed to move out of engagement with at least one indicator retaining foot of an indicator, and subsequently the at least one indicator retaining foot is allowed to move inwardly as a chamfered release surface on the at least one indicator retaining foot slides past a corresponding chamfered retaining surface provided on a sliding channel of a housing within which the indicator is axially movable, and allowing the indicator to move from an initial locked configuration to a released configuration. 
     In some aspects, an indication that the temperature has exceeded first and second predetermined temperature thresholds is provided by using a combination of two temperature sensing and indicating units, a first one of the temperature sensing and indicating units being configured to provide an indication that the temperature has exceeded the first predetermined temperature threshold and a second one of the temperature sensing and indicating units being configured to provide an indication that the temperature has exceeded the second predetermined temperature threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
         FIG. 1  is a side view of an example embodiment of two temperature sensors and indicators installed in an external housing, both in an unactivated configuration. 
         FIG. 2  is an exploded view of the components of an example embodiment of a temperature sensor and indicator, with the external housing omitted for clarity. 
         FIG. 3  is a sectional view of the embodiment of  FIG. 1 , showing one temperature sensor and indicator installed in an external housing. 
         FIG. 4A  is a partial enlarged view of a portion D of  FIG. 3 .  FIG. 4B  is an exploded view of an example embodiment of components that define the first chamber, and  FIG. 4C  is a cross-sectional view of those components in an assembled configuration. 
         FIG. 5A  is a side view of an example embodiment of a temperature sensor and indicator with the external housing omitted for clarity, and  FIG. 5B  is a sectional view thereof taken along line B-B of  FIG. 5A , in the unactivated configuration.  FIG. 5C  is a sectional view thereof when the indicator pin has moved to the activated configuration, but while the indicator feet are still in the locked position and have not yet started to slide relative to the chamfered retaining surfaces of the shell. 
         FIG. 6A  is a sectional view of an example embodiment of a temperature sensor and indicator in the activated configuration, with the indicator in the released position and the external housing omitted for clarity. Although the indicator feet are shown as interfering with the shell in  FIG. 6A , this is an artifact of the drawing model used. In actual construction, the indicator feet are resilient and would flexibly press against and contact the shell, but would not extend therethrough. 
         FIG. 6B  is a sectional view of the embodiment of  FIG. 6A , showing the engagement of a projection on the indicator with a hard stop on the inner surface of the shell to prevent full ejection of the indicator. 
         FIG. 7  is a perspective view of an example embodiment of an actuator pin. 
         FIG. 8  is an example embodiment of a transformer with a temperature sensor and indicator mounted therein. 
         FIG. 9  is a perspective view of an example embodiment of a temperature sensor and indicator that is capable of sensing and indicating that two distinct temperature thresholds have been exceeded, installed in an external housing in its fully unactivated configuration. 
         FIG. 10  is a perspective view of the example embodiment of  FIG. 9 , in which the indicator indicating that the lower temperature threshold has been exceeded is in its released configuration, but the indicator indicating that the higher temperature threshold has been exceeded is in its unactivated configuration. 
         FIG. 11  is a perspective view of the example embodiment of  FIG. 9 , in which both the low temperature threshold indicator and the high temperature threshold indicator are in their released configurations. 
         FIG. 12  is a sectional view of the temperature sensor and indicator shown in  FIG. 11 , with both the low temperature threshold indicator and the high temperature threshold indicator in their released configurations. 
         FIG. 13  is an exploded view of the temperature sensor and indicator shown in  FIG. 9 . 
         FIG. 14  is an example embodiment of a method of sensing and indicating that the temperature has passed a predetermined temperature threshold. 
         FIG. 15  is an example embodiment of a method of sensing and indicating that the temperature has passed one or both of two distinct predetermined temperature thresholds. 
         FIGS. 16A, 16B and 16C  show embodiments of alternative configurations for a temperature sensor. 
         FIGS. 17A, 17B and 17C  show schematically an example embodiment of a temperature sensor using a shape memory material as the temperature-sensitive element.  FIG. 17A  shows the temperature sensor in the unactivated configuration,  FIG. 17B  shows the temperature sensor in the activated configuration but with the actuator pin that would release the indicator still in the locked position, and  FIG. 17C  shows the temperature sensor in the activated configuration with the actuator pin that would release the indicator in the released position. 
     
    
    
     DESCRIPTION 
     Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     The inventors have now developed a temperature sensor for determining when a temperature has exceeded a predetermined value. The sensor includes a heat-sensitive element. In some embodiments, the heat-sensitive element changes phase from solid to liquid at the predetermined temperature value. The heat-sensitive element occupies a space defined by a chamber. A selectively permeable retaining membrane is provided to secure the heat-sensitive element in place within the chamber while the heat-sensitive element is in the solid state. In the solid state, the heat-sensitive element cannot pass through the selectively permeable membrane. In the liquid state, the heat-sensitive element can pass through the selectively permeable membrane and exit the chamber. 
     In one embodiment, a biased member such as a biased pin is supported in its initial position by the solid heat-sensitive element. When the heat-sensitive element melts, the biased pin enters the chamber previously occupied by the heat-sensitive element. The biased pin can thus provide a trigger for a mechanical indicator to provide a visual indication that the heat-sensitive element has melted, and therefore that the predetermined temperature value corresponding to the melting point of the heat-sensitive element has been exceeded. 
     In one embodiment, the indicator is initially retained in a locked, unactivated configuration by the biased pin of the temperature sensor. The indicator has at least one or a plurality of activator feet which are resilient and have chamfered edges in engagement with corresponding chamfered retaining surfaces on the shell. When the spring-driven pin is in the unactivated position, the spring driven pin interposes the plurality of activator feet and prevents the activator feet from moving inwardly together. 
     In the unactivated configuration, the chamfered edges of the activator feet remain in the locked configuration against the corresponding chamfered retaining surfaces on the shell when the biased pin interposes the plurality of feet. A biasing member biases the indicator distally outwards; however, movement of the indicator is prevented by engagement of the activator feet with the shell. When the temperature sensor is activated by melting of the heat-sensitive element, the pin is permitted to enter the chamber, and is thereby removed from between the plurality of activator feet. When the pin is removed from between the plurality of activator feet, the plurality of activator feet can move inwardly, which they do in response to the inward force created by the chamfered retaining surfaces on the shell created by the biasing force of the biasing member that forces the indicator in the distal direction. This allows the chamfered edges on the activator feet to slide past the chamfered retaining surfaces of the shell, so that the indicator is biased longitudinally in the distal direction to move to its released configuration, to provide a visual indication that the temperature sensor has been activated. 
     As used in this specification, the term “proximal” means in a direction towards the end of the temperature sensor and indicator that would be positioned inside a piece of electrical equipment, e.g. inside a transformer, in use, and the term “distal” means the opposite of proximal, i.e. in a direction towards the end of the temperature sensor and indicator that would be positioned outside a piece of electrical equipment. 
     As used in this specification, the term “inner” or “inward” means in a direction towards the interior of the temperature sensor and indicator, and the term “outer” or “outward” has the opposite meaning, i.e. in a direction towards the external surface of the temperature sensor and indicator. As described in greater detail below, depending on the relative orientation of the components at issue, inwards may mean radially inwardly towards an axial centreline of the temperature sensor and indicator, or laterally towards a central plane extending through an axial centreline of the temperature sensor and indicator. 
     With reference to  FIGS. 1, 3, 4A, and 5B , an example embodiment of a temperature sensor and indicator  100  is illustrated in its unactivated configuration. Temperature sensor and indicator  100  has a thermally activated element  102 , an indicator release mechanism  104 , and an indicator  106 , as described in more detail below. An external housing  108  contains and supports the various components of temperature sensor and indicator  100  and can be secured to a piece of electrical equipment for use as described in greater detail below. 
     As most clearly seen in  FIGS. 2, 4B, 4C and 5B , thermally activated element  102  has a heat-sensitive element  110  that occupies the volume of a first chamber  112 . At least a portion of one edge of chamber  112  is defined by a selectively permeable membrane  116 . First chamber  112  is initially fully sealed, so that heat-sensitive element  110  cannot flow or creep out of first chamber  112  when in its solid state. A second chamber  118  is provided on the opposite side of selectively permeable membrane  116 . In the illustrated embodiment, second chamber  118  is defined within a solder washer  119 . 
     Heat-sensitive element  110  is made from a material with a melting temperature selected so that heat-sensitive element  110  will melt or begin to melt at the predetermined temperature at which it is desired to have thermally activated element  102  activate. In some embodiments, heat-sensitive element  110  is a block of solder with a composition selected so that the melting temperature of the solder is the predetermined temperature. By varying the composition of the solder, the melting temperature can be changed. Thus, by selecting a solder with a composition that yields a melting temperature of the predetermined temperature, thermally activated element  102  can be designed to activate at the desired predetermined temperature. 
     Solders having different melting temperatures are commercially available, and a person skilled in the art can select a solder having a composition suitable for melting at a desired predetermined temperature. For example:
         solder having a composition of 52.2 wt % In/46 wt % Sn/1.8 wt % Zn has a solidus temperature of 108° C. and a liquidus temperature of 108° C.;   solder having a composition of 51.6 wt % Bi/41.4 wt % Pb/7.0 wt % Sn has a solidus temperature of 98° C. and a liquidus temperature of 112° C.;   solder having a composition of 52 wt % In/48 wt % Sn has a solidus temperature of 118° C. and a liquidus temperature of 118° C.;   solder having a composition of 57 wt % Bi/43 wt % Sn has a solidus temperature of 139° C. and a liquidus temperature of 139° C.; and   solder having a composition of 95.5 wt % Sn/3.8 wt % Ag/0.7 wt % Cu has a solidus temperature of 217° C. and a liquidus temperature of 217° C.
 
The solidus temperature is the highest temperature at which a composition is completely solid. The liquidus temperature is the lowest temperature at which a composition is completely liquid. In some embodiments, the solder is an eutectic solder. In some embodiments, the solder is a non-eutectic solder.
       

     In the illustrated embodiment, first chamber  112  is generally cylindrical in shape and is defined within a base insert  111 , and heat-sensitive element  110  is provided with a corresponding generally cylindrical shape in its initial solid form. It will be appreciated by those skilled in the art that alternative shapes for both first chamber  112  and heat-sensitive element  110  could be used in alternative embodiments, provided that heat-sensitive element  110  retains actuator pin  120  in the locked configuration until heat-sensitive element  110  melts, and provided that actuator pin  120  is then able to enter first chamber  112  when heat-sensitive element  110  melts. 
     Selectively permeable membrane  116  is in sealing engagement with base insert  111  to define first chamber  112 . In the illustrated embodiment, selectively permeable membrane  116  is sandwiched in position between solder washer  119  and base insert  111 . 
     Any suitable method of securing the selectively permeable membrane  116  in sealing engagement with base insert  111  to seal first chamber  112  could be used in alternative embodiments. 
     In the illustrated embodiment, as best shown in  FIGS. 4B and 4C , a sealing ring  117  is provided on the proximal surface of solder washer  119 . Sealing ring  117  is a generally proximally projecting protrusion that interposes solder washer  119  and membrane  116 , to thereby form a compression seal when solder washer  119  and base insert  111  are compressed together within shell  114  to maintain a good seal between membrane  116  and base insert  111 . In the illustrated embodiment, sealing ring  117  is integrally formed with solder washer  119 . In alternative embodiments, sealing ring  117  could be provided as a separate element coupled to solder washer  119  in any suitable manner, or sealing ring  117  could be provided as a distally extending projection on the distal surface of base insert  111 , either formed integrally or as a separate element secured to base insert  111 . 
     In alternative embodiments, alternative mechanisms of providing a sealing engagement between membrane  116  and base insert  111  to define first chamber  112  could be used, for example suitable adhesives, ultrasonic welding, or the like. 
     In the illustrated embodiment, solder washer  119  is provided with a pair of apertures  142  and base insert  111  is provided with a corresponding pair of projections  144  that are engageable within apertures  142  when assembled, to help facilitate the engagement of these components in the correct orientation. In alternative embodiments, apertures  142 /projections  144  could be omitted, or other mechanisms such as a tongue-and-groove engagement within shell  114  could be used. 
     Selectively permeable membrane  116  is selected to be impermeable to heat-sensitive element  110  when heat-sensitive element  110  is in the solid state, but to be permeable to heat-sensitive element  110  when heat-sensitive element  110  is in its liquid state. The nature of the material used for selectively permeable membrane  116  may vary depending on the nature of the heat-sensitive element  110 . The material used for selectively permeable membrane  116  should be selected to have a pore size suitable to prevent flow or creep of heat-sensitive element  110  through selectively permeable membrane  116  when heat-sensitive element  110  is in the solid state, but to allow flow of heat-sensitive element  110  through selectively permeable membrane  116  when heat-sensitive element  110  is in the liquid state. 
     The material used for selectively permeable membrane  116  should also be selected to be chemically compatible with the material used for heat-sensitive element  110  (e.g. to avoid undesired chemical reactions or the diffusion of chemical elements, ions or molecules between selectively permeable membrane  116  and heat-sensitive element  110 ). The material used for selectively permeable membrane  116  should also be heat resistant, e.g. so that selectively permeable membrane  116  will not itself melt or degrade in the anticipated range of operating temperatures of temperature sensor and indicator  100 . 
     Examples of materials that could be used to provide selectively permeable membrane  116  in various embodiments include sintered stainless steel, ceramic, or a fine mesh or porous membrane made from a suitable material such as plastic, nylon, or metal. 
     In some embodiments, permeable membrane  116  can be made from a porous foam, provided that the foam has a sufficient amount of continuous pores to allow the heat-sensitive element  110  to flow therethrough in its liquid form. In some embodiments, selectively permeable membrane  116  is made from plastic, and the plastic is polytetrafluoroethylene (PTFE) (e.g. Teflon®). 
     In some embodiments, the pore size of selectively permeable membrane  116  is in the range of 0.2 to 10 μm, including any value therebetween, e.g. 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0 or 9.5 μm. In embodiments in which selectively permeable membrane  116  is a plastic membrane, the membrane may have a pore size in the range of 0.2 to 10 μm, including any value therebetween, e.g. 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0 or 9.5 μm. 
     In embodiments such as those illustrated in  FIGS. 1-7 , in which selectively permeable membrane  116  is deformed to allow actuator pin  120  to enter first chamber  112 , selectively permeable membrane  116  should be sufficiently flexible to allow for movement of actuator pin  120  into first chamber  112 . In alternative embodiments, selectively permeable membrane  116  could be permitted to rupture once it starts to deform after heat-sensitive element  110  has started to flow through it and therefore selectively permeable membrane  116  could in such embodiments be made from a non-flexible material. 
     As an example, in embodiments in which heat-sensitive element  110  is solder, selectively permeable membrane  116  can be polytetrafluoroethylene (PTFE) (e.g. Teflon®). In some embodiments, the polytetrafluoroethylene has a pore size in the range of 0.2 to 10 μm, including any value therebetween, e.g. 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0 or 9.5 μm. 
     Thermally activated element  102  further comprises an actuator pin  120  that is biased against heat-sensitive element  110 . In the illustrated embodiment, membrane  116  interposes actuator pin  120  and heat-sensitive element  110 . In the illustrated embodiment, actuator pin  120  is biased against heat-sensitive element by coil spring  122 . A contact surface  123  is provided on a projection towards the proximal end of actuator pin  120  to contact coil spring  122  and allow the coil spring  122  to bias actuator pin  120  towards first chamber  112 . In alternative embodiments, any suitable biasing mechanism could be used to bias actuator pin  120  towards first chamber  112 , such as the weight of the pin under force of gravity, an extension spring, or stretched elastic material. 
     In some embodiments, including embodiments in which heat-sensitive element  110  is solder, even at temperatures below its melting temperature, heat-sensitive element  110  may experience slow deformation or “creep” due to the pressure applied by actuator pin  120 . In such embodiments, membrane  116  acts to prevent or minimize the amount of deformation or creep experienced by heat-sensitive element  110 , and in particular retains heat-sensitive element  110  within first chamber  112  so that the volume of first chamber  112  continues to be occupied by heat-sensitive element  110  so long as heat-sensitive element  110  is in the solid state. Because the volume of first chamber  112  is occupied by heat-sensitive element  110 , actuator pin  120  cannot enter first chamber  112 , notwithstanding the force applied by coil spring  122 . 
     In some embodiments, including the illustrated embodiment shown in  FIG. 7 , actuator pin  120 ′ is provided with structural features to maximize the surface area of actuator pin  120 ′ in contact with membrane  116  (and therefore the surface area of actuator pin  120 ′ that is applying force against heat-sensitive element  110 ), while still allowing heat-sensitive element  110  to flow past the proximal portion of actuator pin  120 ′ when heat-sensitive element  110  is in the liquid phase. In the illustrated embodiment of  FIG. 7 , actuator pin  120 ′ is provided with one or more surface ridges  124  at its proximal end  121 ′. Surface ridges  124  define one or more fluid flow channels  126  therebetween, to allow heat-sensitive element  110  to flow past actuator pin  120  into second chamber  118  when heat-sensitive element  110  is in a liquid state. In alternative embodiments, e.g. as illustrated in  FIG. 2 , actuator pin  120  has no fluid flow channels and no surface ridges. 
     When the predetermined temperature threshold is reached, heat-sensitive element  110  melts, changing from the solid phase to the liquid phase. Selectively permeable membrane  116  is permeable to heat-sensitive element  110  in its liquid form. In some embodiments, a biasing pressure is applied by coil spring  122  forcing actuator pin  120  into first chamber  112  and membrane  116  is sufficiently flexible to allow movement of actuator pin  120  into first chamber  112 , actuator pin  120  begins to move proximally into first chamber  112 , forcing the liquid form of heat-sensitive element  110  to flow through membrane  116  and out fluid flow channels  126  (if present) into second chamber  118 . In embodiments in which fluid flow channels  126  are not present, the liquid form of heat-sensitive element  110  flows out of first chamber  112  into second chamber  118  through gaps and tolerances between actuator pin  120  and solder washer  119 . The evacuation of the liquid form of heat-sensitive element  110  from first chamber  112  allows actuator pin  120  to move into first chamber  112 , thus placing thermally activated element  102  in the activated configuration as shown in  FIGS. 5C, 6A and 6B . 
     In the illustrated embodiment, second chamber  118  is defined at least partially by a solder washer  119 . In the illustrated embodiment, second chamber  118  is defined bys a generally cylindrical internal aperture extending through solder washer  119 , and the proximal end  121  of actuator pin  120  extends though the cylindrical second chamber  118  so that solder washer  119  helps to maintain alignment of actuator pin  120  with first chamber  112 . In the illustrated embodiment, as best seen in  FIG. 4C , the inner wall of solder washer  119  that defines second chamber  118  has a tapered surface  119 A that tapers radially inwardly in the proximal direction. In some embodiments, the tapered surface  119 A of the inner wall of solder washer  119  facilitates assembly. In some embodiments, the inner wall of solder washer  119  is not tapered, i.e. is a generally cylindrical shape with straight sides. Second chamber  118  should have a sufficient volume to receive the volume of liquid heat-sensitive element  110  that is displaced through membrane  116  by actuator pin  120 . 
     With reference to  FIGS. 3, 5B, 5C, 6A and 6B , indicator release mechanism  104  is described in greater detail. Indicator  106  is disposed for axial sliding movement within a sliding channel  113  defined within shell  114 . In the unactivated position shown in  FIGS. 3 and 5B , the distal end  128  of actuator pin  120  sits inwardly of and interposes a plurality of activator feet  130 . In the illustrated embodiment, a pair of laterally opposed activator feet  130  are provided on the proximal portion of indicator  106 . When indicator release mechanism  104  is activated, each activator foot  130  is caused to move laterally inwardly toward the opposing activator foot  130 . In alternative embodiments, any desired number of activator feet can be used, e.g. 1, 2, 3, 4 or more activator feet. In some embodiments, rather than being laterally opposed, activator feet  130  may be distributed around the circumference of the proximal portion of indicator  106 . In such embodiments, when indicator release mechanism  104  is activated, each activator foot is caused to move radially inwardly towards an axial centreline of indicator  106 . 
     Each one of activator feet  130  has a chamfered release surface  132  on an outer edge thereof that tapers outwardly from a distal portion to a proximal portion thereof. Although the indicator feet are shown as interfering with the shell in  FIG. 6A , this is an artifact of the drawing model used. In actual construction, the indicator feet are resilient and would flexibly press against and contact the shell, but would not extend therethrough. 
     As shown in e.g.  FIG. 5B , the space defined between activator feet  130  in indicator release mechanism  104  in the illustrated embodiment has a keyhole shape, having a generally rounded distal portion  146  and generally straight edges  148  at the proximal portion thereof. The keyhole shape shown in the illustrated embodiment reduces stress concentration between the pair of activator feet  130 , and further can be modified in shape to adjust the flexibility of activator feet  130 . 
     Chamfered release surfaces  132  are held in place by engagement with corresponding chamfered retaining surfaces  134  of shell  114 . Chamfered retaining surfaces  134  taper outwardly from a distal portion to a proximal portion thereof in a manner complementary to chamfered release surfaces  132 . Chamfered release surfaces  132  and chamfered retaining surfaces  134  are thus configured to slide past one another when temperature sensor and indicator  100  is activated, as described in greater detail below. 
     Other shapes and configurations for surfaces  132 ,  134  could be used in alternative embodiments, so long as surfaces  132 ,  134  can initially retain indicator release mechanism  104  in an unactivated configuration and allow indicator release mechanism  104  to move to an activated configuration when thermally activated element  102  is activated. 
     In the unactivated position, a biasing mechanism such as coil spring  122  applies an axial biasing force against activator feet  130  in the distal direction, for example via engagement with a contact surface  136  provided on a proximally facing surface of indicator release mechanism  104 . Because distal end  128  of actuator pin  120  prevents activator feet  130  from deflecting inwardly, chamfered release surfaces  132  remain locked in place against chamfered retaining surfaces  134 , and activator feet  130  cannot move when actuator pin  120  is in its unactivated configuration. 
     When thermally activated element  102  is activated by the melting of heat-sensitive element  110 , temperature sensor and indicator  100  moves into the activated configuration, as shown in  FIG. 5C  with indicator release mechanism  104  still in the locked configuration, i.e. in which activator feet  130  are illustrated in the locked configuration. In the activated configuration, actuator pin  120  has moved sufficiently far axially in the proximal direction that its distal end  128  no longer interposes activator feet  130 . In response to the biasing force applied by coil spring  122 , activator feet  130  can thus begin to slide in the distal direction and, because actuator pin  120  has been removed from therebetween, activator feet  130  can begin to deflect inwardly towards one another. Continued movement of activator feet  130  inwardly together and in the distal direction longitudinally as chamfered release surfaces  132  slide past chamfered retaining surfaces  134  allows coil spring  122  to bias indicator release mechanism  104 , and thus indicator  106 , into the released position. 
     Once released, activator feet  130  continue to slide distally within shell  114  until a projection  139  provided on indicator  106  reaches a hard stop  138 . In the illustrated embodiment, hard stop  138  is a radially inwardly extending projection formed on the interior sidewall of shell  114 , and projection  139  is a radially outwardly extending projection formed on a portion of indicator  106 . In the illustrated embodiment, projection  139  is formed on a portion of indicator  106  between activator feet  130 . However, projection  139  could be provided at any desired location in alternative embodiments, so long as hard stop  138  is positioned to contact projection  139 . This prevents complete ejection of indicator  106 , while the biasing force applied by coil spring  122  ensures that indicator  106  remains in its fully extended position, so that it will be visible upon visual inspection. 
     In alternative embodiments, any other structural element suitable for stopping the distal movement of indicator  106  could be used. In alternative embodiments in which it is desired to fully eject indicator  106 , a stop mechanism such as hard stop  138  can be omitted. 
     As a result of the release of activator feet  130 , indicator  106  is ejected axially in the distal direction, away from shell  114 , into its released configuration. In some embodiments, indicator  106  can be provided with a bright colour on its outer surface  140 , which is hidden from view in the locked configuration, but rendered visible in the released configuration. A worker inspecting a piece of electrical equipment can thus readily determine whether temperature sensor and indicator  100  has been activated. In alternative embodiments, indicator  106  can be provided with a surface that is provided with a distinctive pattern, texture or shape in place of a bright colour on outer surface  140 . In some embodiments, the relative length of indicator  106  that is projecting from external housing  108  is used to provide a visual assessment that indicator  106  has been released. In further alternative embodiments, other ways of providing a signal that indicator  106  has been released besides the rendering visible of surface  140  could be used, e.g. the breaking or connection of an electrical circuit between a portion of indicator  106  and another portion of temperature sensor and indicator  100  could be used to provide an electrical signal that indicator  106  has been released, and/or could be used to trigger generation of an audible or visible indication, e.g. a warning tone or activation of a warning light. 
     In the illustrated embodiment, temperature sensor and indicator  100  is secured to a piece of electrical equipment, e.g. a transformer, via external housing  108 . External housing  108  is provided with a radially extending collar  150  and a threaded surface  152 . In use, the distal end of external housing  108  can be passed through a suitable aperture provided in the enclosure of the electrical equipment, illustrated as enclosure  23  of  FIG. 8 , from the inner side of the electrical equipment, so that threaded surface  152  extends outside the outer surface of the enclosure of the electrical equipment, and collar  150  extends adjacent the enclosure of the electrical equipment on the inner surface of the enclosure. In some embodiments, a washer  154  is provided to interpose collar  150  and the enclosure of the electrical equipment, to form a seal between collar  150  and the enclosure of the electrical equipment. 
     To secure temperature sensor and indicator  100  in place, a nut  156  having an inner threaded surface  158  is threadedly engaged with threaded surface  152  on external housing  108  and tightened. In alternative embodiments, any other suitable engagement mechanism could be used to secure nut  156  in place on external housing  108 , e.g. a suitably tight friction fit, suitable adhesives, ultrasonic welding, or the like. In the illustrated embodiment, a cover  160  is provided to secure shell  114  within external housing  108 . 
     In some embodiments, as illustrated schematically in  FIG. 8 , temperature sensor and indicator  100  is mounted to a side of the enclosure  23  of the electrical equipment  21  in which it is installed, to assist in rendering indicator  106  visible to a user inspecting the electrical equipment when indicator  106  is in the released configuration. In some embodiments, temperature sensor and indicator  100  is mounted on an oil-filled transformer at a suitable elevation so that temperature sensor and indicator  100  is measuring a temperature of the oil contained within the oil-filled transformer. In some such embodiments, the temperature sensor and indicator  100  measures the temperature of the upper region of the oil, or the top oil of the transformer. In some example embodiments, mounting the temperature sensor and indicator  100  approximately 5 cm or more below the oil-level is an acceptable position to ensure that oil temperature is being measured. 
     In alternative embodiments, the temperature sensor and indicator  100  can be mounted in the air space above the level of fluid in a fluid-filled transformer, although adjustments to the configuration of the temperature sensor and indicator  100  might need to be made as compared with an equivalent temperature sensor and indicator  100  mounted to measure the temperature of the fluid, because the air temperature and fluid temperature may be slightly different. In alternative embodiments where the temperature sensor and indicator  100  is used with a dry-type transformer, considerations with respect to measuring fluid temperature versus air temperature would be absent. It would be within the expected ability of one skilled in the art to adjust the temperature sensor and indicator to suit its desired position of deployment. 
     In some embodiments, as illustrated in  FIGS. 9-13 , a temperature sensor and indicator  200  comprises a pair of temperature sensor and indicators  200 A,  200 B, which are provided together in a single external housing  208 . Components of temperature sensor and indicators  200 A,  200 B that correspond to components of temperature sensor and indicator  100  are referred to by reference numerals incremented by 100. Each of temperature sensor and indicators  200 A,  200 B are generally similar to temperature sensor and indicator  100 . 
     In some embodiments, each one of temperature sensor and indicators  200 A,  200 B is self-contained within its own shell  214 A,  214 B, so that temperature sensor and indicators  200 A,  200 B are each self-contained modules that are independently replaceable within external housing  208 . In the illustrated embodiment, a separation shield  215 A,  215 B is provided as an interior component of each of shells  214 A,  214 B. Separation shield  215 A/ 215 B can help to prevent interference of coil springs  222 A/ 222 B. Each one of temperature sensor and indicators  200 A,  200 B is provided with a half-cylinder shape that is generally symmetrical, so that they can be easily installed within the generally cylindrical shape of external housing  208 . 
     In the illustrated embodiment, a snap ring  262  and a capsule cover  260  are provided so that shells  214 A,  214 B can be readily engaged together as a single module for insertion into external housing  208 . When capsule cover  260  is removed, either or both of shells  214 A,  214 B can be snapped out of snap ring  262  and replaced, e.g. so that a different temperature sensor and indicator unit that will activate at a different predetermined temperature can be readily installed in external housing  208 . In some embodiments, temperature sensor and indicators like  200 A,  200 B are sold as individual units, so that a purchaser can readily replace a temperature sensor and indicator  200 A,  200 B that has been activated and/or install a temperature sensor and indicator  200 A,  200 B that will activate at a different predetermined temperature. 
     Each one of temperature sensors  200 A,  200 B can be independently selected to have a thermally actuated element  202 A,  202 B that activates at desired predetermined temperature thresholds, and each one of temperature sensors  200 A,  200 B can be independently installed and replaced in external housing  208 . This allows a user to determine and install, for example in the field, a pair of temperature sensors  200 A,  200 B which will actuate at the correct predetermined temperatures for a particular application. This also allows, for example, a user to remove and replace a first temperature sensor and indicator  200 A that has been activated, while leaving an unactivated temperature sensor and indicator  200 B intact and undisturbed within external housing  208  before reinstalling external housing  208  in the piece of electrical equipment from which it was removed. 
     Temperature sensor and indicator  200 A is configured to activate at a first predetermined temperature (referred to herein as the “low temperature threshold”), and temperature sensor and indicator  200 B is configured to activate at a second predetermined temperature (referred to herein as the “high temperature threshold”) that is higher than the first predetermined temperature. That is, the heat-sensitive element  210 A in temperature sensor and indicator  200 A is selected to melt at the first predetermined temperature, and the heat-sensitive element  210 B in temperature sensor and indicator  200 B is selected to melt at the second predetermined temperature. 
     In some embodiments, the low temperature threshold is selected to be a temperature at which the operator considers that the piece of electrical equipment in which temperature sensor and indicator  200  is installed is overloaded. In some embodiments, the high temperature threshold is selected to be a temperature at which the operator considers that the piece of electrical equipment in which temperature sensor and indicator  200  is installed is extremely overloaded. 
     In some embodiments, temperature sensor  200 A and temperature sensor  200 B are provided with two different indicators  206 A,  206 B, that yield perceptibly distinct visual indications that either temperature sensor and indicator  200 A or temperature sensor and indicator  200 B has been activated. For example, in the illustrated embodiment,  FIG. 9  shows temperature sensor and indicator  200  in the fully unactivated position. 
     As shown in  FIG. 10 , once the low temperature threshold has been exceeded, first indicator  206 A is released and extends a first distance  180  ( FIG. 12 ) distally of the distal end of external housing  208 . Outer surface  240 A of first indicator  206 A is thus rendered visible. 
     As shown in  FIGS. 11 and 12 , once the high temperature threshold has been exceeded, second indicator  206 B is released and extends a second distance  182  distally of the distal end of external housing  208 . Outer surface  240 B of second indicator  206 B is thus rendered visible. 
     In the illustrated embodiment, second indicator  206 B is configured so that the second distance  182  is greater than the first distance  180 . That is, indicator  206 B is configured to project farther away from external housing  208  in the released configuration than indicator  206 A. This allows a user to easily visually determine that both indicators  206 A and  206 B have been released, indicating that the temperature inside the piece of electrical equipment has exceeded the high temperature threshold. Alternatively, if only the low temperature threshold has been exceeded, only indicator  206 A will be visible. In the illustrated embodiment, the positioning of projection  239 A/ 239 B on indicators  206 A/ 206 B is used to vary the distance by which the respective indicator projects, that is, projection  239 A is positioned farther from the proximal end of indicator  206 A than projection  239 B is positioned relative to the proximal end of indicator  206 B, so that indicator  206 A will extend farther in the released configuration than will indicator  206 B when projection  239 A is in contact with its corresponding hard stop and when projection  239 B is in contact with its corresponding hard stop. 
     In some embodiments, the orientation in which external housing  208  is installed in a piece of electrical equipment should be selected to ensure that a user will be able to determine upon a visual inspection whether only indicator  206 A or both indicators  206 A and  206 B have been released. For example, if external housing  208  is installed so that a plane extending between indicators  206 A and  206 B extends vertically relative to the ground, a user will be able to see both indicators  206 A and  206 B at the same time. In contrast, if external housing  208  is installed so that a plane extending between indicators  206 A and  206 B extends horizontally relative to the ground, a user will be able to easily see only one of indicators  206 A and  206 B, and extension of indicator  206 B may obscure indicator  206 A, so that a user may be uncertain if one or both indicators have been released. 
     In some embodiments, the orientation in which external housing  208  is installed can be regulated by the shape of external housing  208 ; for example, external housing  208  may be provided with one flat edge, which can engage with a corresponding flat edge provided in the aperture through which external housing  208  is installed in a piece of electrical equipment. In the illustrated embodiment, external housing  208  is provided with one or more gussets  209 , which may be used to help align external housing  208  correctly during installation in a circular aperture. In some embodiments, gussets  209  may also serve as a fill level gauge. 
     In alternative embodiments, other ways of differentiating indicators  206 A and  206 B can be used so that a user will be able to determine whether only indicator  206 A has been released or whether both indicators  206 A,  206 B have been released. For example, first indicator  206 A can have a brightly coloured outer surface  240 A of a first colour, e.g. yellow, and second indicator  206 B can have a brightly coloured surface  240 B of a second colour, e.g. red. In alternative embodiments, different visual indicators than colour may be used to differentiate indicators  206 A,  206 B. For example, in addition to or as an alternative to being provided with a different colour, the two components may be provided with a different shape (e.g. square for one and circular for the other), different textures (e.g. smooth versus rough) and/or different patterns (e.g. stripes having different thicknesses or orientations), different temperature ratings or other printed indicia on the distal ends of indicators  206 A,  206 B (for example, the predetermined temperature at which each indicator is activated, or the wording “LOW” and “HIGH”, or the like) so that a user can readily determine by visual inspection whether no indicator has been released, only first indicator  206 A has been released, or both indicators  206 A,  206 B have been released. 
     In alternative embodiments, each of indicators  206 A,  206 B can be configured to break or connect separate electrical circuits when indicators  206 A,  206 B, respectively, are released, which will allow the generation of two separate electrical signals, a first when indicator  206 A is released and a second when indicator  206 B is released. 
     In the illustrated embodiment of  FIG. 13 , each one of indicators  206 A,  206 B is provided with an end cap  207 A,  207 B. In some embodiments, the first predetermined temperature threshold can be printed on end cap  207 A and the second predetermined temperature threshold can be printed on end cap  207 B. This allows a user to read the relevant temperature thresholds to confirm the temperature or temperatures which have been exceeded. 
     With reference to  FIG. 14 , an example embodiment of a method  300  of using a temperature sensor and indicator to sense and indicate an increase in temperature above a predetermined threshold is illustrated. At step  302 , the temperature reaches or exceeds the predetermined threshold. At step  304 , a heat-sensitive element contained within a chamber melts, thereby changing from a solid to a liquid. At step  306 , the heat-sensitive element in its liquid state passes through a selectively permeable membrane that is impermeable to the heat-sensitive element in its solid state but permeable to the heat-sensitive element in its liquid state. At step  308 , an activator pin that is biased against the membrane and the heat-sensitive element and that is initially retained in position by the heat-sensitive element in its solid state is permitted to move into the chamber as the heat-sensitive element in its liquid phase flows through the selectively permeable membrane, to thereby place the temperature sensor in its activated configuration. 
     In embodiments in which the temperature sensor is operatively engaged with an indicator, the method  300  further includes providing an indication that the temperature has increased above the predetermined threshold. In such embodiments, at step  310 , movement of the activator pin into the chamber displaces the activator pin from an initial configuration in which the activator pin interposes a plurality of activator feet of an indicator release mechanism. This allows the activator feet to be compressed inwardly together. At step  312 , the inward movement of the activator feet allows chamfered activation surfaces of the activator feet to slide axially in the distal direction relative to correspondingly chamfered retaining surfaces provided on an inner surface of a shell of the temperature sensor and indicator. At step  314 , longitudinal movement of the indicator release mechanism releases an indicator in the distal direction. In some embodiments, at step  316 , a hard stop on the inner surface of a sliding channel within which the indicator moves becomes engaged with a corresponding projection formed on the outer surface of the indicator, to prevent full ejection of the indicator. In alternative embodiments, step  316  is omitted. 
     With reference to  FIG. 15 , an example embodiment of a method  400  of using a temperature sensor and indicator having two distinct temperature sensing and indicating units to sense and indicate an increase in temperature above one or both of two distinct predetermined temperature thresholds is illustrated. The first predetermined temperature threshold is a lower temperature than the second predetermined temperature threshold. 
     At step  402 , the temperature reaches or exceeds the first predetermined temperature threshold. At step  404 , the heat-sensitive element contained within a chamber of the first temperature sensing and indicating unit melts, thereby changing from a solid to a liquid. At step  406 , the heat-sensitive element in the first temperature sensing and indicating unit in its liquid state passes through a selectively permeable membrane that is impermeable to the heat-sensitive element in its solid state but permeable to the heat-sensitive element in its liquid state. At step  408 , an activator pin that is biased against the membrane and the heat-sensitive element and that is initially retained in position by the heat-sensitive element in its solid state is permitted to move into the chamber as the heat-sensitive element in its liquid phase flows through the selectively permeable membrane in the first temperature sensing and indicating unit, to thereby place the first temperature sensor in its activated configuration. 
     At step  410 , movement of the activator pin into the chamber displaces the activator pin from an initial configuration in which the activator pin interposes a plurality of activator feet of an indicator release mechanism of the first temperature sensing and indicating unit. This allows the activator feet to be compressed inwardly together. At step  412 , the inward movement of the activator feet allows chamfered activation surfaces of the activator feet to slide axially in the distal direction relative to correspondingly chamfered retaining surfaces provided on an inner surface of the first temperature sensing and indicating unit. At step  414 , longitudinal movement of the indicator release mechanism releases an indicator of the first temperature sensing and indicating unit in the distal direction. In some embodiments, at step  416 , the indicator of the first temperature sensing and indicating unit is stopped in the fully extended position of the indicator by engagement of a hard stop on the inner surface of a sliding channel within which the first indicator moves becomes engaged with a corresponding projection formed on the outer surface of the first indicator, to prevent full ejection of the first indicator. In some embodiments, step  416  is omitted. 
     At step  418 , which may be at the same time as step  402  or at a different time, the temperature reaches or exceeds the second predetermined temperature threshold. At step  420 , the heat-sensitive element contained within a chamber of the second temperature sensing and indicating unit melts, thereby changing from a solid to a liquid. At step  422 , the heat-sensitive element in the second temperature sensing and indicating unit in its liquid state passes through a selectively permeable membrane that is impermeable to the heat-sensitive element in its solid state but permeable to the heat-sensitive element in its liquid state. At step  424 , an activator pin that is biased against the membrane and the heat-sensitive element and that is initially retained in position by the heat-sensitive element in its solid state is permitted to move into the chamber as the heat-sensitive element in its liquid phase flows through the selectively permeable membrane in the second temperature sensing and indicating unit, to thereby place the second temperature sensor in its activated configuration. 
     At step  426 , movement of the activator pin into the chamber displaces the activator pin from an initial configuration in which the activator pin interposes a plurality of activator feet of an indicator release mechanism of the second temperature sensing and indicating unit. This allows the activator feet to be compressed inwardly together. At step  428 , the inward movement of the activator feet allows chamfered activation surfaces of the activator feet to slide axially in the distal direction relative to correspondingly chamfered retaining surfaces provided on an inner surface of the second temperature sensing and indicating unit. At step  430 , longitudinal movement of the indicator release mechanism releases an indicator of the second temperature sensing and indicating unit in the distal direction. In some embodiments, at step  432 , a hard stop on the inner surface of a sliding channel within which the second indicator moves becomes engaged with a corresponding projection formed on the outer surface of the second indicator, to prevent full ejection of the second indicator. In some embodiments, step  432  is omitted. 
     With reference to  FIGS. 16A-16C , alternative embodiments of a thermally activated element  202 A,  202 B and  202 C are illustrated. Elements of thermally activated element  202 A,  202 B and  202 C that have a function similar to components of thermally activated element  102  are described with reference numerals incremented by 100. 
     With reference to  FIG. 16A , thermally activated element  202 A has a heat-sensitive element  210 , which may be any of the materials described for heat-sensitive element  110 . Heat sensitive-element  210  sits within first chamber  212 , which is defined within an insert  211 . First chamber  212  is sealed across at least a portion of its proximal end by a selectively permeable membrane  216  that is impermeable to the heat-sensitive element  210  when heat-sensitive element is in its solid state, but permeable to the heat-sensitive element  210  when the heat-sensitive element is in its liquid state. Any of the materials and properties described for selectively permeable membrane  116  could be used for selectively permeable membrane  216 . 
     Second chamber  218  is defined proximally of selectively permeable membrane  216 . Second chamber  218  can be defined in any suitable manner that allows second chamber  218  to receive heat-sensitive element  210  in its liquid state. In the illustrated embodiment, a surrounding support  170  is provided to secure selectively permeable membrane  216  in place to sealingly define first chamber  212 , i.e. selectively permeable membrane  216  seals first chamber  212 . Second chamber  218  is defined within the interior space of surrounding support  170 . 
     The distal end of first chamber  212  is also sealed by a membrane  172 . Membrane  172  can be a selectively permeable membrane, but in the illustrated embodiment, membrane  172  is an impermeable but flexible membrane. Suitable materials for membrane  172  include any of the materials used for selectively permeable membrane  116 , but also impermeable flexible materials such as rubber or plastic, including non-porous rubber or plastic. In alternative embodiments, membrane  172  could be made from a material that ruptures due to the force applied by actuator pin  220  after heat-sensitive element  210  has started to pass through selectively permeable membrane  216 . An actuator pin  220  is biased against membrane  172 , and cannot enter first chamber  212  when heat-sensitive element is in the solid state. 
     Activation of thermally activated element  202 A is similar to the activation of thermally activated element  102  previously described, except that when heat-sensitive element  210  melts, it passes in the proximal direction through selectively permeable membrane  216  into second chamber  218 . Actuator pin  220  is biased against membrane  172 , and because membrane  172  is flexible, actuator pin  220  is able to enter first chamber  212 , thereby activating thermally activated element  202 . 
     In some embodiments, as illustrated in  FIG. 16B  as an example embodiment  202 B of a thermally activated element, a supplementary support  174  is provided to help retain heat-sensitive element  210  in its initial position when in the solid state. For example, in embodiments in which selectively permeable membrane  216  is flexible, a supplemental retaining element such as supplementary support  174  may be used to hold heat-sensitive element  210  in position when in the solid state. Heat-sensitive element  210  can still flow into second chamber  218  when in its liquid state despite the presence of supplementary support  174 . Thermally activated element  202 B is thus similar to thermally activated element  202 A, except for the presence of supplementary support  174 . 
     In alternative embodiments, rather than providing a supplementary support  174  to retain heat-sensitive element  210  in its initial position, selectively permeable membrane  216  may be made from a less flexible material, and/or may be made thicker, so that selectively permeable membrane  216  can support heat-sensitive element  210  in its initial position when heat-sensitive element  210  is in the solid state. In alternative embodiments, a combination of a supporting element and a thicker and/or less flexible selectively permeable membrane  216  is used to support heat-sensitive element  210  in its initial position when heat-sensitive element  210  is in the solid state. 
     With reference to  FIG. 16C , a further alternative example embodiment of a thermally activated element  202 C is illustrated. Thermally activated element  202 C is generally similar to thermally activated element  202 A, except that instead of selectively permeable membrane  216 , a block of selectively permeable material  216 B is provided proximally of heat-sensitive element  210  to receive heat-sensitive element  210  when in its liquid state. Selectively permeable material  216 B is impermeable to heat-sensitive element  210  when heat-sensitive element  210  is in its solid state, and therefore holds heat-sensitive element  210  in position against the force applied by biasing pin  220  when heat-sensitive element  210  is in the solid state. When the predetermined temperature threshold is exceeded and heat-sensitive element  210  changes to its liquid state, then heat-sensitive element  210  can enter selectively permeable material  216 B, allowing biasing pin  220  to enter first chamber  212 . Examples of suitable material for selectively permeable material  216 B include porous foams made of any material that is chemically compatible with heat-sensitive element  210 . 
     In some embodiments, thermally activated element  202 A,  202 B or  202 C is used in place of thermally activated element  102  in any of the embodiments described in this specification. 
     With reference to  FIGS. 17A, 17B and 17C , an alternative embodiment of a thermally activated element  302  is illustrated schematically. Elements of thermally activated element  302  that perform a function similar to thermally activated element  102  are illustrated with reference numerals incremented by 200. 
     With reference to  FIG. 17A , thermally activated element  302  has a base insert  311 , a guide washer  319  defining a first chamber  312 , and a shape memory material  310  that initially occupies first chamber  312 . Shape memory material  310  is secured in any suitable manner so as to initially occupy first chamber  312 . An actuator pin  320  that functions in generally the same manner as actuator pin  120  to allow inward movement of activator feet of an indicator after thermally activated element  302  is activated by an increase in temperature above a predetermined level is biased towards first chamber  312 , but initially cannot enter first chamber  312  due to the presence of shape memory material  310 . 
     Shape memory material  310  is selected to deform once a predetermined temperature threshold has been reached. The material from which shape memory material  310  is made can be selected so as to deform at the predetermined temperature threshold by one skilled in the art, so that the predetermined temperature threshold can be provided at any desired temperature. 
     Once the predetermined temperature threshold has been reached, shape memory material  310  deforms, as shown in  FIG. 17B . This places thermally activated element  302  into the activated configuration, and allows the biasing force applied against actuator pin  320  to move the proximal end of actuator pin  320  into first chamber  312 , as shown in  FIG. 17C . The release of an indicator can thus occur in the same manner as described for temperature sensor and indicator  100 . 
     While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are consistent with the broadest interpretation of the specification as a whole.