GB2295490A - Thermostat - Google Patents

Abstract

A thermostat 1 has a tube 3 and a rod 9 located inside it. The tube 3 and rod 9 are made of materials respectively having different coefficients of thermal expansions. A fluid-filled sensor 21 is arranged within the tube 3 to provide a safety cut-out function. The fluid can be liquid or vapour/gas. <IMAGE>

Description

THERMOSTAT The present invention relates to a thermostat of the kind which comprises a tube and a rod located within the tube, the tube and rod being made of materials having different coefficients of thermal expansion. Normally, both rod and tube will be made of different metal materials and the rod with have a lower coefficient of thermal expansion than the tube. Thermostats according to the present invention are especially, although not exclusively, suited to use in immersion heaters.

Thermostats of the aforementioned kind are well known. It is common to provide a safety cut-out switch within a head of the thermostat (which supports the tube). In the event of failure of the thermostat, the safety cut-out switch can break the electric current to the heater before the temperature of the water rises too high. One variant of this kind of device entails use of a fuseable link. If the water temperature rises too high, the fusible link melts and breaks the current.

One disadvantage with the aforementioned kind of sensor mounted within the head is that it is remote from the main volume of water and therefore cannot react precisely to changes in water temperature.

This disadvantage has now been overcome by a new design of thermostat. Thus, the present invention provides a thermostat comprising a tube and a rod located within said tube, the tube and rod being made of materials respectively having different coefficients of thermal expansion, and a fluid-filled sensor arranged within said tube for providing a safety cut-out function.

With the thermostat of the present invention, the sensor can be positioned at any point along the tube to avoid, or take advantage of, temperature gradients within the water.

Normally, a thermostat switch is housed within the head and is operable by mean of expansion or contraction of the rod. However, in the case of the present invention, a safety cut-out switch can also be conveniently housed within the head but operable in response to the sensor which itself, is located within the tube.

For the avoidance of doubt, in the context of the present invention, the term "rod" includes elongate members which are solid, or partly or totally hollow.

The tube and rod can independently, be of any convenient shape or shapes in cross-section. However, clearly it is convenient for both to have a substantially circular outer profile.

In a preferred embodiment, the safety cut-out switch is operable by means of an actuator connected to a diaphragm which forms part of a sealed unit. This sealed unit can communicate with the sensor by means of a conduit.

The safety cut-out can be manually re-settable or of self-re-setting type. This provides an advantage over the fusible link kind of cut-out which is obviously a "one shot" device.

In the context of the present invention, the term "fluid" includes both liquids, e.g. non-flammable oils, or gases or vapours. Conveniently, the sensor is in the form of a phial. If the fluid is a liquid, it is expansion of the liquid within the sensor itself which would actuate the cut-out switch. However, in the case of a vapour or gas, if any one portion of the sensor, diaphragm or interconnecting conduit gets hot, then pressure develops in proportion to the temperature at that point, to trip the cut-out switch.

The present invention will now be explained in more detail by the following non-limiting description of a preferred embodiment and with reference to the accompanying drawing, in which Figure 1 shows a part cross-section of a thermostat according to the present invention.

Specifically, Figure 1 shows a thermostat 1 according to the present invention, comprising a metal tube 3, supported at one end 5 on a head 7. Within the tube 3, is located a metal rod 9, which is eccentrically located within the tube to provide a space for the phial.

One end 11 of the rod 9 terminates in an actuator 13 which operates a movable contact 15 of a thermostat switch 17. The metal of the rod 9 has a lower coefficient of thermal expansion than the metal of the tube 3. The tube 3 and rod 9 are connected at their distal ends. Therefore, the temperature to which the rod and tube are exposed will cause the near end 11 of the rod to move inwardly or outwardly, relative to the near end 5 of the tube, and so the actuator 13 will open or close the thermostat switch.

A part 19 of the tube 3 adjacent the rod 9, contains a phial 21 which contains a liquid. It will be appreciated from the foregoing description that an alternative design could be based on a vapour or gas instead of a liquid. The phial 21 communicates with a sealed unit 23, within the head 7, by means of a conduit 25. In the case of a gas or vapour operated system, the sensor could be in the form of a phial or as a single diameter tube.

The sealed unit 23 is provided with a diaphragm 29 (or alternatively a bellows) connected to an actuator 31. A spring 27 is optionally provided if needed to enhance the operating force. The actuator 31 operates a movable sprung contact 33 of a safety cut-out switch 35.

If the thermostat fails and the temperature of the water in which the tube 3 is situated, rises too high, the liquid in the phial 21 will expand by more than a predetermined amount so that the pressure in the sealed unit 23 will increase to a level whereby flexing of the diaphragm 27 will cause the actuator 31 to move, to open the cut-out switch and break the current.

In the event of the thermostat failing to operate and the cut-out switch 35 being tripped in the aforementioned manner, it may subsequently be re-set by means of a manual slidable re-set 37, although this is an optional feature.

It will further be appreciated that although in this preferred arrangement, the sensor mechanism is arranged to operate a separate cut-out switch, different from the main thermostat switch, an alternative arrangement could be realised whereby the sensor is arranged to operate on the same main thermostat switch which is operated by the rod 11, although such a system is normally less preferred.

When the thermostat is manufactured, when the phial 21 and the associated hydraulic components 23, 25 are in the unfilled condition, the diaphragm is held open, i.e.

in an expanded position by its own spring force and any additional spring bias which may be provided. The diaphragm 29 is compressed and the system filled and sealed. Finally, the diaphragm 29 is released leaving the system working under vacuum pressure. It will be appreciated that if the system leaks or is cut, the diaphragm 29 will expand and trip the safety cut-out switch. The optional spring 27 provides an additional force in this event. Similarly, physical damage causing crushing of the hydraulic part of the system would cause a pressure surge to trip the cut-out.

In the light of this description, modifications of the described embodiment, as well as other embodiments, all within the scope of the invention as defined by the appended claims, will now become apparent to persons skilled in the art.

Claims (9)

CLAI MS:

1. A thermostat comprising a tube and a rod located within said tube, the tube and rod being made of materials respectively having different coefficients of thermal expansion, and a fluid-filled sensor arranged within said tube for providing a safety cut-out function.

2. A thermostat according to claim 1, further comprising a head on which said tube is supported, said head incorporating a thermostat switch operable by means of expansion and contraction of the rod, and wherein said head further incorporates a safety cut-out switch operable in response to said fluid-filled sensor.

3. A thermostat according to claim 2, wherein said safety cut-out switch is operable by means of an actuator connected to a diaphragm or bellows, which diaphragm or bellows forms part of a sealed unit.

4. A thermostat according to claim 3, wherein the sealed unit communicates with the fluid-filled sensor by means of a conduit.

5. A thermostat according to any of claims 2, to 4, wherein the safety cut-out switch is manually re-settable.

6. A thermostat according to any preceding claim, wherein the safety cut-out switch is self-re-setting.

7. A thermostat according to any preceding claim, wherein the fluid in the fluid-filled sensor is a liquid.

8. A thermostat according to any of claims 1 to 6, wherein the fluid in the fluid-filled sensor is a gas or a vapour.

9. A thermostat substantially as hereinbefore described with reference to the accompanying drawing.