This invention relates to a thermostatic control and in particular, though not exclusively, to a thermostatic control for responding to temperatures greater than 100° C.

Thermostatic controls are employed in many applications such as to provide "boil-dry" protection for water heaters, washing machines and dishwashers, and also as a high temperature cut-out for central heating boilers. Other common applications include use as a blocked flue sensor and over-heat safety cut-out for compressor discharge pipes.

A well established design of a thermostatic control for use in the

aforementioned applications comprises a sealed enclosure which

incorporates a bellows assembly in combination with a capillary tube which typically is filled with a mixture of mono-ethylene glycol (hereinafter referred to as MEG) and water. In use the capillary tube is exposed to a heat source the temperature of which is being monitored. To ensure good and rapid conduction of heat from the heat source to the liquid within the capillary tube it is conventional to manufacture the tube from a metal such as copper which exhibits good thermal conductivity as well as being suitable for economical manufacture of a small bore capillary tube.

A particular and advantageous feature of the use of a mixture of MEG and water as the fill for the thermostatic control is that there is a very large difference in volume between the liquid and vapour phases of the fill. The boiling point of the fill, and thereby the temperature of operation of the thermostatic control, can be pre-selected by adjustment of the ratio of the MEG to water.

However one undesirable feature of MEG is that over time, and particular when exposed to temperatures above 100°C, it is prone to thermal decomposition which reduces the boiling point of the fill and thus the temperature at which, for example, the thermostatic control effect a switching action. That rate of decomposition increases with the increase in the temperature to which the thermostatic control is exposed. Although the resulting premature operation of the switching action is fail-safe insofar as the equipment protected by the thermostatic control is not exposed to excessive and potentially damaging temperatures, there is an undesirable premature disablement of controlled equipment and an unduly frequent need to replace the thermostatic control.

However, it is not only exposure of MEG to high temperatures that promotes the decomposition and reduction boiling point. The rate of decomposition is adversely affected and accelerated by exposure of the MEG to the commonly employed copper material of the capillary tube. This arises because the copper has a catalytic effect on the rate of decomposition of the MEG. The exposure to copper and many other such metals significantly aggravates the problem of providing a thermostatic control that does not unduly rapidly change in characteristics.

Consideration has been given to manufacturing the capillary tube from a material which does not cause a catalytic effect but up to the present time no material has been identified as being suitable for providing the required thermal conductivity and for being cost effective in manufacture.

The present invention seeks to provide an improved thermostatic control in which at least some of the disadvantages of known thermostatic controls are mitigated or overcome.

In accordance with one aspect of the present invention there is provide a thermostatic control comprising a sealed container wherein the sealed container contains a fluid comprising at least partially a glycol and wherein the sealed container comprises a surface of metal in contact with the fluid, the metal of said surface being a metal which acts as a catalyst to affect adversely the rate of thermal decomposition of glycol, said fluid further comprising a complexing compound of a type which, in use of the

thermostatic control, serves to prevent or inhibit the catalytic action of the metal surface on the thermal decomposition of the glycol by forming a stable complex with said metal surface.

In accordance with another aspect of the present invention there is provided a method for preventing the catalyzed decomposition of glycol fluid in a thermostatic control of the type wherein the thermostatic control comprises a metal surface in contact with the glycol and wherein said metal surface acts as a catalyst to affect adversely the rate of thermal decomposition of the glycol, said method comprising providing a complexing compound in combination with said glycol fluid.

In accordance with a further aspect of the present invention there is provided a method for preventing or inhibiting the catalysed decomposition of glycol fluid in a thermostatic control comprising providing a complexing compound as a constituent of said glycol fluid.

The glycol may comprise mono-ethylene glycol. The fluid may comprise water whereby the boiling point of the fluid is reduced from that of 100% glycol, which typically has a boiling point of 197 °C.

The fluid may be a mixture of at least 62% of a glycol, such as MEG and less than 38% of water (or water and any other additive) thereby to provide a boiling point of at least 130°c.

The complexing agent is selected to provide a degree of chemical isolation of the material of the capillary tube from the fluid within the tube. It may be a heterocyclic compound. Particularly, but not exclusively, for a copper capillary tube a suitable complexing agent is the heterocyclic compound benzotriazole (hereinafter referred to as BTAH). The BTAH is found to form a stable complex with the internal surface of the copper tube and thereby functions advantageously to at least partly suppress the catalytic effect which the copper would otherwise have on the glycol.

Preferably the complexing agent is present in a concentration of at least 5mM.

The thermostatic control preferably contains the fluid substantially at atmospheric pressure when the fluid is below the boiling point thereof.

Preferably the fluid of the thermostatic control is contained within a capillary tube and a variable volume enclosure connected to the capillary tube whereby the variable volume enclosure undergoes expansion as the fluid vapourises and pressurises the enclosure.

Expansion of the enclosure may be employed to operate a switching device that in turn may be employed to de-activate monitored apparatus such as apparatus that requires a boil-dry safety cut-out or other such safeguard.

The variable volume enclosure may comprise relatively moveable

components, such as a piston and cylinder. Alternatively it may be devoid of relatively moveable components such as components which slide relative to one another.

An example of a preferred type of enclosure is a bellows or diaphragm, and an example of a suitable material for the enclosure is stainless steel.

The thermostatic control may comprise a variable volume enclosure in communication with a capillary tube having a first portion for contact with a heat source and a second portion which extends from the first portion to the variable volume enclosure. The length of the first portion usually will be determined by the required application of the thermostatic control. The total length of the capillary tube typically will be in the range 50cm to 150cm.

The capillary tube may be of any known shape appropriate to the required application. Thus it may be of a substantially linear form or may comprise at least in part a section of a spiral or helical shape.

The capillary tube may be a copper tube. The fluid in the liquid phase may be in contact solely with a surface of copper. The quantity of fluid in the liquid phase may be less than the volume of the capillary tube at all temperatures within the working temperature range of the thermostatic control, for example a range of 0 degrees centigrade to 190 degrees centigrade.

Other constructional details of the thermostatic control may be similar to those known per se and include, for example, the constructional features of the fail-safe L series fixed setting temperature limit controls of Drayton (RTM) Controls.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying Figure 1 which is a schematic view of a thermostatic control in accordance with the present invention and Figure 2 shows a thermostatic control which incorporates a diaphragm valve, shown in cross-section, for use instead of the bellows of Figurel .

A thermostatic control 10 comprises a copper capillary tube 1 having a first portion 11a and a second portion 11 b which communicates with a stainless steel bellows 14.

The first portion 11a is positioned adjacent and in good thermal contact with the surface 12 of a monitored heat source 13. The upper end 15 of the bellows bears against a contact arm 16 that is pivotally secured to a support which defines a pivot position 17.

To one side 18 of the pivot position the arm is moveable to operate an electric switch 19 that controls, typically indirectly, operation of the heat source.

The bellows contacts the other side 20 of the arm 16 and that side of the arm also is acted on by a spring 21 that urges the arm to lie in a position in which the electrical switch is closed, thereby to allow operation of apparatus associated with the heat source.

When the fluid within the capillary tube is heated to above the boiling point of the fluid the resulting vapourisation causes the expansion of the bellows which acts against the spring force to move the switch to an open position as shown in Figure 1. Thus when the heat source exceeds a pre-determined temperature the operation of equipment associated with the heat source is terminated.

In a second embodiment of the present invention as shown in Figure 2 a thermostatic control 30 comprises a capillary tube 31 which contains the same fluid as that described in relation to the first embodiment, but the vaporisation of the fluid when heated causes deflection of a diaphragm 32. A sufficient movement of the diaphragm then brings the diaphragm into contact with a plunger 33, typically a spring biased plunger, to cause movement of the plunger and consequential operation of a switch 34.

In accordance with the present invention the reliability of operation of the temperature control as compared to known thermostatic controls, and stability of the boiling point is enhanced. In the aforedescribed embodiments of the invention that is achieved by employing as the fluid a mixture of 38% water with the balance being MEG containing a BTAH concentration of approximately 5mM.

In use the BTAH is found to form a stable complex with the internal surface of the copper capillary tube and thereby prevent or at least inhibit and mitigate the catalytic effect which the copper would otherwise have on the thermal decomposition of the MEG.

Accordingly the propensity for thermal degradation of the MEG is beneficially reduced and an enhanced stability of switching temperature is achieved.