MEASUREMENT AND CONTROL OF TEMPERATURE 



of the body continues to rise after the thermostat has acted, then reaches 

 equihbrium, and then starts to fall towards a temperature at which the thermo- 

 stat is closed. Correspondingly, the heater does not attain full working 

 temperature immediately the supply is restored by the consequences of the 

 body cooling. Thus the temperature of the body continues to fall, then 

 reaches equilibrium, and only then passes into the 'rising' part of the cycle 

 (Figure 29.8). This thermal cycle, then, has to be added on to such thermostat 

 differential as exists, and with an unfortunate choice of components it can 

 amount to several Centigrade degrees. 



Heater design 



The basic fact that a heater must be above the temperature of that which 

 it heats must mean that there is always a temperature gradient from that part 

 of the 'controlled' body nearest the heater, dropping in all directions towards 

 the ambient. The higher the operating temperature of the heater, the greater 

 will be the variation within the various parts of the controlled body. 

 Conversely, the lower its temperature the less efficient will be its transfer of 

 heat energy, into which process the thermal capacity of the heater material 

 as well as its conductivity enter. Choice of the operating temperature of the 

 heater may also involve economic considerations. Generally speaking, low- 

 temperature heaters should be used with gases, while only higher tempera- 

 tures (or very massive heaters) will be able to supply the heat quantity 

 required by typical liquids and solids. Where fluids are force-circulated 

 higher temperatures can be used, but if convective stirring predominates the 

 heater should be at as low a temperature as is possible. Similarly, a form of 

 phase shift is introduced by the accessory parts of the heater, and most 

 especially by any covering over the element; these accessory parts will be 

 heated, and will cool, at a different rate to the heater element itself; they are 

 a mixed blessing. For gases, the secondary parts of the heater must be 

 reduced to the absolute minimum, and a bare heater wire should make 

 direct contact with the gas. For liquids, maximum conductivity from the 

 heater may be achieved by, for example, supporting the heater wire by 

 ceramic beads in an oil-filled metal tube or placing a spiral of wire so that 

 it lines the inner walls of an oil-filled glass tube. It may be found that the 

 'smoothing' of the discontinuous heat supply by the thermal capacity of the 

 heater enclosure does in fact assist in the close control of a large volume of 

 water. 



Heater-thermostat link 



It is essential to realize that the only part of a body whose temperature is 

 actually being 'controlled' is that in direct contact with the T.S.E. of the 

 thermostat ! The information on which the heater is being controlled is that 

 presented at the thermostat; in general it is also here that we can expect the 

 smallest fluctuation in temperature. We can in fact regard the control 

 mechanism as an infinite gain amplifier to which the thermal link between 

 the heater and the thermostat adds overall negative feedback. There is 

 however an appreciable time constant in this link. It may be several seconds 

 after the heater has come up to operating temperature that the temperature 

 of the T.S.E. starts to rise, by which time an excessive amount of heat may 



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