MEASUREMENT AND CONTROL OF TEMPERATURE 



except water as a medium. If one considers the critical vapour pressure of a 

 sealed vessel of suitable liquid, there occurs a narrow range of temperature, 

 when the liquid is vaporized, with a sudden large, and mechanically powerful 

 change in volume. In practice, a bellows-type capsule of copper or bronze 

 is filled with a suitable mixture of liquids, such as alcohol and ether, and 

 sealed. Below a particular temperature the contents are at sufficient pressure 

 to be entirely liquid; on heating a temperature is reached where rapid 

 vaporization occurs, and under considerable pressure the capsule expands, 

 often to many times its original length, and with appreciable force. Above 

 this temperature again, only slight typical expansion takes place. The 

 range covered by any one capsule usually amounts to a few centigrade degrees. 

 It finds a most useful and efficient application in incubators where it operates 

 heater switches directly : another advantage is that a new range is immediately 

 obtained by substituting a different capsule, and the 'fine adjustment' of 

 the system need only cover the few degrees of the individual capsule. On the 

 other hand, though the capsule may appear small in size the energy it absorbs 

 in operation is latent heat of vaporization, usually many times the specific 

 heat of a liquid, so that in this respect it is an element of high 'thermal 

 capacity'. 



Approach characteristics 



So far we have considered only the theoretical aspects of fixed-heater 

 control when in operation at the controlled temperature. In many experimen- 

 tal devices the behaviour of the mechanism when it has had to depart from 

 control is of considerable importance, e.g. the speed of recovery of tempera- 

 ture of an enclosed air space which has temporarily been cooled by opening 

 a door on to the ambient ; experiments in which material might be severely 

 damaged by high temperature overshoot; equipment giving an alternating 

 diurnal temperature cycle, where the temperature must rise and fall between 

 two controlled levels at pre-determined times. For fixed heaters and simple 

 thermostats the heating rate during approach to the controlled temperature 

 is clearly defined by the size of the heater {Figure 29.13a), unless one is 

 operating very close to the maximum temperature obtainable by the heater. 

 If a very rapid return after intermittent cooling is an essential requirement 

 of the equipment it may be necessary to have a heater much larger than 

 would be used for maintaining the desired temperature; there would 

 consequently be poor regulation. These circumstances call for a split heater, 

 of which the larger portion, a booster, is switched out when the thermostat 

 approaches the control temperature. Even so, the thermal capacity of the 

 booster heater and its supports may lead to overshoot unless it is switched 

 off" sufficiently early {Figure 29.13b). 



The behaviour of control gear of an enclosed air space while it is temporarily 

 open to a cool ambient must be closely watched. Regardless of the thermo- 

 static arrangement, this will usually lead to all heaters being fully energized; 

 if opening the box also interferes with the circulation system there may be 

 violent local heating. 



The form of approach in single heater equipment with overshoot is often 

 followed by a typical train of damped oscillations {Figure 29.13c) until 

 control is obtained — this is not a danger sign of instabihty. As a rule the 



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