MEASUREMENT AND CONTROL OF TEMPERATURE 



energy; thermal insulation cannot be at all complete, however, and for all 

 practical purposes heat loss will be appreciable. The primary problem is to 

 compensate for changes in the amount lost because of variations in the 

 ambient (which may be due to radiant and convective loss as well as purely 

 conductive loss). It might be suggested that the control mechanism should 

 therefore follow fluctuations in ambient temperature, but this is not the 

 basis normally used. 



The main technical problem of temperature control, then, is to regulate 

 the amount of, and rate of supplying, heat energy. For this an electric 

 resistive heater is used, fed from a fixed supply voltage. When operating 

 continuously the maximum body temperature will be obtained, in relation 

 to a given ambient temperature ; for all lower body temperatures the heater 

 must operate discontinuQusly, and control consists of mechanisms to open 

 and close the heater circuit appropriately. 



Fixed-heater switching thermostat 



Information on the temperature of the body is obtained by a temperature 

 sensitive element (T.S.E.) — this is not required to present temperature in 

 the sense of a thermometer, an independent thermometer should always 

 be used for that purpose. A T.S.E. linked to a switch, or other control 

 gear for maintaining a constant temperature, is called a thermostat. The 

 simplest switching thermostat works in this way: the T.S.E. is linked to 

 a switch in the heater circuit such that the heater circuit is closed at ambient 

 temperature, and as the body heats, reaches a pre-determined temperature 

 at which the T.S.E. switches the heater 'off'. By loss to the ambient the 

 temperature of the body falls, thus causing the T.S.E. to change and close 

 the heater circuit, introducing more heat into the body and starting a second 

 cycle of a repeated series of switchings. Provided that the temperature of 

 the ambient does not fall so low that continuous operation of the heater 

 cannot achieve switching temperature in the body, a degree of temperature 

 control has been achieved, despite ambient fluctuations. It is also obvious 

 that no control will be obtained if the ambient rises above the selected 

 control temperature. 



This system is, inherently, one of control within limits for the following 

 reasons: 



Thermostat differential — A mechanical system in which the T.S.E. directly 

 opens and closes the heater circuit is converting the continuous movement 

 of the T.S.E. into a make/break action at a defined point. Any appreciable 

 heater current at a practical voltage cannot be interrupted or turned on by 

 a slow-acting contact system; the T.S.E. must thus actuate a 'snap-action' 

 switch. This is most frequently achieved by a bi-stable spring or by biasing 

 with a permanent magnet ; there is no direct mechanical method of achieving 

 a q.m.b. switch from a continuously moving actuator without appreciable 

 hysteresis, for the temperature at which the heater circuit is opened will be 

 appreciably above that at which it is closed, and one is very lucky to get a 

 commercial direct-switching thermostat in which this differential is even 

 as small as l°C. Further, since such a switch will need appreciable acti- 

 vation force, the T.S.E. must consist either of a substantial bimetal rod, or 

 of a liquid-filled bellows, neither of which can (see Thermometers) be at all 



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