TEMPERATURE CONTROL 



a 1 -5 V dry cell should operate for a few days and its change in voltage 

 should not be more than the sensitivity of the measurement. In view of 

 the isolated circumstances under which the machine may be used, a switching 

 system uses the same meter to set up and check a reference battery voltage. 

 Temperatures are displayed in tliree ranges, but by adjusting the values of 

 the individual resistances (Figure 29.7), these can be set to requirement. 



Recording of temperature 



The common thermograph consists merely of a bimetal element directly 

 driving through levers a recording pen on a clock drum. This is about the 

 only thermal element with sufficient energy of change to drive a recorder 

 direct; but the element has a very great thermal capacity and is only useful 

 for meteorological work where changes are slow and accuracy does not 

 need to be better than |°C at best. For more accurate work, or where 

 rapid changes have to be followed, one is virtually limited to using an 

 element giving rise to a proportional electrical output v/hich can be linked 

 to a pen-recorder (see transducers) or string-recorder. Both of these will 

 need at least one d.c. amplifier stage beyond anything that a thermocouple 

 or thermistor bridge can produce. A circuit described below under 'tempera- 

 ture control by variable heater' shows a thermistor controlling a two 

 stage d.c. amplifier; its output (a power tetrode) could be connected to a 

 standard recording device to provide a good temperature recording apparatus. 



TEMPERATURE CONTROL 



Introduction 



To the biologist, temperature control will normally mean the maintaining 

 of a fixed chosen temperature in an enclosure of water or air containing 

 living material; this enclosure may vary from a constant temperature 

 room or incubator running continuously at one pre-determined temperature 

 to a small experimental enclosure in which a wide range of temperatures is 

 to be produced. As accessories, he may be concerned with ovens, hot 

 plates and autoclaves. 



Theory of temperature control 



If we consider a discrete body at ambient temperature and introduce 

 heat energy into it its temperature will rise proportionally with the amount 

 of heat added. As the temperature of the body rises, and a temperature 

 difference is set up between it and the ambient, the body will lose heat into 

 the ambient at an ever increasing rate and eventually a dynamic equilibrium 

 is set up; for any given rate of heat input, and of ambient temperature, 

 there will be a body temperature which will remain constant so long as the 

 ambient temperature also remains so, but it will depend on ambient tempera- 

 ture as much as on heat input. The heat put into the body may be considered 

 in two parts — that needed to raise the temperature of the body, due to its 

 thermal capacity, and that continuously provided to replace losses into 

 the ambient. At equilibrium, the whole heat input equals the heat loss. 



Now if a body could be thermally isolated, the problem of temperature 

 control would be simply a matter of inserting a suitable amount of heat 



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