TEMPERATURE CONTROL 



have been poured into the body in general ; but the solution to this problem 

 is not that of placing the T.S.E. very close to the heater, for if the body is 

 large, the outlying parts of it may be hardly controlled at all. What is 

 essential is to improve the thermal link by such devices as the forced circula- 

 tion of the fluid medium (which will usually make up the body) as well as to 

 ensure the maximum of contact between the T.S.E. and the material of the 

 body. On the other hand, very vigorous stirring, especially of air, will 

 introduce an appreciable heating effect in itself, especially if the stirring 

 motor is inside the environmental enclosure. 



Switching thermostat at different temperatures in its heater range 



For a given fixed heater capacity, the range of available operating tempera- 

 ture will span from that of the ambient to that obtaining for the continuous 

 operation of the heater; intermediate temperatures may vary from circum- 

 stances when the heater is operating for a minimal part of the time in one 

 cycle of operations, to a maximal part of the time. From the foregoing 



Figure 29.9 Thermograph record of a crude switching thermostat showing 

 extreme temperature swing when working close to ambient temperatures 



analysis it follows that the poorest control is obtained when operating at 

 temperatures nearest to the ambient; cooling is slow and heat enters in 

 short bursts of high level (see Figure 29.9), while at the highest levels of 

 temperature the high cooling rate ensures that the effect of cutting the heat 

 supply is rapidly accommodated, with the result that the frequency of a cycle 

 of thermostat operation is much greater here. Though versatility of tempera- 

 ture range may be a requirement of a piece of equipment, it is obvious that 

 the size of the heater should be most carefully chosen where one temperature 

 is particularly required. 



Split and multi-stage heaters 



A great deal of the inherent temperature fluctuation in the switching 

 thermostat system of control is thus due to the characteristics of the heater 

 itself. Especially for higher temperature work, where comparatively large 

 heaters are concerned, the heater may be split into one or more units with 

 advantage. In the simplest system, especially where one particular tempera- 

 ture or small range is required, this takes the form of a continuously 

 operating heater raising the body to a particular level, together with a smaller 

 heater operated by the thermostat which 'tops up' the system. It will be 

 noted that in large equipment this has the added advantage that the major 

 part of the heater current does not have to be switched by the relays of the 

 thermostat; on the other hand, the largest fixed heater which can be enter- 

 tained is that bringing the body below the lowest temperature required 



397 



