TEMPERATURE CONTROL 



sensitive. Where any attempt is made to reduce the mechanical differential, 

 it usually results in a metastable condition of the contacts, producing 

 continuous arcing, and the worst possible conditions for the operation of 

 metal switch contacts. 



Reduction of differential — The two principal methods of reducing the 

 mechanical differential are by the use of mark-to-space systems (see below) 

 or by the use of relays. As soon as the contacts operated by the T.S.E. are 

 freed of the requirement to switch an appreciable heater current, the size 

 and nature of the T.S.E. can be reduced to sensitive proportions, and if 

 the relay current is sufficiently small the thermostat contacts may not need 

 to open and close as quickly. However, it is still not possible to use the 

 most sensitive contact thermostats directly with even the smallest solenoid- 

 type of relay; they should be used with either bimetal or capsule type 

 elements, or with very robust mercury-toluene regulators. 



Ideal make-and-break relay — In the most refined systems of this type, the 

 contacts of the T.S.E. operate through a single-stage valve relay, the anode 

 of which controls a solenoid relay which in turn switches the heater current. 

 This system has been used by the author with complete success, particularly 

 for the control of aquarium tanks to limits of 1/50°C {Figure 29.19). The 

 contacts of the T.S.E. may be handling currents of a few microamps at a 

 few volts, which can thus be interrupted by a minute movement of the 

 element, however slow, without arcing, oxidation or similar attendant 

 problems. The mercury-contact thermometer or sensitive mercury-toluene 

 regulator is usually used in such systems. 



Thermal hysteresis 



It might appear from the above that, with the most refined system, 

 immediately the required temperature had been reached and the heater 



Figure 29.8 Typical form of thermograph record of crude switching thermostat, 

 showing that the total temperature swing is greater than the mechanical 



differential of the switch 



switched off the temperature will fall, and the heater immediately be actuated 

 again, leading to a very rapid oscillation of the contacts (and hence of the 

 relays). These conditions are in fact extremely difficult to achieve. A 

 heater has a finite and often quite substantial thermal capacity; further, 

 if it is to heat a body, the only way it can transfer heat to it is to be at an 

 appreciable temperature above that body. The interruption of power to the 

 heater causes, immediately, only a comparatively gradual decline in heater 

 temperature, and hence of supply of heat to the body; thus the temperature 



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