MEASUREMENT AND CONTROL OF TEMPERATURE 



electric heater on a basis of 10 W per litre and leaving the tank (stirred) to 

 obtain its highest equilibrium temperature. A rough calculation based on 

 this information (W lost per hour per temperature unit above ambient) will 

 enable one to obtain the size of heater for the temperature range required. 

 Up to 50 W might be switched direct on a bimetel thermostat (Figures 29.14 



Figure 29. 14 Layout of tube-enclosed 



heater (lih), stirrer {s) and thermostat 



element (ts) in an aquarium tank 



-• n.j •- 



Heater 



V 



n 



T.S.E. 



Figure 29. 15 Simplest thermostat circuit 



in which the T.S.E. switches the heater 



directly 



tsc 



,_ 







r\ 



y 



X! 



> Heater 



Hg 



Figure 29.16 Schematic layout for control 

 by robust mercury-toluene regulator 

 {t, Hg). Temperature setting by the 

 insulated screw contact {tsc) and relay (r) 

 current adjusted by resistance R 



bmc 



Figure 29.17 Schematic form of proportion- 

 ing head on mercury-contact regulator. The 

 adjusting screw is moimted on a bimetal 

 strip {bm) heated by the winding (h) ener- 

 gized through the contacts (bmc) which are 

 opened and closed by the strip's own 

 movements 



and 29.15). Above this load, and for finer control, a relay should be used, 

 or, better, the relay should be controlled through a robust mercury-toluene 

 regulator (Figure 29.16). The addition of a proportioning head (Figure 29.17) 

 will obtain control to better than 1/10°C. 



Temperatures below ambient in small installations are best obtained by 

 running a cooler coil from a refrigerator exchanger; glycerin/water is advised 

 for the circulating fluid as it is non-corrosive. For mukiple tank systems do 



406 



