MEASUREMENT AND CONTROL OF TEMPERATURE 



not attempt to run cooler spirals in parallel — run them in series, with the 

 coolest tank nearest the outlet of the exchanger reservoir, for the problem of 

 maintaining many flows in parallel from one pump is extremely difficult. For 

 large installations it will prove much more economic in terms both of size 

 of refrigerator unit and also of refrigerator running costs if one has constantly 

 maintained heaters and controls on the refrigerator circulating fluid (Figure 

 29.18) by a solenoid-operated valve. It will be noted that the thermostat 

 now is required to introduce cooling on the 'make' part of the cycle. 



Very fine limits of control are obtained by using a mercury-toluene 

 regulator, or mercury-contact thermostat, operating through a single stage 

 electronic relay which in turn energizes a mercury relay switch — operating 

 the eventual heater or cooler as the case may be {Figure 29.19). Almost any 

 power triode, tetrode or pentode can be used, but there is a great deal to be 

 said for using a very robust valve operating well below its rated anode 

 voltage, in which case its Hfe will be almost indefinite. 



There is the further advantage that in laboratories where 100-0-100 V d.c. 

 is available such a unit can be run without a special power pack ; alternatively, 

 germ.anium rectifiers could be used to obtain suitable plate currents. 



Air enclosures 



The most frequent biological requirement is the plant-animal breeding 

 chamber. More variations in design and principle of these rooms and boxes 

 have been tried — and indeed are in existence — than in any similar piece of 

 laboratory equipment setting out to achieve one object, the provision of 

 constant air temperature with additional lighting equipment. 



Insulation is of great importance, and at least two in. of cork, slag 

 wool, expanded plastic foam or similar material is needed, which must be 

 securely sealed into water-tight compartments, for moisture degrades insula- 

 tion remarkably. An indication of the efficiency of insulation can be gained 

 from the tests which should be carried out to determine heater size; 5-10 W 

 per ft.^ of air should be sufficient to maintain the enclosure at 20-30°C above 

 ambient. An ordinary tungsten filament lamp (which produces 95 per cent 

 of its rated energy consumption as heat) will suffice as a test heater. The 

 eventual heater {Figure 29.20) should be wound of nichrome wire, so that air 

 will circulate through the wire. For simple systems asbestos 'blanket' heaters 

 are ideal. The heater should be so shrouded that the circulating air is 

 positively passed through it. A low-pressure high-volume fan is needed to 

 provide good air circulation — remembering that if the motor is inside the 

 enclosure it also will dissipate its rated consumption in heat, either directly 

 or via the energy of stirring. For temperatures below ambient where 

 refrigeration is used it is again essential to blow the circulated air through a 

 large surface area radiator ; unless a special mark-to-space method is adopted 

 (see below), it is not recommended to control on the cold side. Usually one 

 would place cooler and heater in the same shroud so that the air is cooled, 

 and then heated under thermostatic control; care should be taken that 

 condensation on the cooler does not drip on to the heater. 



It is appreciated that the temperature of air in an incubator can be held 

 to reasonable limits (say d=i°Q using a relatively crude thermostat; incuba- 

 tors normally require many hours to come into equilibrium ; they are slow to 



408 



