PRACTICAL METHODS FOR TEMPERATURE CONTROL 



positive heating used — this is obviously very expensive unless the installations 

 are small. The body may be provided with a continuously operating heater, 

 or cooler, with control of the other factor — the most usual way of doing 

 things, though again this is extravagant of energy ; where economy is essential 

 it is better to have continuous heating with control of cooling, though the 

 reverse process leads to the closer control. The dual control of heat and of 

 coolth is fraught with every possible snag. This is especially true if total 

 heater and cooler switching is used ; either there must be a point at which 

 one switches from heater to cooler or there will be a range over which both 

 heater and cooler are operating simultaneously — and no pack of hounds 

 will ever hunt as efficiently. The author has overcome this problem in a 

 machine controlling the temperature of circulating fluids by using a special 

 mark-to-space system described in the next section; it saves 50 per cent 

 energy at intermediate temperatures, and can move from the high tempera- 

 ture of continuous heating to the low point of continuous cooling. The 

 device is mechanically complex but is particularly useful for such problems 

 as plant-breeding in the laboratory. 



External load changes 



Of the special problems arising in constant temperature enclosures, the 

 commonest is the influx of energy through lighi. The sort of light intensity 

 needed to breed plants may easily introduce energy causing a rise in tempera- 

 ture of several degrees. Some workers have chosen the simple method ot 

 including a fixed heater, which is switched in as the fights are cut, and of a 

 value to keep the heat load constant: as a rule, large-scale cooling is needed 

 continuously to off-set this. The practice seems excessive, and it is far better 

 to place all such lights outside the enclosure and employ every possible 

 means of keeping down the entrant energy, which will be especially heavy in 

 the infra-red. A well designed thermostat should then meet the situation 

 adequately. 



PRACTICAL METHODS FOR TEMPERATURE CONTROL 



It is assumed that the reader is especially interested in the control of tempera- 

 ture of air or water containing biological material, and that ovens, incubators 

 and autoclaves of conventional design wiU usually be purchased ready made. 



Aquaria and water-baths 



Although economically lagging may be worth while, and is advised where 

 such enclosures are run appreciably above or below ambient or where the 

 tank is of metal, it is perfectly possible to obtain temperature control to 

 1/50°C in an unlagged glass tank; however, for normal purposes, control to 

 rh^°C wiU be adequate. The heater should consist of a spiral of nichrome 

 or tungsten wire in a glass tube filled wiih liquid paraffin, supported off the 

 floor of the tank. A simple stirrer should be included, but a stream of 

 compressed air through the water is often sufficient to prevent stratification — 

 the density change with temperature is such that convection in water will 

 not lead to large temperature variations as compared with air. An indication 

 of the required size of heater may be obtained by putting in a smaU immersion 



27 405 



