PRACTICAL METHODS FOR TEMPERATURE CONTROL 



recover from such disturbances as opening doors, and they usually rely on 

 having a water-jacket or similar surround of high thermal capacity to buffer 

 changes and to slow cooling by buffering the insulation. In typical experi- 

 mental enclosures it is usually necessary to have an observation site (for 

 which double glass or plastic walls, with at least an inch of air between, are 

 necessary), a window for admission of light, and a door which may have to 

 be opened frequently; most people would hesitate to build a water-jacketed 

 box with these attributes, while of course high thermal capacity walls are 



Figure 29.20 Suggested sectional layout of a plant-growing cabinet. The 

 heater (/i) and cooler coil (c) are shrouded below a false floor through which 

 the air is blown by a fan (f), the drive motor (m) being outside the box. The 

 lighting equipment (7) is in an aspirated compartment separated by two sheets 



of glass ig) 



directly contrary to a requirement of frequent shifts in actual controlled 

 temperature. Generally speaking, only a small mercury-toluene thermostat 

 or, better, contact thermometer will meet these circumstances, together with 

 an electronic relay operating an electromagnetic relay with mercury contacts. 

 It is true that some not unsuccessful units operate on the principle of placing 

 heaters and/or coolers between a false wall and the main insulation jacket, 

 so that the air is 'enclosed in a box at the right temperature'. Such an 

 extension of the incubator principle is likewise useful for long usage at one 

 fixed temperature, but slow in response to change ; it is also more difficult to 

 construct. If such a system is used for low temperatures, heavy condensation 

 will occur on the walls and may be a great nuisance. 



Large-scale mark-to-space systems 



The author has used the system outlined in Figure 29.21 for a plant- 

 breeding cabinet. It uses circulated air, switched by a mark-to-space 

 thermostat either through a heater or through a cooler, using a solenoid- 

 operated air valve. Its great advantage is of rapid accommodation of 



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