MEASUREMENT AND CONTROL OF TEMPERATURE 



disturbances, together with the economic consideration that for low tempera- 

 tures cooler alone is principally used, while for high temperatures there is 

 little energy drain on the cooler ; further economy can be effected by provid- 

 ing a by-pass so that except on such occasions as the opening of the cabinet 

 doors the major part of the air is re-circulated without reconditioning; 

 humidity control can be independently incorporated on the same principle. 

 The machine can be operated by mechanical or electronic switch-gear, and is 

 particularly suitable for diurnal or continuously variable cyclical operations. 



■r^il-^A^m/s 



Figure 29.21 Layout for fine control of circulated air {large-scale equipment) by 

 mark-to-space control, suitable for temperature andjor humidity control above 

 and below ambient for fixed or programmed characteristics. Air drawn from the 

 cabinet by the pump (p) passes through the solenoid-controlled valve (sv) either to 

 a heater (h) or cooler (c) and into a baffie-chamber {be) for mixing before 

 re-entering the enclosure. A humidifier j drier unit would run in parallel with 

 these. The control thermostat {ct) and mark-to-space injector {mjs) can be 

 sited appropriately at the critical part of the enclosure 



Its accommodation is such that it can readily be made to swamp the load 

 changes of discontinuous illumination. 



Fine control by variable heater 



The circuit for the method of Beament and Machin, using thermistors, is 

 given in Figure 29.22. The heater — 2 kD. of nichrome wire wound on an 

 open frame of mica sheet — is supported in a cylinder of insulating material, 

 and above it a small fan attached to a pot magnet is mounted on watch- 

 maker bearings. A long bar magnet rotated outside the enclosure spins the 

 fan without introducing energy from the heating of the motor or requiring a 

 shaft through the wall of the box. The high resistance thermistor forms a 

 potential divider to control the grid of the first stage valve, and temperature 

 settings are obtained by adjusting the voltage of the upper end of the 

 thermistor. An additional range is made available by switching out part of 

 the resistance network. Two miniature neons provide a voltage drop to the 

 directly coupled grid of the output stage, of which the heater is the anode 

 load. A potentiometer in parallel with this load provides a proportional 

 voltage which is put on to the screen of the first stage, thus giving variable 

 positive feedback and allowing the sensitivity of the amplifier to be adjusted 



410 



