TEMPERATURE CONTROL 



mark-to-space type of control approaches control from lower temperatures 

 smoothly, and without overshoot (Figure 29.13d), but, in general, the more 

 sensitive the equipment the slower will be its approach to the control tempera- 

 ture ; if the sensitivity is increased to the brink of instability a large distur- 

 bance (which is the equivalent to meeting approach conditions) will lead to 

 instability, and it may be advisable to have a variable sensitivity control 

 which is brought into action during such operations as opening doors. Where 

 equipment is needed to alternate in temperature on, for example, a diurnal 



t t 



(c) (d) 



Figure 29. 13 Approach characteristics of controlled bodies heated from a 

 lower temperature, (a) Asymptotic approach of a body to the maximum 

 temperature produced by a fixed wattage heater; (b) overshoot caused when a 

 booster heater of large thermal capacity is not switched off till control tempera- 

 ture is reached; (c) ''damped oscillation^ produced by a thermostatically con- 

 trolled single heater of large thermal capacity; {d) approach of a small thermal 

 capacity heater, controlled by mark-to-space — heating is at maximum rate 

 until control temperature is reached, followed immediately by optimal 



control 



cycle, we must also consider approach to a lower temperature from a higher 

 one at the switch-over point. This is, in general, regulated by the insulation 

 of the body from the ambient, and as it is not impossible to construct an 

 environmental chamber which will lose only a few degrees per hour with its 

 heater switched off, it follows that for such equipment one should have only 

 the minimum of insulation compatible with the necessary cooling rate, 

 bearing in mind that the heater is going to have to be appropriately bigger, 

 and that the machine will be less economic to run. Provided the equipment 

 is always required to run above ambient temperature it is not advisable to 

 introduce active cooling for this (see below). 



Continuously variable heater system 



In all the foregoing the effect of the control apparatus has been to distri- 

 bute the action of a fixed-power heater in order to obtain the required mean 

 power necessary for a temperature lower than that produced by continuous 



403 



