TEMPERATURE CONTROL 



In order to obtain a new temperature setting the entire needle mechanism is 

 of course moved to an appropriate new mean position. 



Detailed consideration of mark-to-space systems — The size and form of the 

 oscillation of the needle is critical. Compensation is due to the change in 

 the mark-to-space-the 'on-to-off ' ratio. Suppose the form of the contact 

 oscillation is a sine wave, such as might be derived from a motor, and that 

 the thermostat is set in its mid-range for the heater so that the on-off periods 



Hg contact 



make /break 



^. Q^^ ./^. Q^^ 



(a) 



Original contact /u\ 

 line ^°' 



Original contact (c) 



New 

 Hg line 



line 



Figure 29.10 The effect of superimposing a proportioning head on a mercury- 

 contact thermostat . (a) Unmodified thermostat ; movement of the mercury 

 meniscus {equivalent to temperature fluctuation) across the stationary contact 

 position; {b) the contact is now oscillated by the proportioning head and its 

 movement superimposed on the previous meniscal fluctuation, the intersection 

 of meniscus and needle still determines the on I off periods, 'on' being indicated 

 by the thickened parts of the curve; (c) condition of final control, the more even 

 distribution of heat in time has achieved a much smaller temperature fluctuation 



are about equal (Figure 29.11a). The intersecting 'mercury line' will have to 

 move appreciably across this form of oscillation in order to change the ratio 

 and produce compensation ; the sensitivity will thus be low. If the apparatus 

 is, on the other hand, working near to its upper or lower limits so that the 

 oscillation curve is cut near its peaks {Figure 29.11b), the ratio will be rapidly 

 changed by a small shift in the temperature line — so much so that over- 

 compensation and instability may result. Uniform and adequate compensa- 

 tion over the entire available temperature range can only be obtained from a 

 triangular type waveform {Figure 29.11c) which is not so readily obtainable. 

 The wave front must be appreciably steep, but the more rapid within reason 

 is the frequency of its oscillation the less need is there to approach the 

 triangular form. However, one must still retain a sense of proportion about 

 this, for presumably heater currents are being switched via relays, whose 

 frequency of operation — and life — are limited. 



Instability in mark-to-space control — Over-compensation may lead to in- 

 stability — the regular form of large temperature cycles which may occur 

 under these circumstances is called 'hunting'. It can often be cured either 

 by a reduction in the sensitivity of the T.S.E. or by increasing the amplitude 

 of the hunting head; but in stubborn cases an analysis of the entire apparatus 

 is often necessary. Theoretically {Figure 29.12a), the temperature of the 



399 



