MEASUREMENT AND CONTROL OF TEMPERATURE 



(a) 



(b) 



l A A A A 



(C) 



Figure 29.11 Forms of mark-to-space cycle, (a) Contact oscillating in a 

 sine wave, a large temperature change gives rise to a small change in the ratio 

 intercept, when operating centrally on the sine wave; (b) when operating near the 

 apices of the sine wave, a small temperature change produces a large change 

 in intercept ratio, (c) the change in the mark-to-space ratio is directly pro- 

 portional to temperature change when a triangular wave is used 



(a) 



(b) 



Figure 29.12 



Effect of phase shift on temperature control by mark-to-space 

 systems, {a) Ideal state in which temperature rises in the 'on' period and falls 

 in the 'off'; {b) form of large-scale hunting due to 180 degree phase shift, i.e. 

 a delay of half a mark-to-space cycle between the heater being energized and 



the corresponding temperature rise in the T.S.E. 



400 



