MEASUREMENT AND CONTROL OF TEMPERATURE 



when the ambient is at its highest. In more exacting systems, where a wide 

 temperature range is also required, the position of a T.S.E. may select an 

 appropriate number of sections of 'fixed' heater, in addition to which either 

 the same T.S.E. or a separate one may complete the topping up process. 



Ambient compensators 



For the largest installations, where economy of power becomes an essential, 

 it is advisable to switch in extra sections of heater via a separate thermostat 

 in the ambient, especially if this is subject to large fluctuations, and/or 

 insulation is poor. Great care must be taken that this heat load can never 

 produce temperatures which could overlap the working range of the internal 

 thermostat, otherwise the most complex cycles of instability will be set up. 



Mark-to-space systems 



So far, control methods have aimed at minimizing the cyclical temperature 

 fluctuations due to the method of heating — obtaining a fractional mean 

 power from a discontinuously operating heater. The frequency of the 

 operating cycle is a characteristic of the control machinery; in coarse control 

 a cycle of several minutes is not unusual. Now approximately the same 

 mean power emission could be obtained by any frequency of cycle, provided 

 that the ratio of heater 'on' to 'off' remains constant (it is not exactly the 

 same because the dissipated power also depends to some extent on the 

 duration of actual heating and cooling of the heater element itself which 

 will be greater in long frequency cycles) ; but the higher the frequency, the 

 more even will the temperature control be. All the refinements outlined 

 above do in fact tend to increase the frequency of the thermostat cycle, but 

 a much more satisfactory method is by the use of a 'mark-to-space' system. 

 The simplest of these is now dealt with. 



Proportioning head — Consider {Figure 29.10a) the movement of the 

 meniscus of a mercury-contact thermometer or toluene regulator; the 'on' 

 and 'off"' portions of the cycle are determined by a line representing the 

 position of the needle contact. Now suppose the needle be made itself to 

 oscillate vertically {Figure 29.10b) with a frequency appreciably greater than 

 that of the mercury cycle. The sum of the 'on' periods per mercury cycle — 

 and likewise of the 'off' — have not been changed, the mean power dissipation 

 will not have altered but the distribution of power in time has been made very 

 much more even. Because of this, of course, the amplitude of the mercury 

 meniscus will be so materially reduced that the relative roles of meniscus and 

 contact in determining the mark-to-space ratio will be reversed {Figure 29.10c). 

 The much smaller movement of the mercury in itself indicates the improve- 

 ment in control. Should the temperature of the body rise for any reason 

 (e.g. rise of ambient) the 'on' period per needle cycle is at once reduced, 

 resulting in rapid compensation. Perhaps the greatest advantage of the 

 method is the ease with which it can be attached to existing mercury regula- 

 tors; movement is usually obtained by mounting the needle on a bimetal 

 strip wound with a small independent heater element whose circuit is broken 

 by the strip's own movement, and remade as the strip cools again. It could 

 be derived mechanically from a motor (but see next paragraph), and the 

 application of the principle to electronic methods is of particular interest. 



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