Page 52 - Instrumentation and Measurement

Because the control action is discontinuous and there are time lags in the system, oscillations, i.e.
cycling, of the controlled variable occur about the required condition. Thus, with temperature control
using the bimetallic thermostat, when the room temperature drops below the required level there is a
significant time before the heater begins to have an effect on the room temperature and, in the
meantime, the temperature has fallen even more. When the temperature rises to the required
temperature, since time elapses before the control system reacts and switches the heater off and it
cools, the room temperature goes beyond the required value. The result is that the room temperature
oscillates above and below the required temperature (Figure 4.3).

FIGURE 4.3 Fluctuation of temperature about set value.
There is also a problem with the simple on-off system in that when the room temperature is hovering
about the set value the thermostat might be reacting to very slight changes in temperature and almost
continually switching on or off. Thus, when it is at its set value a slight draught might cause it to
operate. This problem can be reduced if the heater is switched on at a lower temperature than the one
at which it is switched off (Figure 4.4).
The term dead band or neutral zone is used for the values between the on and off values. For example,
if the set value on a thermostat is 20 ͦ C, then a dead band might mean it switches on when the
temperature falls to 19.5 ͦ and off when it is 20.5 .ͦ The temperature has thus to change by one degree
for the controller to switch the heater on or off and thus smaller changes do not cause the thermostat
to switch. A large dead band results in large fluctuations of the temperature about the set temperature;
a small dead band will result in an increased frequency of switching. The bimetallic thermostat shown
in Figure 4.2 has a permanent magnet on one switch contact and a small piece of soft iron on the
other; this has the effect of producing a small dead band in that, when the switch is closed, a
significant rise in temperature is needed for the bimetallic element to produce sufficient force to
separate the contacts.

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