7341
Users Guide
9-10
S + E
Access heater power in secondary menu
12 Pct Heater power in percent
To exit out of the secondary menu press “EXIT”. To continue on to the proportional band
setting function press “SET”.
E
Return to temperature display
Proportional Band
In a proportional controller such as this, the heater output power is proportional to the
bath temperature over a limited range of temperatures around the set-point. This range of
temperature is called the proportional band. At the bottom of the proportional band the
heater output is 100%. At the top of the proportional band the heater output is 0. Thus as
the bath temperature rises the heater power is reduced, which consequently tends to lower
the temperature back down. In this way the temperature is maintained at a fairly constant
temperature.
The temperature stability of the bath depends on the width of the proportional band (see
Figure 9-2). If the band is too wide, the bath temperature deviates excessively from the
set-point due to varying external conditions. This is because the power output changes
very little with temperature and the controller cannot respond very well to changing
conditions or noise in the system. If the proportional band is too narrow, the bath
temperature may swing back and forth because the controller overreacts to temperature
variations. For best control stability the proportional band must be set for the optimum
width.
haz006.eps
Figure 9-2. Bath Temperature Fluctuation At Various Proportional Band Settings
The optimum proportional band width depends on several factors among which are fluid
volume, fluid characteristics (viscosity, specific heat, thermal conductivity), heater power
setting, operating temperature, and stirring. Thus the proportional band width may require
adjustment for best bath stability when any of these conditions change. Of these, the most
significant factors affecting the optimum proportional band width are heater power
setting and fluid viscosity. The proportional band should be wider when the higher power
setting is used so that the change in output power per change in temperature remains the
same. The proportional band should also be wider when the fluid viscosity is higher
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