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Proportional Band
In a proportional controller like this, heater output power is proportional to the Bath temperature
over a limited range of temperatures around the set-point. This temperature range is the
proportional band. At the bottom of the proportional band, heater output is 100 %. At the top of
the proportional band, heater output is 0 %. As the Bath temperature rises, the heater power
reduces, which usually lowers the temperature. These actions maintain a fairly-constant
temperature.
Temperature stability of the Bath depends on the width of the proportional band, see
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 a minor amount with temperature,
and the controller does not respond well to changing conditions or noise in the system. If the
proportional band is too narrow, Bath temperature can oscillate because the controller overreacts
to temperature variations.
Figure 5. Bath Temperature Fluctuations at Various Proportional Band Settings
Optimum proportional band width depends on factors that include:
•
fluid volume
•
fluid characteristics (viscosity, specific heat, thermal conductivity)
•
heater power setting
•
operating temperature
•
stirring
The proportional band width can require adjustment for best Bath stability when any of these
conditions change. Of these conditions, the most significant factors that affect the optimum
proportional band width are heater power setting and fluid viscosity. The proportional band
should be wider when you use the higher power setting so that the change in output power per
change in temperature remains the same.
Calibration Bath
Controller Operation
Table
5. If
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