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6.9 Proportional Band
In a proportional controller such as this, the heater output power is proportional
to the well temperature over a limited range of temperatures around the
set-point. This range of temperature is called the proportional band. At the bot
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tom of the proportional band, the heater output is 100%. At the top of the pro
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portional band, the heater output is 0 to 100 (hot side) and –100 to 100 (cold
side). Thus, as the temperature rises the heater power is reduced, which conse
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quently tends to lower the temperature back down. In this way the temperature
is maintained at a fairly constant temperature.
The temperature stability of the well and response time depend on the width of
the proportional band. If the band is too wide, the well temperature deviates ex
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cessively 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 propor
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tional band is too narrow, the 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.
The proportional band width is set at the factory and printed on the Report of
Calibration. The proportional band width may be altered by the user if desired
to optimize the control characteristics for a particular application.
The proportional band width is easily adjusted from the front panel. The width
may be set to discrete values in degrees C or F depending on the selected units.
The proportional band adjustment is accessed within the secondary menu. Press
“EXIT” while holding down “SET” and then releasing both to enter the sec
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ondary menu and show the heater power. Then press “SET” to access the pro
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portional band.
S+E
Access heater power in secondary menu
9011 Dual-Well Calibrator
User’s Guide
28
Figure 5 Temperature fluctuations at various proportional band settings