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7.8 Thermal Electric Devices (TED)
The temperature controller controls the temperature of the well by pulsing the
TED on and off. The total power being applied to the TED is determined by the
duty cycle or the ratio of TED on time to the pulse cycle time. By knowing the
amount of heating the user can tell if the calibrator is heating up to the
set-point, cooling down, or controlling at a constant temperature. Monitoring
the percent heater power lets the user know how stable the well temperature is.
With good control stability the percent heating power should not fluctuate more
than ±5% within one minute.
The heater power display is accessed in the secondary menu. Press “SET” and
”EXIT” simultaneously and release. The heater power is displayed as a percent
-
age of full power.
100.0 Well temperature
S+E
Access heater power in secondary menu
SEC Flashes
12.0 P Heater power in percent
To exit out of the secondary menu press and hold “EXIT”. To continue on to
the proportional band setting function press “EXIT” momentarily or “SET”.
7.9 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. Thus as the 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 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 deviation
is because the power output changes very little with temperature and the con
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troller cannot respond very well to changing conditions or noise in the system.
If the proportional band is too narrow the temperature may swing back and
forth because the controller overreacts to temperature variations. For best con
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trol stability the proportional band must be set for the optimum width.
25
7 Controller Operation
Thermal Electric Devices (TED)
1.888.610.7664 sales@GlobalTestSupply.com
Fluke-Direct.com
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