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11 Calibration Procedure
At times the user may want to calibrate the unit to improve the temperature
set-point accuracy. Calibration is done by adjusting the controller probe cali
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bration constants CE1, CE2, and CE3 so that the temperature of the unit as
measured with a standard thermocouple agrees more closely with the set-point.
The thermometer used must be able to measure the well temperature with
higher accuracy than the desired accuracy of the unit.
11.1 Calibration Points
In calibrating the unit, CE1, CE2, and CE3 are adjusted to minimize the
set-point error at each of three different well temperatures. Any three reason
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ably separated temperatures may be used for the calibration. Improved results
can be obtained for shorter ranges when using temperatures that are just within
the most useful operating range of the unit. The farther apart the calibration
temperatures, the larger the calibration range but the calibration error is also
greater over the range. Choosing a range of 150°C to 500°C may allow the cali-
brator to have a better accuracy of maybe ±2.0°C but outside that range the ac-
curacy may be greater than ±10.0°C.
11.2 Calibration Procedure
1. Choose three set-points to use in the calibration of CE1, CE2, and CE3
parameters. These set-points are generally CT1 = 150°C, CT2 = 675°C
and CT3 = 1200°C but other set-points may be used if desired or neces-
sary. Using these three temperature set-points may result in ±4.0°C
accuracy.
2. If the normal set-points are not used, initialize CT1, CT2, and CT3 to the
desired set points. Where CT1 is the low-set point and CT3 is the high
set-point.
3. Set the unit to the low set-point. When the unit reaches the set-point and
the display is stable, wait 15 minutes or so and take a reading from the
thermometer. Repeat step 3 for the other two set-points recording them
as Tm1, Tm2, and Tm3.
4. Retrieve the original calibration errors from the unit or the Report of
Calibration.
5. Record the previous values for CE1, CE2, and CE3 and calculate new
values for CE1, CE2, and CE3 using the following formula.
Tmn –Tsn +CEn = CEm
Where:
Tmn is the temperature measured, Tsn (CTn) is the set-point tempera
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11 Calibration Procedure
Calibration Points