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10 Calibration Procedure
Sometimes the user may want to calibrate the instrument to improve the tem
-
perature set-point accuracy. Calibration is done by adjusting the controller
probe calibration constants R0 , ALPHA, DELTA, and BETA (cold side) so that
the temperature of the calibrator as measured with a standard thermometer
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 calibrator. By using a good thermometer and following this procedure the
instrument can be calibrated to an accuracy of better than 0.5°C up 100°C.
10.1 Calibration Points
In calibrating the instrument, R0, ALPHA, DELTA, and BETA (cold side) are
adjusted to minimize the set-point error at each of three different temperatures.
Any three reasonably 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 instrument. The far-
ther apart the calibration temperatures, the larger will be the calibrated temper-
ature range but the calibration error will also be greater over the range. If for
instance –15°C to 100°C is chosen as the calibration range then the calibrator
may achieve an accuracy of ±0.2°C over the range –15 to 100°C. Choosing a
range of 50°C to 90°C may allow the calibrator to have a better accuracy than
±0.2°C over that range but outside that range the accuracy may be only ±1.5°C.
10.2 Calibration Procedure
1. Choose four set-points to use in the calibration of the R0, ALPHA, DELTA,
and BETA parameters. These set-points are generally –15°C, 0°C, 60°C and
110°C but other set-points may be used if desired or necessary.
2. Set the instrument to the low set-point. When the calibrator reaches the
set-point and the display is stable, wait 15 minutes or so and then take a read
-
ing. Sample the set-point resistance by holding down the SET key and pressing
the DOWN key. Write these values down as T
1
and R
1
respectively.
3. Repeat step 2 for the other two set-points recording them asT
1
,R
1
,T
2
,R
2
,
T
3
,R
3
,T
4
,andR
4
respectively.
4. Using the recorded data, calculate new values for R0, ALPHA, DELTA,
BETA parameters using the equations given below:
10.2.1 Compute DELTA
AT T=−
43
BTT=−
32
37
10 Calibration Procedure
Calibration Points
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