2
8 Calibration Procedure
8
Calibration Procedure
Sometimes the user may want to calibrate the instrument to improve the temperature set-
point accuracy. Calibration is done by adjusting the controller probe calibration constants
R0,
ALPHA, and DELTA
so that the temperature of the dry-well as measured with a standard ther-
mometer 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 dry-well. By using a
good thermometer and following this procedure the dry-well can be calibrated to an accuracy of
better than 0.8
°
C over its full range.
8.1
Calibration Points
In calibrating the instrument,
R0, ALPHA, and DELTA
are adjusted to minimize the set-point
error at each of three different dry-well temperatures. Any three reasonably separated tempera-
tures 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 dry-well. The
farther apart the calibration temperatures, the larger the calibrated temperature range is, but the
calibration error is greater over the range. If, for instance, 150
°
C to 350
°
C is chosen as the cali-
bration range then the calibrator may achieve an accuracy of say
±
0.3
°
C over the range 150 to
350
°
C. Choosing a range of 200
°
C to 300
°
C may allow the calibrator to have a better accuracy
of maybe
±
0.2
°
C over the range 175 to 325
°
C but outside that range the accuracy may be only
±
0.5
°
C.
8.2
Calibration Procedure
Choose three set points to use in the calibration of the R0, ALPHA, and DELTA
parameters. These set points are generally 50.0
°
C, 250
°
C, and 500.0
°
C but other set
points may be used if desired or necessary.
Set the calibrator to the low set-point. When the instrument reaches the set-point and the
display is stable, wait 15 minutes or so and then take a reading from the thermometer.
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.
Repeat step 2 for the other two set points recording them as T
2
and R
2
and T
3
and R
3
respectively.
Using the recorded data, calculate new values for the R0, ALPHA, and DELTA
parameters using the following equations.
8.2.1 Compute DELTA
1.
2.
3.
4.
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