User manual

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

and R

respectively.

Repeat step 2 for the other two set points recording them as T

and R

and T

and R

respectively.

Using the recorded data, calculate new values for the R0, ALPHA, and DELTA

parameters using the following equations.

8.2.1 Compute DELTA