User manual

Table Of Contents

9 Calibration Procedure

Sometimes the user may want to calibrate the instrument to improve the

temperature set-point accuracy. Calibration is done by adjusting the control

ler probe calibration constants R0, ALPHA, DELTA, and BETA 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 ac

curacy of the calibrator. By using a good thermometer and following this

proceduretheinstrumentcanbecalibratedtoanaccuracyofbetterthan

0.5°C up to 150°C.

9.1 Calibration Points

In calibrating the instrument, R0, ALPHA, DELTA, and BETA, are adjusted to

minimize the set-point error at each of three different temperatures. Any

three appropriately separated temperatures may be used for the calibration.

Improved results can be obtained for shorter ranges when using tempera-

tures that are just within the most useful operating range of the instrument.

The farther apart the calibration temperatures, the larger the calibrated

range. However, the calibration error will also be greater over that range. If,

for instance, –20°C to 100°C is chosen as the calibration range, the calibra-

tor may achieve an accuracy of ±0.3°C over that range (–20 to 100°C).

Choosing a range of 50°C to 90°C may allow the calibrator to have a better

accuracy of maybe ±0.2°C over the narrower range but, outside that range,

theaccuracymaybeonly±1.5°C.

9.2 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 –25°C,

0°C, 60°C, and 125°C but other set-points may be used if desired or

necessary.

2. Set the instrument 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. Sample the set-point resistance by hold

ing down the “SET” key and pressing the “DOWN” key. Write these

values down as T

and R

respectively.

3. Repeat step 2 for the other three set-points recording them as T

, R

, T

and R

respectively.

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

and BETA parameters using the equations given below.

Calibration Procedure