Calibration Procedure
Calibration Points11
11-3
In some instances the user may want to calibrate the bath to improve the temperature set-
point accuracy. Calibration is done by adjusting the controller probe calibration constants
R0 and ALPHA so that the temperature of the bath as measured with a standard
thermometer agrees more closely with the bath set-point. The thermometer used must be
able to measure the bath fluid temperature with higher accuracy than the desired accuracy
of the bath. By using a good thermometer and carefully following procedure the bath can
be calibrated to an accuracy of better than 0.2°C over a range of 100 degrees.
Calibration Points
In calibrating the bath R0 and ALPHA are adjusted to minimize the set-point error at
each of two different bath temperatures. Any two reasonably separated bath temperatures
may be used for the calibration however best results will be obtained when using bath
temperatures which are just within the most useful operating range of the bath. The
further apart the calibration temperatures, the greater the calibrated temperature range
and the calibration error. If, for instance, 0°C and 100°C are chosen as the calibration
temperatures, the bath may achieve an accuracy of ±0.3°C over the range –10 to 110°C.
Choosing 30°C and 70°C may allow the bath to have a better accuracy of ±0.1°C over the
range 25 to 75°C, but, outside that range the accuracy may be only ±0.5°C.
Measuring the Set-point Error
The first step in the calibration procedure is to measure the temperature errors (including
sign) at the two calibration temperatures. First set the bath to the lower set-point, t
L
. Wait
for the bath to reach the set-point and allow 15 minutes to stabilize at that temperature.
Check the bath stability with the thermometer. When both the bath and the thermometer
have stabilized, measure the actual bath temperature and compute the temperature error,
err
L
(the actual bath temperature minus the set-point temperature). For example, set the
bath to 0°C. The bath reaches a measured temperature of –0.3°C giving an error of –
0.3°C.
Next, set the bath for the upper set-point, t
H
, and after stabilizing, measure the bath
temperature and compute the error err
H
. For example, set the bath to 100°C, the
thermometer measures 100.1°C giving an error of +0.1°C.
Computing R0 and ALPHA
Before computing the new values for R0 and ALPHA the current values must be known.
The values may be found by either accessing the probe calibration menu from the
controller panel or by inquiring through the digital interface. The user should keep a
record of these values in case they may need to be restored in the future. The new values
R0′ and ALPHA′ are computed by entering the old values for R0 and ALPHA, the
calibration temperature set-points t
L
and t
H
, and the temperature errors err
L
and err
H
into
the following equations,
If R0 and ALPHA were previously set for 100.000 and 0.0038500 respectively and the
data for t
L
, t
H
, err
L
, and err
H
were as given above then the new values R0′ and ALPHA′
would be computed as 110.116 and 0.0038302 respectively. Program the new values R0
and ALPHA into the controller. Check the calibration by setting the temperature to t
L
and
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