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11
Bath Calibration
In some instances the user may want to calibrate the bath to improve the tem-
perature set-point accuracy. Calibration is done by adjusting the controller
probe calibration constants D0 and DG 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 temper-
ature 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 0.1°C over a range of 50 degrees.
11.1
Calibration Points
In calibrating the bath D0 and DG 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 ob-
tained when using bath temperatures which are just within the most useful op-
erating range of the bath. The farther apart the calibration temperatures the
larger will be the calibrated temperature range but the calibration error will also
be greater over the range. If for instance 20°C and 80°C are chosen as the cali-
bration temperatures then the bath may achieve an accuracy of say ±0.2°C over
the range 20 to 80°C. Choosing 30°C and 70°C may allow the bath to have a
better accuracy of maybe ±0.05°C over the range 30 to 70°C but outside that
range the accuracy may be only ±0.5°C.
11.2
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 setpoint which we will call t
allow 15 minutes to stabilize at that temperature. Check the bath stability with
the thermometer. When both the bath and the thermometer have stabilized mea-
sure the bath temperature with the thermometer and compute the temperature
error err
If for example the bath is set for a lower set-point of t
reaches a measured temperature of 19.7°C then the error is –0.3°C.
Next, set the bath for the upper set-point t
bath temperature and compute the error err
the bath was set for 80°C and the thermometer measured 80.1°C giving an error
of +0.1°C.
11.3
Computing D0 and DG
Before computing the new values for D
known. The values may be found by either accessing the probe calibration
which is the actual bath temperature minus the set-point temperature.
L
. Wait for the bath to reach the set-point and
L
=20°C and the bath
L
and after stabilizing measure the
H
. For our example we will suppose
H
and D
the current values must be
O
G
Bath Calibration
51
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