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Calibration Procedure
This procedure is to be considered a general guideline. Each
laboratory should write their own procedure based on their
equipment and their quality program. Each procedure should be
accompanied by an uncertainty analysis also based on the
laboratory's equipment and environment.
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 0.02 °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 are obtained
when using bath temperatures which are just within the most useful operating
range of the bath. The farther apart the calibration temperatures the larger the
calibrated temperature range, but the calibration error is also greater over the
range. For instance, if 50 °C and 150 °C are chosen as the calibration
temperatures, then the bath may achieve an accuracy of ±0.03 °C over the range
40 to 160 °C. Choosing 80 °C and 120 °C may allow the bath to have a better
accuracy of maybe ±0.01 °C over the range 75 to 125 °C but outside that range
the accuracy may be only ±0.05 °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
. Wait for the bath to reach the set-point and allow 15 minutes to
L
stabilize at that temperature. Check the bath stability with the thermometer.
When both the bath and the thermometer have stabilized, measure the bath
temperature with the thermometer and compute the temperature error, err
which is the actual bath temperature minus the set-point temperature. For
example, if the bath is set for a lower set-point of t
a measured temperature of 49.7 °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
was set for 150 °C and the thermometer measured 150.1 °C giving an error of
+0.1 °C.
Note
=50 °C and the bath reaches
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, and, after stabilizing, measure the
H
. For example, suppose the bath
H
Calibration Bath
Calibration Procedure
,
L
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