11
Calibration Procedure
Note:
This procedure is to be considered a general guideline. Each labo-
ratory should write their own procedure based on their equipment and
their quality program. Each procedure should be accompanied by an un-
certainty analysis also based on the laboratory's equipment and environ-
ment.
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 R
0
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 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 0.02°C over a range of 100 degrees.
11.1
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 ob-
tained when using bath temperatures that are just within the most useful operat-
ing range of the bath. The further apart the calibration temperatures, the greater
the calibrated temperature range, but, the calibration error will also be greater
over that range. For example, 50°C and 150°C are chosen as the calibration
temperatures, the bath may possibly 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
±
0.01°C over the range 75 to 125°C, but outside that range,
the accuracy may be only
±
0.05°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 chosen 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 a ther-
mometer. When both the bath and the thermometer have stabilized, measure the
bath temperature with the thermometer and compute the temperature error, the
actual bath temperature minus the set-point temperature, err
L
. If the bath is set
for a lower set-point of t
L
=50°C and the bath reaches a measured temperature
of 49.7°C, then the error is –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 our example the bath was set
for 150°C and the thermometer measured 150.1°C giving an error of +0.1°C.
65
11 Calibration Procedure
Calibration Points
