Table of Contents
Advertisement
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 DO 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 the procedure the bath can be cali-
brated to an accuracy of better than 0.1°C over a range of 50 degrees.
11.1
Calibration Points
In calibrating the bath DO 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 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 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 calibration tem-
peratures, 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 set-point which we will call t
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 bath temperature with the thermometer and compute the tempera-
ture error err
ature. If for example the bath is set for a lower set-point of t
bath 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
which is the actual bath temperature minus the set-point temper-
L
11 Calibration Procedure
. Wait for the bath to reach the set-point
L
and after stabilizing measure the
H
. For our example we will suppose
H
Calibration Points
=20°C and the
L
59
Advertisement
Table of Contents
