Table of Contents
Advertisement
11
Calibration Procedure
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
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 R
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 which are just within the most useful op-
erating range of the bath. The further 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 0°C and 100°C are chosen as the cali-
bration temperatures then the bath may achieve an accuracy of maybe ±0.03°C
over the range -10 to 110°C. Choosing 30°C and 70°C may allow the bath to
have a better accuracy of maybe ±0.01°C over the range 25 to 75°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 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
reaches a measured temperature of -0.3°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 100°C and the thermometer measured 100.1°C giving an
error of +0.1°C.
11.3
Computing R
Before computing the new values for R
be known. The values may be found by either accessing the probe calibration
and ALPHA so that the temperature of the bath
0
and ALPHA are adjusted to minimize the set-point
0
which is the actual bath temperature minus the set-point temper-
L
and ALPHA
0
11 Calibration Procedure
. Wait for the bath to reach the set-point
L
and after stabilizing measure the
H
. For this example we will suppose
H
and ALPHA the current values must
0
Calibration Points
=0°C and the bath
L
59
Advertisement
Table of Contents
