Calibration Procedure; Calibration Points; Measuring The Set-Point Error; Computing R0 And Alpha - Fluke 7341 User Manual

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 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 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 0.2°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 will be
obtained when using bath temperatures which are just within the most useful
operating range of the bath. The further apart the calibration temperatures, the
greater the calibrated temperature range and the calibration error. If, for in-
stance, 0°C and 100°C are chosen as the calibration temperatures, the bath may
achieve an accuracy of ±0.3°C over the range –10 to 110°C. Choosing 30°C
and 70°C may allow the bath to have a better accuracy of ±0.1°C over the range
25 to 75°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, t
to stabilize at that temperature. Check the bath stability with the thermometer.
When both the bath and the thermometer have stabilized, measure the actual
bath temperature and compute the temperature error, err
perature minus the set-point temperature). For example, set the bath to 0°C.
The bath reaches a measured temperature of –0.3°C giving an error of –0.3°C.
Next, set the bath for the upper set-point, t
bath temperature and compute the error err
100°C, the thermometer measures 100.1°C giving an error of +0.1°C.
11.3

Computing R0 and ALPHA

Before computing the new values for R0 and ALPHA the current values must
be known. The values may be found by either accessing the probe calibration
menu from the controller panel or by inquiring through the digital interface.
The user should keep a record of these values in case they may need to be re-
stored in the future. The new values R0' and ALPHA' are computed by entering
. Wait for the bath to reach the set-point and allow 15 minutes
L
11 Calibration Procedure
(the actual bath tem-
L
, and after stabilizing, measure the
H
. For example, set the bath to
H
Calibration Points
65