Fluke 7381 - Measure the Set-Point Error; Compute R and ALPHA

7381
User’s Guide
48
Measure 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.
1. Set the Bath to the lower set-point, t
L
.
2. Wait for the Bath to reach the set-point and allow 15 minutes to stabilize at that temperature.
3. Check the Bath stability with the thermometer.
4. 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
). 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.
5. Set the Bath for the upper set-point, t
H
, and after the Bath stabilizes, measure the Bath
temperature and compute the error, err
H
. For example set the Bath to 100 °C. If the
thermometer measured 100.1 °C, this would mean an error of +0.1 °C.
Compute R
0
and ALPHA
Before you can compute the new R
0
and ALPHA values, you must know the current values.
Access the Probe Calibration menu from the controller panel or inquire through the digital
interface to find these values. Keep a record of these values in case they need to be restored in
the future. To compute the new R
0
′ and ALPHA′ values, enter the old values for R
0
and ALPHA,
the calibration temperature set-points, t
L
and t
H
, and the temperature errors, err
L
and err
H
, into
these equations,
For example, if R
0
and ALPHA were previously set for 100.000 and 0.0038500, respectively, and
the data for t
L
, t
H
, err
L
, and err
H
were as given above, then the new values R
0
′ and ALPHA′ are
computed as 100.193 and 0.0038272, respectively. Add the new values for R
0
and ALPHA into
the controller. Check the calibration by setting the temperature to t
L
and t
H
and measuring the
errors again. If desired, the calibration procedure can be repeated again to further improve the
accuracy.
R
err t err t
tt
ALPHA R
HL LH
HL
00
1‘



ALPHA
ALPHA t err ALPHA t err
tt
HLLH
HL
‘



()()11
1 ALPHA