11.3
Computing R0 and ALPHA
Before computing the new values for R
0
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 need to be restored
in the future. The new values R
0
′
and ALPHA
′
are computed by entering 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 the following equations,
R
err t
err t
t
t
ALPHA
R
H
L
L
H
H
L
0
1
0
′ =
−
−
+
⎡
⎣
⎢
⎤
⎦
⎥
ALPHA
ALPHA t err
ALPHA t err
t
t
H
L
L
H
H
L
′ =
+
− +
−
+
⎡
⎣
⎢
⎤
⎦
(
)
(
)
1
1
1
⎥
ALPHA
For example, if R0 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 R0
′
and ALPHA
′
would be computed as 100.193
and 0.0038272 respectively. Program the new values R0 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 may be re-
peated again to further improve the accuracy.
11.4
Calibration Example
The bath is to be used between 75 and 125°C and it is desired to calibrate the
bath as accurately as possible for operation within this range. The current val-
ues for R0 and ALPHA are 100.000 and 0.0038500, respectively. The calibra-
tion points are chosen to be 80.00 and 120.00°C. The measured bath
temperatures are 79.843 and 119.914°C respectively. Refer to Figure 8 for ap-
plying equations to the example data and computing the new probe constants.
6331 Calibration Bath
User’s Guide
66
