Fluke 7312, Руководство пользователя

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The user should keep a record of these values in case they may need to be re-
stored in the future. The new values DO’ and DG’ are computed by entering the
old values for DO and DG, the calibration temperature set-points, t
L
and t
H
, and
the temperature errors, err
L
and err
H
, into the following equations,
D
err t D err t D
t t
D
L H H L
H L
0
0 0
0
′ =
− − −
−
+
( ) ( )
DG
err err
t t
DG
H L
H L
′ =
−
−
+
⎡
⎣
⎢
⎤
⎦
⎥ 1
For example, if DO and DG were previously set for –25.229 and 186.9740 re-
spectively and the data for t
L
, t
H
, err
L
, and err
H
were as given above then the
new values D0
′
and DG
′
would be computed as –25.831 and 188.220 respec-
tively. Program the new values D0 and DG into the controller. The new con-
stants are used the next time the bath temperature is set. Check the calibration
by setting the temperature to t
L
and t
H
and measuring the errors again. If de-
sired, the calibration procedure may be repeated again to further improve the
accuracy.
11.4 Calibration Example
The bath is to be used between 25 and 75°C and it is desired to calibrate the
bath as accurately as possible for operation within this range. The current val-
ues for D0 and DG are –25.229 and 186.974 respectively. The calibration
points are chosen to be 25.00 and 75.00°C. The measured bath temperatures are
24.869 and 74.901°C respectively. Refer to Figure 10 for applying the equa-
tions to the example data and computing the new probe constants.
11.5 Single Point Calibration
When calibrating the bath for applications emphasizing a single temperature, a
single point calibration can be quicker, easier and more accurate. These appli-
cations include maintenance of a triple point of water cell near zero degrees
Celsius or for the gallium point near 30
°
C. An adjustment of only the D0 cali-
bration constant is made in this process. The advantage of calibration is to have
the bath set-point temperature more closely reflect the desired bath tempera-
ture. While this procedure improves the accuracy at a specific set-point, the re-
sults may not be accurate enough alone to make the baths final temperature
adjustment. For example, for the maintenance of a triple point of water cell, the
bath temperature must be known to within one thousandth of a degree Celsius.
The calibration of the bath is not intended to get that close. Final adjustments
7312 TPW Maintenance Bath
User’s Guide
66