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1575/1590 Thermometer Readout
User's Guide
7.4.4.2.5
58
sistor for the UUT. The temperature measurement need not be repeated be-
tween resistance measurements so TEMP is set to ONCE.
Table 9 SPRT Resistance
Comparison Calibration
Setup
REFR: PROBE 2
CHANNEL: INPUT 2
TEMP: ONCE
MEAS: PROMPT
RREF: SPRT
Calibration With W Values
Calibration of an SPRT can also be performed using fixed points by obtaining
the W value at each point. The W value is obtained by measuring the resistance
of the SPRT at the fixed point and at the triple point of water at each point and
then computing the ratio. This process is more time consuming but may give
greater precision. It helps to reduce error caused by the SPRT drifting during
the calibration process. The effect of self-heating can also be observed during
calibration and zero-power coefficients produced.
For this calibration process, the W values must be obtained outside of the CAL
ITS-90 function instead of using the MEASURE soft-key. The steps are as
follows:
1.
At each calibration point, you measure the resistance of the probe at the
fixed point and again at the triple point of water. W is calculated by di-
viding the fixed-point resistance by the triple point of water resistance,
R T
( )
=
W T
( )
R TPW
(
)
The self-heating can be determined while measuring resistance by in-
creasing the current by a factor of 1.414 and noting the increase in the
measurement (see Section 7.4.4.3). Subtract the difference from the mea-
sured value (with normal current) to obtain the zero-power resistance.
2.
You must now calculate resistances again to provide the necessary data
for the 1575/1590 to calculate the coefficients. The resistances are calcu-
lated using the formula,
=
R T
( )
R TPW W T
(
) ( )
The R(TPW) value used for these calculations is not to critical as long as
the same value is used for each point. You may use the most recent water
triple point resistance measurement.
3.
Set up the probe definition in preparation for calculation of coefficients.
Select the ITS-90 conversion and select the ranges you want to calibrate
(see Section 7.4.3.2.1).
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