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525B
Users Manual
3-8
Default RTD Coefficients
The USR_DEF function of the calibrator uses the Calendar-Van Dusen equation
for outputting and measuring custom RTDs. The C coefficient is only used for the
subrange –260 to 0 degrees Celsius. Only the A and B coefficients are needed for
the subrange 0 to 630 degrees. The R0 is the resistance of the probe at 0 degrees
Celsius. All of the USR_DEF will be set to PT385 as shown in Table 3-1.
Table 3-1. Default RTD Coefficients
Sub range R
0
Coefficient A Coefficient B Coefficient C
USR_DEF1 0 to 630 100 3.908X10-3 -5.775e-7
USR_DEF2 -260 to 0 100 3.908X10-3 -5.775e-7 -4.183e-12
USR_DEF3 0 to 630 100 3.908X10-3 -5.775e-7
USR_DEF4 -260 to 0 100 3.908X10-3 -5.775e-7 -4.183e-12
USR_DEF5 0 to 630 100 3.908X10-3 -5.775e-7
If other types of RTD are required, Table 3-2 shows the coefficients for PT391 and
PT392. Again, C is only used for temperatures below 0 degrees Celsius.
Table 3-2. Other Commonly Used RTDs
RTD Type Coefficient A Coefficient B Coefficient C
PT392 3.9848X10-3 -5.87X10-7 -4X10-12
PT391 3.9692X10-3 -5.8495X10-7 -4.2325X10-12
The SPRT function of the calibrator uses ITS-90 standard coefficients for
measuring a SPRT. Since the five coefficients are deviations all of them will be set
to 0.
The coefficients A- and B- represent the A4 and B4 coefficient, obtained when the
SPRT is calibrated at the triple points of argon, mercury, and water. This covers
the 83.8058K to 273.16K subrange. Coefficients A, B, and C can represent
different coefficients based on which subranges of the SPRT has been calibrated.
For example, if the 273.15K to 933.473K subrange was used, A, B, and C would
represent A7, B7, and C7 whereas if the 273.15K to 692.67K subrange was used,
A and B would represent A8 and B8 and C=0.
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