7.9.15.1 Measuring PT100s (100 Ω PRTs)
PT100s (100 Ω PRTs) are readily available. The CR1000 can measure PT100s in
several configurations, each with its own advantages.
7.9.15.1.1 Self-Heating and Resolution
PRT measurements present a dichotomy. Excitation voltage should be maximized
to maximize the measurement resolution. Conversely, excitation voltage should
be minimized to minimize self-heating of the PRT.
If the voltage drop across the PRT is ≤ 25 mV, self-heating should be less than
0.001°C in still air. To maximize measurement resolution, optimize the excitation
voltage (Vx) such that the voltage drop spans, but does not exceed, the voltage
input range.
7.9.15.1.2 PRT Calculation Standards
Two CRBasic instructions are available to facilitate PRT measurements.
PRT() — an obsolete instruction. It calculates temperature from RTD
resistance using DIN standard 43760. It is superseded in probably all cases
by PRTCalc().
PRTCalc() — calculates temperature from RTD resistance according to one
of several supported standards. PRTCalc() supersedes PRT() in probably all
cases.
For industrial grade RTDs, the relationship between temperature and resistance is
characterized by the Callendar-Van Dusen (CVD) equation. Coefficients for
different sensor types are given in published standards or by the manufacturers for
non-standard types. Measured temperatures are compared against the ITS-90
scale, a temperature instrumentation-calibration standard.
PRTCalc() follows the principles and equations given in the US ASTM E1137-04
standard for conversion of resistance to temperature. For temperature range 0 to
650 °C, a direct solution to the CVD equation results in errors < ±0.0005 °C
(caused by rounding errors in CR1000 math). For the range of –200 to 0 °C, a
fourth-order polynomial is used to convert resistance to temperature resulting in
errors of < ±0.003 °C.
These errors are only the errors in approximating the relationships between
temperature and resistance given in the relevant standards. The CVD equations
and the tables published from them are only an approximation to the true linearity
of an RTD, but are deemed adequate for industrial use. Errors in that
approximation can be several hundredths of a degree Celsius at different points in
the temperature range and vary from sensor to sensor. In addition, individual
sensors have errors relative to the standard, which can be up to ±0.3 °C at 0 °C
with increasing errors away from 0 °C, depending on the grade of sensor. Highest
accuracy is usually achieved by calibrating individual sensors over the range of
use and applying corrections to the R
S
/R
O
value input to the PRTCalc()
instruction (by using the calibrated value of R
O
) and the multiplier and offset
parameters.
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