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© National Instruments Corp. 7 FP-RTD-122 and cFP-RTD-122
Converting Resistance Measurements
to Temperature Measurements
The [c]FP-RTD-122 has built-in linearization algorithms for
platinum RTDs of either 100 or 1,000 Ω nominal resistance,
and for six TCR (or alpha,
α) values. The TCR is the average
temperature coefficient of resistance of an RTD from 0
to 100 °C.
This document specifies TCR in units of mΩ/Ω/°C.
The [c]FP-RTD-122 linearizes resistance values and returns
readings in units of temperature. The available ranges are
73 to 1,123 K, –200 to 850 °C, and –328 to 1,562 °F. You can
configure each channel independently, so you can connect different
types of RTDs to each channel.
Note You must configure each channel of the
[c]FP-RTD-122 for the RTD type connected to it. The
module does not automatically recognize RTD types.
RTD Types
RTD types are specified by material composition, nominal
resistance at 0 °C, and TCR. The [c]FP-RTD-122 can directly
measure the temperature of platinum RTDs of either 100 or
1,000 Ω nominal resistance. These RTDs are commonly referred to
as PT100 or PT1000 RTDs, respectively.
Different types of platinum RTDs have different TCRs. The
[c]FP-RTD-122 supports the following TCRs: 3.750, 3.851, 3.911,
3.916, 3.920, and 3.928 mΩ/Ω/°C. The most common TCR for
RTDs is 3.851 mΩ/Ω/°C. It is defined in international standards
such as IEC-751, DIN 43760, BS 1904, and ASTM E1137.
The TCR of 3.928 mΩ/Ω/°C is used in the reference function for
platinum thermometers in the International Temperature Scale
of 1990 (ITS-90) for high-accuracy metrology applications.
Unfortunately, not all TCR values are as well defined by standards
organizations, and the behavior of RTDs with the same TCR value
may vary from vendor to vendor. The variations are usually small,
and the built-in linearization algorithms of the [c]FP-RTD-122 are
appropriate for nearly all applications.