National Instruments cFP-RTD-124 - Manual (Page 7)

© National Instruments Corp. 7 FP-RTD-124 and cFP-RTD-124
Measuring Resistance
You can use the [c]FP-RTD-124 to measure resistance in ohms.
In this way, you can take measurements from RTDs of types that
the [c]FP-RTD-124 does not directly support (such as 120
Ω nickel
RTDs) and from resistive devices other than RTDs. You can
measure resistances up to 400
Ω. Resistance values greater than
400 Ω, including open circuits, result in an
Out of range error
for the affected channels. The [c]FP-RTD-124 ignores any
configuration of RTD type for channels with resistance selected.
Converting Resistance Measurements
to Temperature Measurements
The [c]FP-RTD-124 has built-in linearization algorithms for
platinum RTDs of either 100 or 1,000
Ω nominal resistance,
and for 6 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 m
Ω/Ω/°C.
The [c]FP-RTD-124 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-124 for the type of RTD 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-124 can directly
measure the temperature of platinum RTDs of 100 Ω nominal
resistance. These RTDs are commonly referred to as PT100 RTDs.
Different types of platinum RTDs have different TCRs. The
[c]FP-RTD-124 supports the following TCRs: 3.750, 3.851, 3.911,
3.916, 3.920, and 3.928 m
Ω/Ω/°C. The most common TCR for
RTDs is 3.851 m Ω/Ω/°C. It is defined in international standards
such as IEC-751, DIN 43760, BS 1904, and ASTM E1137. The
TCR of 3.928 m
Ω/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.