National Instruments cFP-RTD-124, FP-RTD-124 Measuring Resistance, Converting Resistance Measurements to Temperature Measurements

Measuring Resistance

You can use the [c]FP-RTD-124 to measure resistance in ohms.

Inthis way, you can take measurements from RTDs of types that

the [c]FP-RTD-124 does not directly support (such as 120Ω nickel

RTDs) and from resistive devices other than RTDs. You can

measure resistances up to 400 Ω. Resistance values greater than

400Ω, 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,

andfor 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 mΩ/Ω/°C.

The [c]FP-RTD-124 linearizes resistance values and returns

readings in units of temperature. The available ranges are 73to

1,123K, –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.

Themodule does not automatically recognize 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 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, DIN43760, 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

of1990 (ITS-90) for high-accuracy metrology applications.