National Instruments cFP-RTD-124 User Manual download pdf (Page 9)

Effects of Lead Resistance

The [c]FP-RTD-124 measures resistance by driving a current

through a circuit and sensing the resulting voltage. Any resistance

in the lead wires that connect to the RTD can add errors to your

readings if the measured voltage includes the drop across these

wires.

For example, consider a two-wire RTD element connected as

previously shown in Figure 2. The resistance of the lead wires is

indistinguishable from the resistance of the RTD element. In a

typical application using 10 m of 22 AWG copper wire, each of the

two leads adds approximately 0.5 Ω of error to the measurement.

The 0.5 Ω of error corresponds to a temperature error of about

2.6

°C. Using the three-wire RTD connection in Figure 2 causes

the inclusion of only one of the leads in the sensed measurement.

The resultant error caused by lead resistances is half that of the

error in the two-wire example.

The most accurate way to measure temperature from RTDs is the

four-wire method shown in Figures 2 and 3. With this method, one

pair, the EXCITE and COM leads, carries the current through the

RTD. The other pair, the SENSE leads, measures the voltage

developed by the RTD element independent of the lead resistance.

Status Indicators

The [c]FP-RTD-124 has two green status LEDs, POWER and

READY. After you insert the [c]FP-RTD-124 into a terminal base

or backplane and apply power to the connected network module,

the green POWER indicator lights and the [c]FP-RTD-124

informs the network module of its presence. When the network

module recognizes the [c]FP-RTD-124, it sends initial

configuration information to the [c]FP-RTD-124. After receiving

this initial information, the green READY indicator lights and the

[c]FP-RTD-124 is in normal operating mode.

Isolation and Safety Guidelines

Caution Read the following information before

attempting to connect the [c]FP-RTD-124 to any circuits

that may contain hazardous voltages.

This section describes the isolation of the [c]FP-RTD-124 and its

compliance with international safety standards. The field wiring

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National Instruments cFP-RTD-124 User Manual Page 9