National Instruments cFP-RTD-124 - Manual (Page 9)

© National Instruments Corp. 9 FP-RTD-124 and cFP-RTD-124
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