National Instruments Fieldpoint cFP-RTD-124 Bedienungsanleitung

323349A-01 October 2002

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FieldPoint Operating Instructions

FP-RTD-124

AND CFP-RTD-124

Eight-Channel Four-Wire RTD

and Resistance Input Modules

These operating instructions describe how to install and use the

National Instruments FP-RTD-124 and cFP-RTD-124 three-wire

RTD and resistance input modules (referred to inclusively as the

[c]FP-RTD-124). For details on configuring and accessing the

[c]FP-RTD-124 over a network, refer to the user manual for the

FieldPoint network module you are using.

Features

The [c]FP-RTD-124 is a FieldPoint RTD and resistance input

module with the following features:

• Inputs for 100 Ωplatinum RTDs (resistance temperature

detectors)

• Built-in linearization for six TCR (temperature coefficient of

resistance, or alpha) values of RTDs

• Direct resistance measurements with a 400 Ωrange

• Four-wire measurement for high accuracy

• 16-bit resolution

• Filtering against 50 and 60 Hz noise

• 2,300 Vrms transient overvoltage protection

• –40 to 70 °C operation

• Hot plug-and-play

FP-RTD-124 and cFP-RTD-124 2 ni.com

Installing the FP-RTD-124

The FP-RTD-124 mounts on a FieldPoint terminal base (FP-TB-x).

Hot plug-and-play enables you to install the FP-RTD-124 onto a

powered terminal base without disturbing the operation of other

modules or terminal bases. The FP-RTD-124 receives operating

power from the terminal base.

To install the FP-RTD-124, refer to Figure 1 and complete the

following steps:

1. Slide the terminal base key to either position X, used for any

module, or position 1, used for the FP-RTD-124 module.

2. Align the FP-RTD-124 alignment slots with the guide rails on

the terminalbase.

3. Press firmly to seat the FP-RTD-124 on the terminal base.

When the module is firmly seated, the terminal base latch locks

it into place.

Figure 1. Installing the FP-RTD-124

Installing the cFP-RTD-124

The cFP-RTD-124 mounts on a Compact FieldPoint backplane

(cFP-BP-x). Hot plug-and-play enables you to install the

cFP-RTD-124 onto a powered backplane without disturbing the

operation of other modules or connector blocks. The cFP-RTD-124

receives operating power from the backplane.

Alignment

Slot

Key

Latch

Guide Rails

Terminal BaseI/O Module

To install the cFP-RTD-124, refer to Figure 2 and complete the

following steps:

1. Align the captive screws on the cFP-RTD-124 with the holes

on the backplane. The alignment keys on the cFP-RTD-124

prevent backward insertion.

2. Press firmly to seat the cFP-RTD-124 on the backplane.

3. Using a number 2 Phillips screwdriver with a shank of at least

64 mm (2.5 in.) length, tighten the captive screws to 1.1 N ⋅m

(10 lb ⋅in.) of torque. The nylon coating on the screws

prevents them from loosening.

Figure 2. Installing the cFP-RTD-124

1 cFP Backplane

2 cFP Controller Module

3 cFP I/O Module

4CaptiveScrews

5 Screw Holes

2 1

FP-RTD-124 and cFP-RTD-124 4 ni.com

Wiring the [c]FP-RTD-124

The FP-TB-xterminal bases have connections for each of the eight

input channels on the FP-RTD-124. The cFP-CB-xconnector

blocks provide the same connections for the cFP-RTD-124.

Table 1 lists the terminal assignments for the signals associated

with each channel.

If you are using shielded wiring, you can reduce input signal noise

by connecting one end of the shield to the COM terminal. Do not

connect the shield to any of the wires at the signal end.

Taking RTD Measurements

with the [c]FP-RTD-124

The [c]FP-RTD-124 has eight input channels. All eight channels

share a common ground that is isolated from other modules in the

FieldPoint system. Each channel pulses a 2 mA excitation current

out of the EXCITE+terminal. The excitation current returns

through the COM terminal. The SENSE+ and SENSE–terminals

measure resistance and compensate for lead resistance errors. Each

channel is filtered, then sampled by a 16-bit analog-to-digital

converter.

Table 1. Terminal Assignments

Channel

Terminal Numbers

SENSE+ SENSE– EXCITE+ COM

0 1 2 17 18

1 3 4 19 20

2 5 6 21 22

3 7 8 23 24

4 9 10 25 26

511 12 27 28

613 14 29 30

715 16 31 32

Figure 3 shows the input circuitry on one channel.

Figure 3. FP-RTD-124 Input Circuit

Taking Measurements from Four-Wire RTDs

Four-wire RTDs generally have two wires of one color and two of

another color (usually red and white). Connect the wires of one

color to the positive terminals (EXCITE+ and SENSE+). Connect

the wires of the other color to the negative terminals (COM and

SENSE–).

Figure 4. Four-Wire RTD Connections on One Channel

Taking Measurements from Three-Wire RTDs

NI recommends the [c]FP-RTD-122 for three-wire RTD

measurements because it includes special sensing circuitry to

compensate for up to 95% of the lead resistance of the third wire.

Most three-wire RTDs have two wires of one color (usually red,

sometimes black) and one wire of another color (usually white,

sometimes red). Connect the two wires of the same color to the

SENSE+

[c]FP-RTD-124

SENSE–

COM

EXCITE+

Amplifier

Pulsed 2 mA

Four-Wire

RTD

16-bit

ADC

SENSE+

[c]FP-RTD-124

SENSE–

COM

EXCITE+

Four-Wire

RTD

FP-RTD-124 and cFP-RTD-124 6 ni.com

negative terminals (COM and SENSE–). Connect the third wire to

the positive terminal (SENSE+). Use a short jumper wire to

connect the EXCITE+ terminal to the SENSE+ terminal.

Figure 5. Three-Wire RTD Connections on One Channel

Taking Measurements from Two-Wire RTDs

Connect one wire of a two-wire RTD to the SENSE+ terminal and

the other wire to the SENSE–terminal. Use a short jumper wire to

connect the SENSE+ terminal to the EXCITE+ terminal. Use

another short jumper wire to connect the SENSE–terminal to the

COM terminal.

Figure 6. Two-Wire RTD Connections on One Channel

SENSE+

[c]FP-RTD-124

SENSE–

COM

EXCITE+

Three-Wire

RTD

Short Jumper Wire

SENSE+

[c]FP-RTD-124

SENSE–

COM

EXCITE+

Two-Wire

RTD

Short

Jumper

Wire

Short

Jumper

Wire

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 Ωnickel

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 Ωnominal 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 mΩ/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 Ωnominal

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, 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.

FP-RTD-124 and cFP-RTD-124 8 ni.com

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-124 are

appropriate for nearly all applications.

The [c]FP-RTD-124 uses a linearization curve known as the

Callendar-Van Dusen equation to measure the temperature of

RTDs. The equation is as follows:

Temperatures below 0 °C:

RT= R0[1 +A × T+B × T2+C × T3× (T–100 °C)]

Temperatures above 0 °C:

RT= R0[1 +A × T+B × T2]

T= temperature in °C

RT= RTD resistance at temperature T

R0= RTD nominal resistance at 0 °C

A, B, and C are coefficients given in Table 2.

Table 2 lists the coefficients used in this equation for each of the

TCR values that the [c]FP-RTD-124 supports. If you have a

nonstandard RTD that does not match one of these linearization

curves, measure the resistance with the [c]FP-RTD-124 and

convert the resistance to temperature in the manner suggested by

the RTD vendor.

Table 2. Callendar-Van Dusen Coefficients Used by the [c]FP-RTD-124

TCR

mΩ/Ω/°C

(°C)–1

(°C)–2

(°C)–4

3.750 3.81 ×10–3–6.02 ×10–7–6.0 × 10–12

3.851 3.9083 ×10–3–5.775 ×10–7–4.183 ×10–12

3.911 3.9692 ×10–3–5.8495 ×10–7–4.233 × 10–12

3.916 3.9739 × 10–3–5.870 ×10–7–4.4 × 10–12

3.920 3.9787 × 10–3–5.8686 ×10–7–4.167 ×10–12

3.928 3.9888 × 10–3–5.915 ×10–7–3.85 ×10–12

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 Ωof error to the measurement.

The 0.5 Ωof 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

FP-RTD-124 and cFP-RTD-124 10 ni.com

connections are isolated from the backplane and the inter-module

communication bus. The isolation is provided by the module,

which has optical and galvanic isolation barriers designed and

tested to protect against transient fault voltages of up to 2,300 Vrms.

Follow these guidelines to ensure a safe total system:

•The [c]FP-RTD-124 has a safety isolation barrier between the

I/O channels and the inter-module communication bus. There

is no isolation between channels unless otherwise noted. If any

of the channels on a module are wired at a hazardous potential,

make sure that all other devices or circuits connected to that

module are properly insulated from human contact.

•Do not share the external supply voltages (the V and C

terminals) with other devices (including other FieldPoint

devices), unless those devices are isolated from human contact.

•For Compact FieldPoint, you must connect the protective earth

(PE) ground terminal on the cFP-BP-xbackplane to the system

safety ground. The backplane PE ground terminal has the

following symbol stamped beside it: . Connect the

backplane PE ground terminal to the system safety ground

using 14 AWG (1.6 mm) wire with a ring lug. Use the 5/16 in.

panhead screw shipped with the backplane to secure the ring

lug to the backplane PE ground terminal.

•As with any hazardous voltage wiring, make sure that all

wiring and connections meet applicable electrical codes and

commonsense practices. Mount terminal bases and backplanes

in an area, position, or cabinet that prevents accidental or

unauthorized access to wiring that carries hazardous voltages.

•Operate the [c]FP-RTD-124 only at or below Pollution

Degree 2. Pollution Degree 2 means that only nonconductive

pollution occurs in most cases. Occasionally, however, a

temporary conductivity caused by condensation must be

expected.

•Refer to the FieldPoint product label for regulatory

certification under hazardous location standards. If the

FieldPoint product is not certified for operation in hazardous

locations, do not operate it in an explosive atmosphere or

where there may be flammable gases or fumes.

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