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 Ω platinum 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 Ω range
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
FieldPoint™, National Instruments™, NI™, and ni.com™ are trademarks of National Instruments Corporation.
Product and company names mentioned herein are trademarks or trade names of their respective companies.
For patents covering National Instruments products, refer to the appropriate location: Help»Patents in your software,
the patents.txt file on your CD, or ni.com/patents.
323349A-01
October 2002
© 2002 National Instruments Corp. All rights reserved.
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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.
5
2
1
4
5
4
3
1
2
cFP Backplane
cFP Controller Module
3
4
cFP I/O Module
Captive Screws
5
Screw Holes
Figure 2. Installing the cFP-RTD-124
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Wiring the [c]FP-RTD-124
The FP-TB-x terminal bases have connections for each of the eight
input channels on the FP-RTD-124. The cFP-CB-x connector
blocks provide the same connections for the cFP-RTD-124.
Table 1 lists the terminal assignments for the signals associated
with each channel.
Table 1. Terminal Assignments
Terminal Numbers
Channel
SENSE+
SENSE–
EXCITE+
COM
18
0
1
2
3
4
5
6
7
1
3
2
4
17
19
21
23
25
27
29
31
20
5
6
22
7
8
24
9
10
12
14
16
26
11
13
15
28
30
32
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.
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Figure 3 shows the input circuitry on one channel.
Pulsed 2 mA
EXCITE+
SENSE+
Amplifier
16-bit
ADC
Four-Wire
RTD
SENSE–
COM
[c]FP-RTD-124
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–).
EXCITE+
SENSE+
Four-Wire
RTD
SENSE–
COM
[c]FP-RTD-124
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
© National Instruments Corp.
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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.
Short Jumper Wire
EXCITE+
SENSE+
Three-Wire
RTD
SENSE–
COM
[c]FP-RTD-124
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.
Short
Jumper
Wire
EXCITE+
SENSE+
Two-Wire
RTD
SENSE–
COM
Short
Jumper
Wire
[c]FP-RTD-124
Figure 6. Two-Wire RTD Connections on One Channel
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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 rangeerror
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 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 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, 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.
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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
A
(°C)–1
B
(°C)–2
C
(°C)–4
TCR
mΩ/Ω/°C
3.750
3.851
3.911
3.916
3.920
3.928
3.81 × 10–3
3.9083 × 10–3
3.9692 × 10–3
3.9739 × 10–3
3.9787 × 10–3
3.9888 × 10–3
–6.02 × 10–7
–5.775 × 10–7
–5.8495 × 10–7 –4.233 × 10–12
–5.870 × 10–7 –4.4 × 10–12
–5.8686 × 10–7 –4.167 × 10–12
–5.915 × 10–7 –3.85 × 10–12
–6.0 × 10–12
–4.183 × 10–12
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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
© National Instruments Corp.
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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-x backplane 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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Specifications
These specifications are typical for the range –40 to 70 °C unless
otherwise noted.
Input Characteristics
Number of channels..........................8
ADC resolution.................................16 bits
Type of ADC.....................................Delta-sigma
Input signal ranges (software-selectable by channel)
Temperature................................73 to 1123 K
–200 to 850 °C
–328 to 1562 °F
Resistance...................................0 to 400 Ω
Temperature accuracy
Error
15 to 35 °C
–40 to 70 °C
Measured Value
–200 to 150 °C
150 to 850 °C
Typical
Maximum
0.25
Typical
Maximum
0.60
0.15
0.25
0.30
0.80
0.50
1.50
Resolution.........................................0.016 °C
Resistance accuracy
Offset error .................................0.03 Ω typical, 0.1 Ω max
Gain error ...................................0.06% typical, 0.1% max
Resolution...................................0.0061 Ω
Excitation current..............................135 ms pulses of 2 mA every
second
Input noise ........................................ 1 bit peak-to-peak
Signal input bandwidth.....................3 Hz
Update rate........................................Each channel is updated
once a second
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Physical Characteristics
Indicators ..........................................Green POWER and
READY indicators
Weight
FP-RTD-124...............................140 g (4.8 oz)
cFP-RTD-124 .............................110 g (3.7 oz)
Power Requirements
Power from network module ............350 mW
Isolation Voltage
Channel-to-channel isolation............No isolation between
channels
Transient overvoltage........................2,300 Vrms
Environmental
FieldPoint modules are intended for indoor use only. For outdoor
use, FieldPoint modules must be mounted inside a sealed
enclosure.
Operating temperature ...................... –40 to 70 °C
Storage temperature.......................... –55 to 100 °C
Humidity...........................................10 to 90% RH,
noncondensing
Maximum altitude.............................2,000 m
Pollution Degree ...............................2
Shock and Vibration
Operating shock (IEC 68-2-27)
cFP-RTD-124.............................. 50 g, 3 ms half sine, 3 shocks;
30 g, 11 ms half sine,
3 shocks
Operating vibration, random (IEC 60068-2-34)
FP-RTD-124...............................10–500 Hz, 2.2 grms
cFP-RTD-124 .............................10–500 Hz, 5 grms
Operating vibration, sinusoidal (IEC 60068-2-6)
[c]FP-RTD-124...........................10–500 Hz, 5 g
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Safety
The [c]FP-RTD-124 is designed to meet the requirements of the
following standards for safety and electrical equipment for
measurement, control, and laboratory use.
•
•
•
EN 61010-1, IEC 61010-1
UL 3121-1
CAN/CSA C22.2 No. 1010.1
For certifications under regulatory standards, including hazardous
location standards, refer to the product label or to ni.com.
Electromagnetic Compatibility
CE, C-Tick, and FCC Part 15 (Class A) Compliant
Electromagnetic emissions ...............EN 55011 Class A at 10 m
FCC Part 15A above 1 GHz
Electromagnetic immunity................Evaluated to EN 61326:
1997/A1: 1998, Table 1
Note For full EMC compliance, you must operate this
device with shielded cabling. Refer to the Declaration
of Conformity (DoC) for this product for any additional
regulatory compliance information. To obtain the DoC
for this product, click Declaration of Conformity at
ni.com/hardref.nsf/. Refer to the Read Me First
document that accompanies the product for regulatory
statements regarding FCC compliance.
© National Instruments Corp.
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Mechanical Dimensions
Figure 7 shows the mechanical dimensions of the FP-RTD-124
installed on a terminal base. Dimensions are given in millimeters
[inches]. If you are using the cFP-RTD-124, refer to the Compact
FieldPoint controller user manual for the dimensions and cabling
clearance requirements of the Compact FieldPoint system.
107.19
[4.22]
109.5
[4.31]
91.44
[3.60]
Figure 7. FP-RTD-124 Mechanical Dimensions
Where to Go for Support
For more information about setting up your FieldPoint modular I/O
system, refer to these National Instruments documents:
•
•
•
FieldPoint network module user manual
Other FieldPoint I/O module operating instructions
FieldPoint terminal base operating instructions
If you no longer have the print versions of these documents, you
can find them online at ni.com.
If you have searched the technical support resources on our Web
site and still cannot find the answers you need, contact your local
office or National Instruments corporate. For telephone support in
the United States, dial 512 795 8248. For telephone support
outside the United States, contact your local branch office:
Australia 03 9879 5166, Austria 0662 45 79 90 0,
Belgium 02 757 00 20, Brazil 55 11 3262 3599,
Canada (Calgary) 403 274 9391,
Canada (Montreal) 514 288 5722,
Canada (Ottawa) 613 233 5949, Canada (Québec) 514 694 8521,
Canada (Toronto) 905 785 0085, China 86 21 6555 7838,
Czech Republic 02 2423 5774, Denmark 45 76 26 00,
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Finland 09 725 725 11, France 01 48 14 24 24,
Germany 089 741 31 30, Greece 01 42 96 427,
Hong Kong 2645 3186, India 91 80 4190000,
Israel 03 6393737, Italy 02 413091, Japan 03 5472 2970,
Korea 02 3451 3400, Malaysia 603 9596711,
Mexico 001 800 010 0793, Netherlands 0348 433466,
New Zealand 09 914 0488, Norway 32 27 73 00,
Poland 22 3390 150, Portugal 210 311 210, Russia 095 238 7139,
Singapore 65 6 226 5886, Slovenia 3 425 4200,
South Africa 11 805 8197, Spain 91 640 0085,
Sweden 08 587 895 00, Switzerland 056 200 51 51,
Taiwan 02 2528 7227, United Kingdom 01635 523545
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