National Instruments SCXI SCXI-1102 Instrucciones de instalación y funcionamiento

NATIONAL 

INSTRUMENTS®

The Software is the Instrument

Application Note 076

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How to Use Thermocouples with an

SCXI-1102 Module

This application note describes how to set up and use the SCXI-1102 with thermocouples. It also describes the

example programs that use the SCXI-1102 to make thermocouple measurements. These example programs are

included on the SCXI Application Examples disk included in the SCXI Getting Started Pack (P/N 777132-01).

You can also download these programs from the National Instruments BBS. These programs, written in

LabVIEW, LabWindows/CVI, and C specifically for the configuration described in this application note, are

easy to understand and use. They serve as a starting point for more complicated programs.

Following the programming instructions, this note includes some useful background information – a brief

description of thermocouple operation and special signal conditioning requirements for thermocouples.

SCXI-1102 Overview

The SCXI-1102 is a 32-channel thermocouple amplifier module. Each of the 32 channels includes a gain

amplifier and lowpass filter. The block diagram in Figure 1 shows the basic function blocks of the SCXI-1102.

Thermocouple Vout to DAQ

board or 

SCXI bus

Ground

Reference

CJ Sensor

Input

Protection

Gain

(1 or 100)

Multiplexer

SCXI-1102

SCXI-1303

Terminal

Block

32 Channels

–Filter

Filter

–

•

•

•

•

Buffer

Figure 1. Block Diagram of the SCXI-1102

Each input channel includes input protection and a lowpass noise filter with a fixed bandwidth of 1 Hz. Each

channel also includes a software programmable gain amplifier that can be set for a gain of 100 (for

thermocouple and other millivolt signals) or 1 (for ±10 V inputs). The outputs of the 32 amplifiers are

connected to a multiplexer, which passes the conditioned signals to the DAQ board or device, or the SCXI bus

in the backplane of the SCXI chassis.

You connect thermocouples to the SCXI-1102 via the SCXI-1303 or SCXI-1300 terminal blocks, which include

an onboard temperature sensor for cold-junction compensation. Alternatively, you can use the TBX-1303 DIN-

rail-mountable terminal block, and cable the TBX-1303 to the SCXI-1102 with a SH96-96 or R96-96 cable.

The SCXI-1303 and TBX-1303 are recommended for thermocouples because they include isothermal

construction, ground-referencing connections for floating thermocouples, and a high-precision cold-junction

sensor.

Thermocouple Measurement Example

For your convenience, a J-type thermocouple wire is included with the SCXI Getting Started Pack. Use this

thermocouple sensor (or substitute your own thermocouple) in the example described in this note.

This section describes step-by-step how to connect the J-type thermocouple to your SCXI system, configure

and install your SCXI module, and take measurements with the thermocouple. The SCXI Application Examples

diskette includes example programs written in LabVIEW, LabWindows/CVI, and C for this application.

Step 1. Connect The Thermocouple

Wire the thermocouple to channel 0 on your SCXI terminal block. For the J-type thermocouple included with

the SCXI Getting Started Pack, connect the positive (white) lead to the positive channel and the negative (red)

lead to the negative channel. Figure 2a shows the thermocouple connection to the SCXI-1303, and Figure 2b

shows the thermocouple connection to the SCXI-1300.

You do not need to connect the negative terminal to ground on the SCXI-1303 because the negative

connection of each channel on the terminal block is already wired to ground through a bias resistor.

Note: Most thermocouples, like this one, are floating with respect to ground. For floating

thermocouples, the signal connections of Figure 2 are correct. If, however, the

thermocouples that you will use in your application are not floating, but connected to a

ground point, then a) if you are using the SCXI-1303, disable the ground-referencing

circuitry on the SCXI-1303 (see the SCXI-1303 Terminal Block Installation Guide); or b) if

using the SCXI-1300, do not connect the negative lead of the thermocouple to chassis

ground of the SCXI-1300 terminal block. These steps are necessary to prevent the creation

of ground loops.

MTEMP

Thermocouple

- (red)

+ (white)

Thermocouple

This wire grounds the

thermocouple to the

chassis ground of the

terminal block.

Chassis Ground

- (red)

+ (white)

MTEMP

a) SCXI-1300 b) SCXI-1303

Figure 2. Connecting a Thermocouple to Channel 0 of the SCXI Terminal Block

Step 2. Assemble Your SCXI System

A. Configure the SCXI-1102 Module and Terminal Block—Most configuration

settings of the SCXI-1102 are set via software. The SCXI-1102 does include

one jumper, W1, which you should leave in the factory-default position

(position P). You do not need to modify the setting of W1 jumper unless you

are daisy-chaining multiple SCXI chassis and a cable is attached directly to

the SCXI-1102.

Note: The SCXI-1300 and SCXI-1303 include a jumper to select the MTEMP or DTEMP channel

for the cold-junction sensor input. When used with the SCXI-1102, the position of this

jumper is irrelevant; MTEMP and DTEMP are connected internally in the SCXI-1102.

B. Insert the SCXI-1102 module into your SCXI chassis.

C. Cable SCXI module to your DAQ board—If you are using a plug-in DAQ

board, connect the SCXI-1102 to your DAQ board using the appropriate

SCXI-134X cable assembly.

If you are using the SCXI-1200 module, install the module into your SCXI

chassis and connect the SCXI-1200 to the parallel port of your PC, using the

cable included in the SCXI-1200 kit.

D. Attach terminal block—Finally, plug the SCXI-1300 or SCXI-1303 terminal

block onto the front of the SCXI-1102 module.

Finally, power up your SCXI chassis and computer. Figure 3 shows the

components of an SCXI system cabled to a DAQ board.

For more complete information on how to set up an SCXI system (multiple modules, configuration of different

types of modules, and so on.), refer to the Getting Started with SCXI manual.

SCXI

Cable 

Assembly

SCXI-1303 or

SCXI-1300

Terminal Blocks

SCXI-1102

Module

SCXI Chassis

SCXI-1001

MAINFRAME

SCXI

DAQ Board

Thermocouple

SCXI 1100

Figure 3. SCXI-1102 System Diagram (with DAQ Board)

Step 3. Configure Your Software

Note: If you are using a plug-in DAQ board, and have not installed and configured the board, you

should do so now. Refer to your hardware user manual for instructions.

To configure your sytem software, first make sure that you have installed NI-DAQ software. Refer to your

NI-DAQ manual for detailed installation instructions.

After you install NI-DAQ software, run either wdaqconf.exe for Windows or NI-DAQ Utilities for Mac OS; then

configure your DAQ board and SCXI chassis. The following figures show configuration settings for Windows.

Run the appropriate configuration utility, selecting the parameters shown below. The following figures include

an AT-MIO-16E-2; your device may differ.

The Windows system settings are shown below:

1. Configure your

plug-in DAQ board

(if used) as device

number 1.

2. Click on the SCXI

menu to open the

SCXI Configuration

window.

1. Select your chassis type.

4. Click here to

configure Module 1.

These settings will be set auto-

matically for you.

2. Select chassis slot 1.

3. Select the SCXI-1102 module.

1. Select your DAQ board

(Dev #1).

3. Set channel to 0.

2. Select your terminal block.

4. Set gain to 100.0.

5. Save your settings.

The Macintosh system settings are shown below.

1. Click here and choose

SCXI configuration.

1. Select your chassis type.

2. Select the SCXI-1102 module.

3. Select your DAQ board.

4. Select chassis slot number.

5. Choose multiplex mode.

6. Set gain to 100.

7. Set channel to 0.

Step 4. Run the Program Example

A diskette labeled SCXI Application Examples is included in the Getting Started with SCXI pack. This diskette

includes three example programs written to illustrate the use of the SCXI-1102 with thermocouples. The three

example programs are written using LabVIEW, LabWindows/CVI, and C programming language.

Each of the example programs perform the measurement of the J-type thermocouple and displays the

temperature reading in degrees Celsius. Below is a general flow chart of the example programs that shows the

fundamental steps in programming the SCXI-1102 for thermocouples.

Start

Read

Cold-Junction

Temperature Voltage

(CJTEMP)



Read

Thermocouple

Voltage

Convert Thermocouple

Reading to Temperature

Units

Read

Again

Stop

Plot

Temperature

Yes

Figure 4. Flow Chart of Programming the SCXI-1102 for Thermocouple Measurement

The SCXI Applications Example diskette includes several basic programming examples, including three that

illustrate the use of the SCXI-1102 with thermocouples in three different programming environments.

• LabVIEW Example: 1102tc.vi

File Location: a:\labview\1102\1102tc.vi (on SCXI Applications Examples diskette)

• LabWindows/CVI Example: 1102tc.prj

File Location: a:\cvi\1102\1102tc.prj (on SCXI Applications Examples diskette)

• C Language Example: 1102tc.c

File Location: a:\c_ex\1102\1102tc.c (on SCXI Applications Examples diskette)

Thermocouple Overview

A thermocouple is created whenever two dissimilar metals touch and the contact point produces a small open-

circuit voltage that varies as a function of temperature. This thermoelectric voltage is known as the Seebeck

voltage and is nonlinear with respect to temperature. However, for very small changes in temperature, the

voltage is approximately linear and can be expressed in the following equation:

∆

V = S

∆

where

∆

Vis the change in voltage, Sis the Seebeck coefficient, and

∆

Tis the change in temperature.

Svaries with changes in temperature, causing the output voltages of thermocouples to be nonlinear over their

operating ranges.

Several types of thermocouples are generally available; these thermocouples are designated by capital letters

that indicate their composition according to American National Standards Institute (ANSI) conversions. For

example, the J-type thermocouple included in this package consists of one iron wire and one constantan

(a copper-nickel alloy) wire.

Signal Conditioning Considerations

When using thermocouples, you should be aware of several measurement issues such as the following:

• Cold-junction compensation

• Nonlinear data

• Low-voltage signals

• Noisy signals

Cold-Junction Compensation

When you connect a thermocouple wire to your terminal block, you create one or two additional

thermocouples where the sensor wire contacts the terminal. You must account for the voltage generated at this

junction, called the cold junction. Use the following equation:

VTC (TTC)= VMEAS + VTC (Tcjc)

where VMEAS is the thermocouple voltage measured by your data acquisition (DAQ) system, VTC (Tcjc)is the

cold-junction voltage at the terminal block, and VTC (TTC)is the compensated thermocouple voltage that you

can scale to temperature units.

First, measure the temperature of the cold junction, Tcjc, and use this reading to calculate VTC (Tcjc). The

SCXI terminal blocks include a special cold-junction temperature sensor for this purpose. The first thing that

your temperature measurement program does is take a reading of the cold-junction temperature sensor. If you

are using the SCXI-1300, this sensor is an IC temperature sensor, which outputs 10 mV/˚C.

The SCXI-1303 isothermal terminal block includes a high-precision thermistor temperature sensor that outputs

1.91 V at 0˚ C to 0.58 V at 55˚ C. Because the thermistor output is nonlinear, you must use a thermistor

linearizing function to convert the measured voltage to temperature. National Instruments software supplies

you with a function to linearize thermistor outputs.

After you determine the cold-junction temperature, your software must convert this temperature into the

voltage for the appropriate thermocouple type. National Instruments conversion functions include thermocouple

temperature-to-voltage routines for this conversion. You can also use standard look-up tables or polynomials

listed in National Institute of Standards and Technology (NIST) NBS Monograph 175.

Finally, add the voltage, VTC (Tcjc), to your measured thermocouple voltage, VMEAS, to yield the compensated

thermocouple voltage, VTC (TTC).

Nonlinear Data

After determining the compensated thermocouple voltage, you must convert this voltage into temperature

units. The voltage-temperature relationship of thermocouples is very nonlinear. To convert the nonlinear

voltage readings into temperature values, you can use either look-up tables or the following polynomial

equation:

T = a0+ a1x + a2x2... + anxn

where Tis the temperature in degrees Celsius, xis the thermocouple voltage in volts, and a0through anare

coefficients that are specific to each thermocouple type. National Instruments software provides thermocouple

linearization functions that use this polynomial equation.

Low-Voltage Signals

Thermocouples generate low-voltage signals, typically in the millivolt range. For example, a J-type

thermocouple outputs –8.1 mV at –210˚ C and 21.8 mV at 400˚ C. Therefore, you must amplify the signal to

accurately read and digitize it. The SCXI-1102 has programmable gain amplifiers that can be programmed for

a gain of 1 or 100. This amplification yields an input voltage range of ±10 V or ±100 mV. This gain can be

combined with gain on the DAQ board or module for higher amplification. For example, an AT-MIO-16E-2

configured for a gain of 5 and an SCXI-1102 configured for a gain of 100 yields an input range of ±20 mV.

Noisy Signals

• Low-voltage signals are susceptible to noise corruption. Thermocouple wire acts as an antenna and picks

up stray electromagnetic signals in the environment. The most common sources of stray electromagnetic

waves are power lines, electric motors, and computer monitors. Poor grounding of your system also

produces noise. Use the following methods to diminish the effects of noise on your thermocouple.

• Use a shielded cable from the SCXI chassis to the plug-in DAQ board and apply extra shielding to your

thermocouple wire.

• Use lowpass noise filters to attenuate high-frequency noise. The SCXI-1102 includes 1 Hz lowpass filters

on every channel to maximize rejection of 50 Hz and 60 Hz noise.

• Use the SCXI-1102 programmable gain instrumentation amplifiers (PGIA) to amplify the signal and

increase the noise immunity before the signal leaves the SCXI module.

• Make sure you have only one point of ground for your thermocouple circuit.

• In extreme cases, you may find it helpful to average your thermocouple voltage readings to improve noise

rejection. For example, acquire 100 samples from a single thermocouple and use the average value of all

the samples as one data point.

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