Archived: How to Use Thermocouples with an SCXI-1102 Module

Publish Date: Jan 10, 2019 | 35 Ratings | 3.97 out of 5 | Print | Submit your review

Overview

This document has been archived and is no longer updated by National Instruments.

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.

Table of Contents

  1. SCXI-1102 Overview
  2. Thermocouple Measurement Example
  3. Thermocouple Overview
  4. Signal Conditioning Considerations
  5. Thermocouple Measurements for Newer Designs

1. 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.



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.

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2. Thermocouple Measurement Example

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.



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

 

Step 2. Assemble Your SCXI System

A. Configure the SCXI Module and Terminal Block— Some versions of SCXI Modules are jumper-less and all settings are done through software. Other modules will have jumpers. Please refer to your user manual for the jumper settings.

B. Insert the SCXI module into your SCXI chassis. The particular slot in which the module is placed does not matter.

 

C. Cable the SCXI module to your DAQ board—If you are using a plug-in DAQ board, connect the SCXI module to your DAQ board using the appropriate SCXI-134X cable assembly. If you have multiple modules in the same chassis, see step 5 of the SCXI Quick Start Guide to determine to which module you should connect your DAQ board.

 

D. Attach terminal block—Plug the SCXI terminal block onto the front of the SCXI module. The SCXI-1112 does not require a terminal block, please see users manual for all other setups.

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.

 

E. Power up your SCXI chassis and computer. The order in which you power up your SCXI chassis and computer does not matter but the setup will not work unless both are on.

Figure 3 shows the components of an SCXI system cabled to a DAQ board. See users manual for directions on installation and configuration of a DAQ card.

 

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.



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 system software, first make sure that you have installed NI-DAQ software. Refer to your NI-DAQ manual for detailed installation instructions. The following screenshots include a PCI-MIO-16E-4, SCXI-1000, SCXI-1102, and SCXI-1303. Your setup may be different, but can be configured using this same procedure.


1. Launch Measurement & Automation Explorer
2. Expand “Devices and Interfaces” under “My System”
3. Expand “NI-DAQmx Devices” and check to make sure you see your installed DAQ card



4. Right-click on the “NI-DAQmx Devices” folder under “Devices and Interfaces”
5. Choose Create New NI-DAQmx Device and continue on to your specific signal conditioning device (SCXI-1000 shown)




6. Create New SCXI Chassis
The “Chassis Communicator” is the DAQ card to which your chassis is cabled. If you only have one DAQ device installed it will be selected by default.
The “Communicating Module Slot” is the slot in which you have the cabled module (usually 1)
Click Save







7. Your modules should auto-detect (Chassis must be powered on)
Accessories will not auto-detect: select an accessory for each module










8. Your device will now appear under “NI-DAQmx Devices”







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3. 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:

DV = SDT


DV = SDT where DV is the change in voltage, S is the Seebeck coefficient, and DT is the change in temperature. S varies 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.

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4. 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

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 calculateVTC (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 T is the temperature in degrees Celsius, x is the thermocouple voltage in volts, and a0 through an are 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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5. Thermocouple Measurements for Newer Designs

For newer designs, NI recommends the use of the PXIe-4353 for thermocouple measurements. The PXIe-4353 is specifically designed for thermocouple measurements and maintains the 32-channels per module slot. Since this module is designed for thermocouples, any calculations and conversions are automatically done for you. Also, the PXIe-4353 is more accurate for every thermocouple type. 

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