Choosing a Thermocouple Measurement Device

Publish Date: Nov 07, 2014 | 42 Ratings | 3.45 out of 5 |  PDF

Overview

This document outlines considerations when building instrumentation systems for thermocouple temperature sensors. It also provides an overview of NI measurement devices that meet the needs of these sensor types.

Table of Contents

  1. Signal Conditioning for Thermocouple Sensors
  2. NI-Compatible Thermocouples
  3. Platforms for Measuring Thermocouples

Learn more about the Fundamentals of Thermocouples before you begin.

1. Signal Conditioning for Thermocouple Sensors

Thermocouple output signals are typically in the millivolt range, and generally have a very low temperature to voltage sensitivity, which means that you must pay careful attention to the sources of errors that can impact your measurement accuracy. The primary sources of error for thermocouple measurements are:

  • Cold-junction compensation (CJC) errors
  • Offset and gain errors
  • Noise errors
  • Thermocouple errors

CJC Errors

CJC errors represent the difference between the actual temperature at the point where the thermocouple is connected to the measurement device (the cold-junction temperature) and the measured temperature by the device. The CJC error is roughly a 1:1 contributor to the accuracy of the temperature measurement of the thermocouple, and it is often one of the largest single contributors to the overall accuracy. The overall CJC error includes the error from the CJC temperature sensor (often a thermistor) used to sense the cold-junction temperature, the error from the device measuring the CJC sensor, and the temperature gradient between the cold junction and the CJC sensor. Of these three errors, the temperature gradient between the cold junction and the CJC sensor is generally the largest, and it typically has the largest variation. The error from the CJC sensor can be a large contributor in many devices; however, high-accuracy thermistors or resistance temperature detectors (RTDs) with small errors are common in many high-end thermocouple measurement devices.

The error from the temperature gradient between the cold junction and the CJC sensor is where you generally have the most control. A well-designed thermocouple device can often minimize this error considerably; however, the magnitude of this error still often depends on the environment in which the thermocouple device is used. Because the error comes from the temperature difference between the cold junction and the CJC sensor, anything that can introduce a temperature gradient in a thermocouple device influences this error. Maintaining your thermocouple device in a stable environment with minimal temperature variation and low air flow is the best way to improve CJC accuracy. Adjacent heat sources, such as other instruments, can also affect CJC accuracy. Some devices have a single CJC sensor for many channels, while others may have multiple CJC sensors. As a general rule, devices with a low ratio of channels to CJC sensors are less susceptible to errors from temperature gradients. Refer to the device documentation for specifics about CJC accuracy and other recommendations for improving overall CJC accuracy.

Offset and Gain Errors

Because thermocouples often output signals very close to 0 V and have a full input range that is measured in millivolts, offset errors from the measurement device can be a large contributor to overall accuracy. Many devices support a built-in autozero function that measures the internal offset of the circuit automatically. If a device supports built-in autozero, this is often the best way to compensate for offset errors and offset drift in the measurement device. Read the device documentation to determine if autozero is supported. If autozero is not supported, pay careful attention to the contribution of the offset error specification to the overall accuracy of the measurement device, and ensure that the device is regularly calibrated.

Gain errors are proportional to the input voltage, so they generally have the largest impact when thermocouples are measuring temperatures at the edge of their supported ranges.

Noise Errors

Thermocouple output signals are typically in the millivolt range, making them susceptible to noise. Noise can be introduced either by the external environment or by the measurement device. Lowpass filters are commonly used in thermocouple DAQ systems to effectively eliminate high-frequency noise in thermocouple measurements. For instance, lowpass filters are useful for removing the 50 Hz and 60 Hz power line noise that is prevalent in many laboratory and manufacturing settings.

For measurement devices with a large input range, you can also improve the noise performance of your system by amplifying the low-level thermocouple voltages near the signal source (measurement point) to match the output range of the thermocouples. Because thermocouple output voltage levels are very low, you should choose a gain or input range for the device that optimizes the input limits of the analog-to-digital converter (ADC). The output range of all thermocouple types falls between -10 mV and 80 mV.

Another source of noise is caused by thermocouples being mounted or soldered directly to a conductive material such as steel or being submerged in conductive liquids such as water. When connected to a conductive material, thermocouples are particularly susceptible to common-mode noise and ground loops. Isolation helps prevent ground loops from occurring, and can dramatically improve the rejection of common-mode noise. With conductive materials that have a large common-mode voltage, isolation is required because nonisolated amplifiers cannot measure signals with large common-mode voltages.

Thermocouple Errors

These errors are introduced by the thermocouple. The voltage generated by the thermocouple is proportional to the temperature difference between the point where the temperature is measured and the point where the thermocouple connects to the device. Temperature gradients across the thermocouple wire can introduce errors due to impurities in the metals, which can be large relative to most measurement devices. Refer to the thermocouple documentation to understand its accuracy impact on the overall measurement.

 

Back to Top

2. NI-Compatible Thermocouples

Ready-Made

 

For cost-sensitive applications, National Instruments offers ready-made thermocouples and individual packets of thermocouple wire with the measuring junction provided at one end. These thermocouples are available in 1 m (39.4 in.) and 2 m (78.7 in.) lengths. For shorter lengths, trim the thermocouple to the desired length and then install it into your application. Ready-made thermocouples are ideal for starter or educational applications.

Features

  • Available in 1 m and 2 m lengths
  • Available in J, K, T, and E types
  • Ideal for starter and educational applications
  • Measures up to 900 °F (482 °C)
  • Measuring junction provided

Field-Cuttable

 

National Instruments field-cuttable thermocouples meet a wide variety of temperature application needs. With field-cuttable thermocouples, you can cut the probe to the desired length from 3.5 in. to 24 in. (8.9 cm to 61 cm). Therefore, you can use one stock of thermocouples for all your temperature measurements, which reduces inventory cost and production downtime. These probes can measure up to 900 °F(482 °C), and are available in grounded and ungrounded versions. For applications using thermal wells, the optional spring-loaded fitting reduces installation time while ensuring contact with the thermal well. Each probe has glass braid insulated leads and is sold individually.

Features

  • Available in J, K, T, and E types
  • Cuttable from 24 in. to 3.5 in.
  • Measures up to 900 °F (482 °C)
  • Reduces inventory cost and downtime

Extension Wire

 

Use extension wire to connect your thermocouple elements to your measurement and automation systems in less harsh environments. It is important to match your extension wire type to your thermocouple element type. Otherwise, errors can be introduced into your system.

Features

  • Available in 30 m and 300 m spools
  • Available in Jx, Kx, Tx, and Ex types

 

Back to Top

3. Platforms for Measuring Thermocouples

Compact Measurements

Benchtop or Field Measurements

An NI CompactDAQ system consists of a chassis, NI C Series I/O modules, and software. Chassis can connect to a host computer over USB, Ethernet, or 802.11 WiFi or operate stand-alone with a built-in controller. With over 50 measurement-specific modules and 1-, 4-, and 8-slot chassis, NI CompactDAQ provides a flexible, expandable platform to meet the needs of any electrical or sensor measurement system.

  • Multiple timing engines for multiple acquisition rates
  • Advanced counter functionality from NI-STC3 technology

What Is NI CompactDAQ?

Embedded Data-Logging Measurements

NI CompactDAQ controllers offer a high-performance platform for embedded measurements and data logging. Controllers feature an integrated computer and nonvolatile storage, so that NI CompactDAQ can be deployed without an external computer.

  • Includes Intel multicore processing, up to 32 GB nonvolatile storage, and 2 GB RAM
  • Simultaneously stream continuous measurements with sample rates up to 1 MS/s per channel

What Is NI CompactDAQ?

Extreme Ruggedness and Advanced Control

NI CompactRIO is a reconfigurable embedded control and acquisition system. The CompactRIO hardware architecture includes a reconfigurable FPGA chassis and an embedded controller. You can use CompactRIO in a variety of embedded control and monitoring applications.

  • A variety of reconfigurable chassis featuring an FPGA for custom timing, analysis, and control
  • Open embedded architecture with small size and extreme ruggedness

What Is NI CompactRIO?

 

C Series I/O Modules With Integrated Signal Conditioning for Temperature With Thermocouples

NI 9214 

For C Series I/O modules specifically designed for the measurement of thermocouples, National Instruments offers the NI 9211, NI 9213, and the NI 9214. These modules contain all of the signal conditioning required to measure thermocouples simultaneously. They also feature 24-bit ADCs for up to 0.02 °C measurement sensitivity. The NI 9213 and NI 9214 have a higher channel count, up to 16 channels, and 250 Vrms channel-to-earth ground safety isolation. The NI 9214 also provides an isothermal terminal block for measurement accuracy up to 0.45 °C, several CJC sensors, and an autozero channel for offset error compensation.

NI 9211, NI 9213, NI 9214 Measurement Systems

Thermocouple Data Logger—Up to 16 Channels

This thermocouple data logger consists of one 4-channel (NI 9211), 16-channel (NI 9213), or isolated 4-channel (NI 9219) C Series thermocouple module and one USB (NI 9171), 802.11a/b wireless (NI 9191), or Ethernet (NI 9181) single module carrier. Each module is specifically designed for thermocouple measurements and features integrated signal conditioning that provides direct connectivity to your sensors. The 24 bits of resolution along with built-in CJC offer superior accuracy. The isolated 4-channel module includes 250 Vrms channel-to-earth ground isolation for safety, noise immunity, and high common-mode voltage range.

Thermocouple Data Logger

 

High-Performance and High-Channel-Count Systems

 

The PXI platform offers a variety of chassis, controller, and module options so you can create a measurement system to meet your specific application needs. The NI SC Express strain modules can provide up to 0.02 percent accuracy at up to 102.4 kS/s. PXI can scale to thousands of strain and bridge-based measurement channels for large applications, and provides both wired and wireless synchronization options.

  • Configurable desktop, rack-mount, and distributed multichassis solutions
  • Compatibility with a wide range of sensors and transducers
  • More than 1,500 PXI modules available, including CAN, vision, motion, and GPS

PXI I/O Modules With Integrated Signal Conditioning for Measuring Thermocouples

The NI SC Express family features PXI Express DAQ modules with integrated signal conditioning for sensor measurements, such as thermocouples and other temperature transducers.

 

NI PXIe-4353 

32-Channel, 24-Bit, Thermocouple Input Module

The NI PXIe-4353 thermocouple input module provides integrated DAQ and signal conditioning for temperature measurements. This module includes increased accuracy and synchronization features for scalable measurement systems from low- to high-channel counts.

The NI PXIe-4353 features 32 channels with three 24-bit analog-to-digital converters (ADCs) and operates with high-speed (90 S/s per channel) and high-resolution (1 S/s per channel) modes. It is designed to be highly accurate with 0.3 °C typical accuracy for the module and terminal block. The module has two autozero channels for offset compensation as well as open thermocouple detection to identify disconnected thermocouples.

The high accuracy is attributed to the NI TB-4353 isothermal terminal block that includes eight CJC channels. It has a unique design that optimizes thermal conductivity between the thermocouple terminals and the CJCs for an isothermal error as low as 0.25 °C. The TB-4353 is a front mount terminal block with screw terminal connectivity.

NI PXIe-4353 product details

 

NI InstantDAQ Technology

NI USB-TC01 

 

The NI USB-TC01 thermocouple measurement device features NI InstantDAQ technology so you can instantly take temperature measurements with your PC. Just plug it in and built-in software for viewing and logging data automatically loads. No driver installation is necessary. Connect to any USB port to use your PC as a display and monitor data in real time. The USB-TC01, which is compatible with J, K, R, S, T, N, and B thermocouples, uses a standard miniplug for easy thermocouple connection.

Additional applications for generating alarms, scheduled logging, logging to a spreadsheet file, and logging under threshold conditions are available as free downloads. For even further customization, you can build your own applications with NI LabVIEW graphical programming and NI-DAQmx driver software (requires version 9.1 or later).

 

Learn more about Making a Thermocouple Measurement With LabVIEW.

Back to Top

Bookmark & Share


Ratings

Rate this document

Answered Your Question?
Yes No

Submit