Choosing an RTD or Thermistor Measurement System

Publish Date: Jun 26, 2014 | 145 Ratings | 3.70 out of 5 |  PDF

Overview

This tutorial introduces and explains the concepts and techniques of measuring temperature with a resistance temperature detector (RTD) or thermistor.

Table of Contents

  1. Signal Conditioning Requirements for RTD and Thermistor Sensors
  2. NI-Compatible RTDs
  3. Platforms for Measuring RTDs 
  4. NI Platforms for Measuring Thermistors

Learn more about Understanding and Choosing RTDs or Understanding and Choosing Thermistors before you begin.

 

1. Signal Conditioning Requirements for RTD and Thermistor Sensors

Thermistor output signals are typically in the millivolt range, which means that you must pay careful attention to the sources of error that can impact your measurement accuracy. The primary signal conditioning factors to consider for thermistor measurements are: 

RTD and Thermistor Excitation

Because RTDs and thermistors are resistive devices, you must supply them with an excitation current and then read the voltage across their terminals. If extra heat cannot be dissipated, I2R heating caused by the excitation current can raise the temperature of the sensing element above that of the ambient temperature. Self-heating actually changes the resistance of the RTD or thermistor, causing error in the measurement. Self-heating effects can be minimized by supplying lower excitation current.

Noise Considerations

Lowpass filters are commonly used in RTD and thermistor DAQ systems to effectively eliminate high-frequency noise in RTD and thermistor measurements. For instance, lowpass filters are useful for removing the 60 Hz power line noise that is prevalent in most laboratory and plant settings.

Connection Configuration

The easiest way to connect an RTD or thermistor to a measurement device is with a 2-wire connection.


Figure 1. Making a 2-Wire RTD/Thermistor Measurement


With this method, the two wires that provide the RTD or thermistor with its excitation current are also used to measure the voltage across the sensor. Because of the low nominal resistance of RTDs, measurement accuracy can be drastically affected by lead wire resistance. For example, lead wires with a resistance of 1 Ω connected to a 100 Ω platinum RTD cause a 1 percent measurement error.

A 3-wire or 4-wire connection method can eliminate the effects of lead wire resistance. The connection places leads on a high-impedance path through the measurement device, effectively eliminating error caused by lead wire resistance. It is not necessary to use a 3- or 4-wire connection method for thermistors because they typically have much higher nominal resistance values than RTDs. A diagram of a 4-wire connection is shown in Figure 2.



Figure 2. Making a 4-Wire RTD Measurement

 

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2. NI-Compatible RTDs

Ready-Made RTD

 

Ready-made RTDs offer solutions for cost-sensitive temperature measurement applications. Each RTD element is sealed in an Alumina tube with three Teflon-coated leads, and can measure up to 400 °F (204 °C). Teflon offers good resistance to abrasion and moisture. Each RTD is sold individually and is available in lead lengths of 1 m (39.4 in.) and 2 m (78.7 in.).

Features

  • 3-wire leads
  • 1 m and 2 m lengths
  • Ideal for starter and educational applications
  • Measure up to 400 °F (204 °C)
  • Platinum 100 Ω at 0 °C (alpha=0.00385)

Field-Cuttable RTD

 

National Instruments field-cuttable RTDs are ideal for a wide variety of temperature applications. With field-cuttable RTDs, you can cut the probe to your desired length, from 3.5 in. to 24 in. (8.9 cm to 61 cm). Therefore, you can use one stock of RTDs for all of your temperature measurements to reduce inventory cost and production downtime. These RTD probes can measure up to 900 °F (482 °C). 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

  • Reduces inventory cost and downtime
  • 3-wire
  • Measures up to 900 °F (482 °C)
  • Platinum 100 Ω at 0 °C (alpha=0.00385)
  • Probes can be field cut to within 3.5 in. of tip (maximum length 24 in.)
  • Lead length is 6 in.

 

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3. Platforms for Measuring RTDs 

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. CompactRIO can be used 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 RTDs

 

NI 9217 Measurement System

The NI 9217 C Series RTD analog input module features four channels and 24 bits of resolution for 100 Ω RTD measurements. You can configure the NI 9217 for two different sampling rate modes. With the high-sampling-rate mode, you can sample at up to 400 S/s (100 S/s per channel). Using the high-resolution mode, you can sample at 5 S/s (1.25 S/s per channel) with built-in 50/60 Hz noise rejection.

The NI 9217, compatible with 3- and 4-wire RTD measurements, automatically detects the type of RTD (3- or 4-wire) connected to the channel and configures each channel for the appropriate mode. The module can provide 1 mA of current excitation per channel and has less than a 1 °C accuracy error over its entire operating temperature range. The NI 9217 contains NIST-traceable calibration and includes a channel-to-earth ground double isolation barrier for safety, noise immunity, and high-common-mode voltage range.

NI 9217 Measurement System

NI 9219 Measurement System 

The NI 9219 Measurement System measures four universal channels at 100 S/s with 24-bit resolution, sending data to a host PC over USB, Ethernet, or 802.11 WiFi.

The NI 9219 is a 4-channel universal C Series module designed for multipurpose testing in any NI CompactDAQ or CompactRIO chassis. With the NI 9219, you can measure several signals from sensors such as strain gages, RTDs, thermocouples, load cells, and other powered sensors. The channels are individually selectable, so you can perform a different measurement type on each of the four channels. Measurement ranges differ for each type of measurement and include up to ±60 V for voltage and ±25 mA for current. See the manual for detailed specifications and ranges.

 

 

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4. NI Platforms for Measuring Thermistors

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

 

 

 

 

NI 9215 and NI 9265 Measurement Systems 

The NI 9215 C Series module for use with NI CompactDAQ and CompactRIO chassis includes four simultaneously sampled analog input channels and successive approximation register (SAR) 16-bit analog-to-digital converters (ADCs). The NI 9215 features NIST-traceable calibration, a channel-to-earth ground double isolation barrier for safety and noise immunity, and high-common-mode voltage range.

Coupled with the NI 9265 for thermistor excitation, this kit is the ideal solution for taking measurements with thermistors.

NI 9215 and NI 9265

 

NI 9219 Measurement System 

The NI 9219 can theoretically be used for thermistor measurements thought it was not specifically designed for them. The KnowledgeBase article Thermistor Measurements Using the NI 9219 covers the details and limitations of using the NI 9219 along with thermistors.

 

 

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