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How To Select Thermocouples for Temperature Measurement Systems

November 4, 2019

For many test applications, measuring the temperature is a necessity. For example, when testing an aircraft engine, you may need to measure manifold temperature or the exhaust gas temperature. To help you take those measurements, VTI Instruments offers a number of options, including the EX1000A series of precision voltage measurement instruments and the RX1032 rugged thermocouple measurement system.

Thermocouples are arguably the most common type of temperature sensor, although you can also use resistance temperature detectors (RTDs), or thermistors. The EX1000A series supports all three types of sensors, while the RX1032 supports only thermocouples.

Thermocouples are made by welding two different types of metals together. This junction of dissimilar metals will generate a voltage that's proportional to the temperature. There are many different types of thermocouples, and the different types are designated with a letter. Some of the more common thermocouple types are J, K, T, E, S, R, B, and N.

Because they come in so many varieties and measure temperature over wide temperature ranges (up to 1800 °C), thermocouples are the sensor of choice for many applications requiring the use of intense heat. When used in a high-temperature application, the sensor is housed in a metal probe, which protects the sensor from the heat. Thermocouples are generally inexpensive, though prices can escalate dramatically if your application requires a special housing.

The type of thermocouple that you should choose depends on the accuracy and temperature range that you need. Type J thermocouples for example, have a temperature range of -200°C to 1200°C, while Type K thermocouples have a temperature range of -200°C to 1372°C. When connected to the RX1032, both Type J and Type K thermocouples have a typical accuracy of ±0.53°C. Other thermocouple types cover different temperature ranges and have different accuracies, as shown in the figure below.

Thermocouples, Temperature measurement

One thing to consider when using thermocouples is the cold-junction compensation provided by the instrument. When you connect a thermocouple to the input of an instrument, you actually form another thermocouple there. If the temperature at the input connection is not known, the voltage generated by the connection of dissimilar metals at the terminal strip will throw off the measurement.

To achieve the highest accuracy and stability possible, the VTI temperature-measurement instruments provide embedded isothermal input sections that are monitored by precision thermistors, one for every four input channels. To ensure that the cold junction temperature measurements are current and time-correlated with the input channels, the thermistor channels are measured with every scan, providing a maximum time separation of less than 4 ms between the measurement of an input channel and its associated cold junction temperature measurement. For users that prefer using an external cold junction, the EX10xxA also allows for the programming input of up to 48 unique external cold junction temperatures, one for every input channel, and internal and external cold junction thermistor inputs can be mixed throughout the unit on a per channel basis.

For more information on thermocouples and how to use them in your test system, contact one of our sales representative by visiting https://www.vtiinstruments.com/sales/vti-instrument-sales. You can also email us at sales.ppd@ametek.com or call 800-733-5427 or 858-450-0085.