Cold Chain

Series on Measuring Temperature: RTDs, Thermistors and Thermocouples (Part 4)

Our series left off over the US Thanksgiving holiday when we last talked about deciding between the use of a thermistor or thermocouple for ensuring the proper temperature of your favorite meal from an oven. And, now that we are back from the end of the year 2019 holidays, with temperatures trending downwards for at least the next few weeks, it’s time to wrap up our series on temperature measuring devices with a deeper look at thermocouples and how they stack up against the likes of RTDs and thermistors.

As was illustrated in Part 3 of our series, thermistors are highly advantageous in extreme temperature monitoring environments where reliability and stability are paramount. Thermistors can be configured to operate in a multitude of shapes and sizes and are, therefore, commonly chosen for use in monitoring temperatures of surfaces, liquids, and gases in process environments where space is at a premium and external “noise” (such as electrical currents) is prevalent.

Thermocouples, as we introduced in Part 1 of the series, rely on what are known as “hot” and “cold” junctions between two different types of metals. The hot junction is the location of the thermocouple’s sensor where the metals (referred to as thermoelements) are fused or connected, and the temperature is measured.

When the hot junction is heated (or cooled), voltage is produced that can be correlated to temperature using “thermocouple curves,” which employ extensive coefficient equations to compute temperature. “Base metal” thermocouples, known as types J, K, E, and T, provide the lowest cost solutions for the broadest range of temperature measurement. Specifically, the K Type Thermocouple measures temperatures between -200°C and 1,250°C (-328° to 2282°F) with +/-2.2°C accuracy and is frequently the solution of choice in controlled temperature storage monitoring.

Because of their wide-ranging measurement capabilities, the use of exposed junction thermocouples (where the tip of the thermocouple protrudes from the protective sheath wall) is most employed for measuring changes in air temperature, such as inside of ultra-low temperature units. While relatively costly, thermocouples can meet the demands of a spectrum of applications for measuring temperature in transportation and warehousing environments.

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