Cold Chain

Series on Measuring Temperature: RTDs, Thermistors and Thermocouples (Part 2)

In today’s blog, we take a deeper look at how Resistance Temperature Detectors (RTDs) function and the factors that make this type of temperature monitor a good choice for your respective temperature monitoring needs.

As we introduced in Part 1 of our series on measuring temperature, RTDs function by measuring the change in resistance across a metal (copper, nickel, and platinum are common) with defined resistive characteristics. Of notable importance is the relatively exceptional accuracy of the RTD in measuring temperature.

A common “Class B” RTD device can measure the temperature at ±0.35°C at 0°C (increasing linearly to ±2.0°C at 400°C). Compared to the ± 2.2°C of a standard type thermocouple, that is reliable accuracy. Accuracy of an RTD device is dependent on the number of lead wires used in measuring resistivity between the resistance element (platinum, in many cases) and the source (also known as the “cold-end termination”).

Industrial applications, such as those for the measurement of the temperature of medicines in storage and distribution, typically utilize a “3 wire” configuration. The resistance between the set of 3 copper wires establishes the degree of accuracy that can be attributed to the RTD device itself. The more wires, the higher the accuracy.

In a three-wire RTD, the copper lead wire between the cold-end termination (where the temperature is to be measured) and the platinum resistance element ( where the temperature is “sensed”) is a bundle of 3 copper wires insulated with Teflon or glass (depending on the degree of temperature exposure). RTD devices are common in applications where a “probe” is best suited for measuring temperatures, such as inside of a liquid nitrogen vessel or the interior of a validated storage location. The probe provides a measure of the temperature inside the controlled environment, while the transmitter display is maintained outside of the storage chamber.

As a result of reliable accuracy, resistance temperature detectors are commonly found in life sciences settings where storage and distribution of temperature-sensitive materials are monitored on an ongoing basis and can be affixed to a location for extended use.

Check in later this week when we look more closely at thermocouples and their applications.

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