Thermocouples vs. Resistance Temperature Detectors (RTD)

08/15/2019
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Two of the most common temperature measurement devices are RTDs and thermocouples. Although they perform a similar function, in many ways these two devices couldn’t be more different. 

When choosing a device for temperature measurement or calibration some key factors to consider are:
  • Accuracy and stability
  • Environment
  • Temperature range

In a lab setting where the environment is less rugged and accuracy is of the highest importance, RTDs are most commonly used. Resistance temperature detectors (RTDs) are sensors that measure temperature by correlating the RTD element with a temperature value. This is usually done through a coil of metal wire wrapped around a core made of ceramic or glass.

The coil is typically constructed out of a pure material such as platinum, nickel or copper. The coil material is verified by international standards for having a certain resistance based on temperature. The coil’s properties are therefore predictable. 

When an RTD is made of platinum, it is sometimes referred to as a platinum resistance thermometer (PRT). Platinum is known as an industry standard in RTDs because it provides the widest temperature range, longest stability and highest accuracy. As the temperature varies, it produces a change in resistance. This change is used to determine temperature. 

Due to their high accuracy and stability, RTDs are ideal for calibration labs. These instruments are not ideal for industrial settings because they are typically more fragile. Since they are immune from electrical interference, they also do well in automated processes.

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Thermocouples also measure temperature, but by a different method. A thermocouple consists of two (a couple) probes of different metals such as nickel, copper or iron. An electrical voltage is produced between the probes which is dependent on temperature and used to determine the temperature value.

While they often have lower accuracy and stability than RTDs, thermocouples do typically feature a wider temperature range. Thermocouples can read temperatures as low as -450 degrees F to as high as 4200 degrees F. Generally speaking, a thinner thermocouple probe will not reach as wide a range of temperatures as a thicker probe.

Based on materials used, thermocouples are calibrated to certain ranges. Some of the most common types are J, K, T and E which are considered “base metal” thermocouples. Meanwhile “noble metal” thermocouples, such as R, S and B, are less common and are used for high temperature applications.

Some other pros are in the durability, speed and cost. Thermocouples are ideal for industrial settings due to their robust design, quick response time and lower price point.

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Author

Kellie Bucy

Kellie Bucy

Technical Content Writer

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