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What is an RTD Sensor?

Resistance Temperature Detector (RTD) is a temperature sensor that contains a resistor whose resistance value varies as its temperature changes. RTDs have long been used for industrial temperature measurement and in precise laboratory work. Their accuracy—within 0.1 to 0.2°C at 0°C—makes them ideal for use in many applications.

The invention of a resistance thermometer was made possible by the discovery that the conductivity of metals decreases predictably with increases in their temperatures. The first-ever resistance thermometer was assembled from insulated copper wire, a battery and a galvanometer in 1860. However, its inventor, C.W. Siemens, soon discovered that a platinum element yielded more accurate readings at a much wider range of temperatures. Platinum remains the most commonly used material in temperature measurement using RTD sensing elements today.

How Does an RTD Sensor Work?

An RTD sensor consists of a sensing element, usually made of highly pure metal, housed within a protective probe and connected to monitor or control equipment by insulated lead wires. Because the relationship between resistance and temperature is stable and consistent for the metals employed in RTD sensors, measuring the resistance across the sensing element at any given temperature allows that temperature to be calculated with a high degree of accuracy.

Since temp measurement using RTD is based on resistance, the resistance in the lead wires and connectors must be taken into account when calculating the overall resistance in the system circuit. Using a 3-wire circuit design allows lead wire resistance to be factored out of the overall calculation. A 4-wire construction is also available and offers even greater precision.

Common Applications of RTDs

RTD sensors are employed in a broad variety of commercial and industrial applications. They are suitable for use whenever accuracy and stability of measurement are needed across a wide temperature range, since they can reliably measure temperatures as low as -200°C and as high as 850°C. They also remain stable over time, retaining their accuracy despite repeated use. Depending on the type of sensor housing chosen, RTDs can be used to measure the temperatures of liquids, surfaces, air streams or gasses.

To give a few examples among many, our customers have chosen RTDs for use in:

  • Measuring temperatures of materials and equipment in industrial manufacturing facilities
  • Measuring air temperature in laboratories, clean rooms, and other locations
  • Measuring temperature inside air ducts
  • Measuring temperatures in harsh or highly pressurized environments, including plating baths
  • Measuring temperatures inside ovens, stoves and food processing equipment

RTDs are known to be sensitive to vibration, which can apply repeated mechanical stress to the sensing element, eventually resulting in damage or breaking. When using RTDs in environments where they will be subjected to machine, acoustical or flow-induced vibrations, specially-designed RTDs employing thin-film elements and vibration-resistant probes should be selected.
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