Questions and answers about how RTDs measure temperature changes by tracking the variation in electrical resistance in metal wires. The resistance values of the metal wires are known, so resistance value changes are an accurate proxy for temperature changes. A resistance temperature detector (RTD) is a temperature sensor that is based on a metal resistor whose resistance value varies as the temperature changes. Sometimes referred to as resistance thermometers, RTDs are commonly used in laboratory and industrial applications because they provide accurate, reliable measurements across a wide temperature range.
How do RTDs work?
A metal wire in the RTD sensor has a known change in electrical resistance relative to variation in temperature, so the RTD sensor can output a precise measurement of temperature changes. This reading is either sent directly to a local display, or a transmitter sends the reading to a remote control location.
What are the advantages and drawbacks of RTDs?
- Accurate, reliable temperature measurement across a wide temperature range
- Easier to install, calibrate, and maintain than thermocouples, bimetal thermometers, and digital thermometers
- Highly stable, repeatable readings compared to other temperature measurement technologies
- Cost about twice as much as thermocouples
- Cannot be used at temperatures above 900–1000°F (482–538°C)
- Susceptible to vibration or incidental damage
What are the different types of RTDs?
Three types of RTDs are commonly used in industrial applications today:
- Wire-wound RTD elements comprise a metal wire – usually platinum, but also copper or nickel – wrapped around a glass or ceramic bobbin and sealed with glass.
- Partially supported wound RTD elements are produced with a small coil of wire inserted and attached to one side of a hole in the ceramic insulator.
- In thin-film RTD elements, a thin layer of platinum or metal-glass slurry is deposited onto a ceramic substrate. These sensors offer the most accuracy and the longest lifetimes, but are typically more expensive than the other two types.
Why is platinum the most commonly used metal in RTDs?
This noble metal has several beneficial properties:
- It is chemically inert.
- It has a nearly linear temperature-versus-resistance relationship.
- Its temperature coefficient of resistance large enough to produce measurable resistance changes with temperature variation.
- It is high stability; the resistance of platinum changes very little over time.
What is the difference between two-, three-, and four-wire RTDs? The greater the number of wires used, the more accurate the RTD is.
- With a two-wire RTD element, the two wires that provide the excitation current and the two wires across which the voltage is measured are the same.
- The most common type for modern industrial applications, a three-wire RTD uses a Wheatstone bridge measurement circuit to compensate for the resistance of the copper lead wire (copper because it’s less expensive than the platinum main wires). The two main wires, A and B, are the same length and typically made of platinum. This is because the Wheatstone bridge has to make the impedances of wires A and B – each acting as an opposite leg of the bridge – cancel the other out, and then wire C can act as a sense lead with a microampere current.
- Four-wire RTDs more completely compensate for the resistance of the wires, with the wire length being a less important constraint. This provides notably improved accuracy for just the cost of a small amount of copper extension wire.
You can learn more about how RTDs are used for temperature measurement in the food and pharmaceutical industries from WIKA’s special report on Pressure, Temperature and Level Measurement for Sanitary Applications (p. 24).