Machine Design

Sensor Sense: RTDs

RTD stands for resistance-temperature device or detector.

It is the generic term for any device that measures temperature through changes in resistance.

Though iron, nickel, copper, and tungsten have been used as temperature-sensing elements, platinum is the preferred material. Pure platinum has a nearly linear coefficient over a wide temperature range. It's not uncommon to use platinum to measure temperatures from –200 to over 800°C with a precision of ±0.1°.

Platinum come in either wire-wound or thin-film formats. As the name implies, wire-wound designs use a thin platinum wire wound around an insulating core. Thin-film devices contain a thin layer of platinum on a base material such as ceramic or glass. Wire-wound is the more common form, but thin-film types react faster to temperature changes. For either style the sensing element is enclosed in a probe body for protection against physical damage and chemical contamination.

Most platinum are designed to a standard value of 100 Ω. Simply stated, at 0°C the RTD has a resistance of 100 Ω. The standard permits interchanging and temperature monitors from different manufacturers.

An external current must pass through the RTD to read its resistance. However, too much current will self-heat the RTD, producing an error in the reading. Other error sources are created by long leads or corroded connections. Both add resistance to the RTD circuit making the temperature read higher than it should.

The resistance of an RTD is directly related to its temperature. The simplest method of measuring RTD resistance uses two wire leads, L1 and L2, and a simple ohmmeter circuit calibrated for temperature. Unfortunately, it's also the most error prone as the resistance of the leads adds to the RTD resistance leading to higher temperature readings. Three-wire sensing uses three identical wire leads. The L1 and L3 leads connect to the RTD the same as a two-wire connection. But the L1 and L2 pair is used to measure just the resistance of the wires. Since L1 + L2 has the same resistance as L1 + L3, the L1 + L2 value is subtracted from the reading, entirely eliminating wire resistance. The four-wire system measures the current supplied to the RTD by one wire pair, while the other wire pair measures the voltage drop across the RTD element alone. Ohm's Law provides the resistance of the RTD and, thus, the temperature.
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