Resistance Temperature Detector
Resistance Temperature Detector
Resistance Temperature Detector
Resistance
thermometers,
also
called resistance
temperature
detectors (RTDs),
are sensors used to measure temperature by correlating the resistance of the RTD element with
temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or
glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to
protect it. The RTD element is made from a pure material, typically platinum, nickel or copper. The
material has a predictable change in resistance as the temperature changes and it is this predictable
change that is used to determine temperature.
They are slowly replacing the use of thermocouples in many industrial applications below 600 C,
due to higher accuracy and repeatability.
Common RTD sensing elements constructed of platinum, copper or nickel have a repeatable
resistance versus temperature relationship (R vs T) and operating temperaturerange. The R vs T
relationship is defined as the amount of resistance change of the sensor per degree of temperature
change.[3] The relative change in resistance (temperature coefficient of resistance) varies only slightly
over the useful range of the sensor.
Platinum was proposed by Sir William Siemens as an element for resistance temperature detector at
the Bakerian lecture in 1871:[4] it is a noble metal and has the most stable resistance-temperature
relationship over the largest temperature range. Nickel elements have a limited temperature range
because the amount of change in resistance per degree of change in temperature becomes very
non-linear at temperatures over 572 F (300 C). Copper has a very linear resistance-temperature
relationship, however copper oxidizes at moderate temperatures and cannot be used over 302 F
(150 C).
Platinum is the best metal for RTDs because it follows a very linear resistance-temperature
relationship and it follows the R vs T relationship in a highly repeatable manner over a wide
temperature range. The unique properties of platinum make it the material of choice for temperature
standards over the range of -272.5 C to 961.78 C, and is used in the sensors that define the
International Temperature Standard, ITS-90. Platinum is chosen also because of its chemical
inertness.
The significant characteristic of metals used as resistive elements is the linear approximation of the
resistance versus temperature relationship between 0 and 100 C. This temperature coefficient of
resistance is called alpha, . The equation below defines ; its units are ohm/ohm/C.