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
R vs T relationship of various metals
Common RTD sensing elements constructed of platinum, copper
or nickel have a repeatable resistance versus temperature relationship (R vs T)
and operating temperature range. The R vs T relationship is defined as the
amount of resistance change of the sensor per degree of temperature change.[2]
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:[3] 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, α. The equation below defines α; its units are ohm/ohm/°C.
\
the resistance of the sensor at 0°C
the resistance of the sensor at 100°C
Pure platinum has an alpha of 0.003925 ohm/ohm/°C in the 0
to 100 °C range and is used in the construction of laboratory grade RTDs.
Conversely two widely recognized standards for industrial RTDs IEC 60751 and
ASTM E-1137 specify an alpha of 0.00385 ohms/ohm/°C. Before these standards
were widely adopted several different alpha values were used. It is still
possible to find older probes that are made with platinum that have alpha
values of 0.003916 ohms/ohm/°C and 0.003902 ohms/ohm/°C.
These different alpha values for platinum are achieved by
doping; basically carefully introducing impurities into the platinum. The
impurities introduced during doping become embedded in the lattice structure of
the platinum and result in a different R vs. T curve and hence alpha value
