The resistance thermometer, often called a Resistance Temperature Detector (RTD) is a common thermometer, well known for its stability and accuracy. The most common and most reproducible is the Platinum Resistance Thermometer (PRT).
The sensing element of the RTD is metallic and its electrical resistance varies with temperature. A RTD can be manufactured in many configurations and used over a wide temperature range. The particular design and shape of the sensing element, protection and connection leads depend on the application and the required robustness, sensitivity and accuracy.
The resistance of a metal depends on the temperature. It can be expressed as
where ρ(T) is the resistivity of the metal at temperature T, and L0 and S0 are the length and the cross section of the wire at a specified temperature T0. A high resistivity ρ(T) allows small sensing elements.
The temperature coefficient of resistance α is defined by the equation:
In many applications the temperature dependence of the resistance R(T) can be written
Standards, requiring the highest accuracy, express the resistance of platinum sensors to higher orders of T, depending on the temperature range, as follows:
where R0 is the resistance at 0 °C, and A, B and C are constants.
The choice of metal depends on characteristics such as its temperature coefficient of resistance α, resistivity ρ, melting point and its resistance to corrosion and pollution.
Platinum is widely used but gold, nickel and silver can also be used because of their oxidation resistance. The following table gives values of the resistivity and the coefficient of resistance for a variety of metals:
Platinum is highly corrosion and oxidation resistant and stable over a wide temperature range (-250°C to 850°C). It can be refined to high purity, making for consistent sensors. The length and diameter of the platinum wire used in a thermometer are often chosen so that the resistance of the sensor around 0°C is 100 Ω. Such a sensor is called a Pt100 sensor and is the most commonly used resistance thermometer in industry. Pt25 (25 Ω at 0 °C) is used at the highest level of accuracy as the interpolation instrument for the International Temperature Scale, ITS-90, between the triple point of hydrogen (13.8023 K) and the freezing point of silver (1234.93 K).
Rhodium-iron resistance thermometers are widely used at cryogenic temperature. The resistivity of rhodium-iron exhibits an anomaly leading to an increase of temperature coefficient as the temperature falls, in contrast to the usual loss of sensitivity in a pure metal. Their excellent stability and good sensitivity down to 0.5 K make them ideal as standards for realising the ITS-90 at the lowest temperatures.
The resistance of a metal wire is measured by passing a current through it and measuring a voltage across it according to Ohm's law
R(T)= V / I
Two kinds of instruments are used for measuring electrical resistance, one is the Wheatstone bridge (in balanced mode or a current excitation source). The bridge is mainly used in metrology laboratories where accurate measurements are needed.
One of the main sources of uncertainty is due to the additional parasitic resistance introduced by the leads that connect the sensing element to the measurement device. The error in the temperature measurement depends on the resistance value of the leads, which is proportional to their length and section.
Various wiring configurations are available for measuring the electrical resistance of the RTD: 2, 3 and 4 wire configurations are possible, as shown in the following figure. The choice of a particular wire configuration depends on the required accuracy of the temperature measurement.
Another measurement uncertainty results from the self-heating caused by current flow through the resistance. To minimise this, the current used must be fitted with the value of the measured resistance.
The wire type is generally more accurate than the film type due to the better control of metal purity, but they are more expensive. They cannot be used in their basic sensing element form because they are not robust enough. Nevertheless, they are widely used in industrial applications with suitable manufacturing.
The platinum resistance detector is used as an interpolating standard between the fixed points of the International temperature Scale ITS-90.