An RTD (resistance temperature detector) is a temperature sensor that operates on the measurement principle that a material’s electrical resistance changes with temperature.

The relationship between an RTD’s resistance and the surrounding temperature is highly predictable, allowing for accurate and consistent temperature measurement.

By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD’s resistance can be calculated and the temperature can be determined.

The variation of resistance R with temperature T is given by,

$R = R_0 ( \alpha_ 1 + \alpha_1T + \alpha_ 2T_2+…..+ \alpha_ nT_n)$

Where R0 is resistance at temperature T = 0, and $\alpha_1$,$\alpha_2$,.. $\alpha_n$ are constants.

• For narrow temperature range generally following relation is used,

$R =R_0 (1 + \alpha T)$

Where $\alpha$ is resistance temperature coefficient in /˚C

$\alpha$= 0.00392 Ω/Ω/˚C for Platinum at -240 to 650˚C

• Different materials used in the construction of RTDs offer a different relationship between resistance and temperature.

• Temperature sensitive materials used in the construction of RTDs include platinum, nickel, and copper, platinum being the most commonly used.

• These metals are best suited for RTD applications because of their linear resistance-temperature characteristics (as shown in the figure), their high coefficient of resistance, and their ability to withstand repeated temperature cycles.

• The coefficient of resistance is the change in resistance per degree change in temperature, usually expressed as a percentage per degree of temperature.

• The material used must be capable of being drawn into fine wire so that the element can be easily constructed.

• Construction and Working of RTD

• RTD sensing elements come in two basic styles, wire wound and film.

• Wire wound elements contain a length of very small diameter wire (typically .0008 to .0015 inch diameter) which is either wound into a coil and packaged inside a ceramic mandrel, or wound around the outside of a ceramic housing and coated with an insulating material.

• Larger lead wires (typically 0.008 to 0.015 inch diameter) are provided which allow the larger extension wires to be connected to the very small element wire.

Film type sensing elements are made from a metal substrate which has a resistance pattern cut into it.

This pattern acts as a long, flat, skinny conductor, which provides the electrical resistance.

Lead wires are bonded to the metal coated substrate and are held in place using a bead of epoxy or glass.

• Advantages

• RTDs are most accurate temperature sensors. They provide very accurate measurements even over comparatively narrow temperature range.

• Stability and repeatability is excellent.

• RTDs provide immunization against electrical noise. Therefore, they are considered ideal for temperature measurements in industrial environments. For example, around motors, generators and other high voltage equipment etc.

• RTDs are very simple to recalibrate and installation is easy.

• It has linear characteristics and has high sensitivity.

• Disadvantages

• Their overall temperature range is very small.

• Their application involves high initial cost.

• They are not rugged enough to be used in high vibration environments.