Temperature Sensors

TEMPERATURE SENSORS

Bimetallic Strips:

A Bimetallic thermostat consists of two different metal strips bounded together and they cannot move relative to each other.

These metals have different coefficients of expansion and when the temperature changes the composite strips bends into a curved strip, with the higher coefficient metal on the outside of the curve.

The basic principle in this is all metals try to change their physical dimensions at different rates when subjected to same change in temperature.

This deformation may be used as a temperature- controlled switch, as in the simple thermostat.

Resistance Temperature Detectors (RTDs):

The materials used for RTDs are Nickel, Iron, Platinum, Copper, Lead, Tungsten, Mercury, Silver, etc.

The resistance of most metals increases over a limited temperature range and the relationship between Resistance and Temperature is shown below.

The Resistance temperature detectors are simple and resistive elements in the form of coils of wire The equation which is used to find the linear relationship in RTD is

Constructional Details of RTDs:

The platinum, nickel and copper in the form wire are the most commonly used materials in the RTDs.

Thin film platinum elements are often made by depositing the metal on a suitable substrate wire-wound elements involving a platinum wire held by a high temperature

glass adhesive inside a ceramic tube.

Thermistors:

Thermistor is a semiconductor device that has a negative temperature coefficient of resistance in contrast to positive coefficient displayed by most metals.

Thermistors are small pieces of material made from mixtures of metal oxides, such as Iron, cobalt, chromium, Nickel, and Manganese.

The shape of the materials is in terms of discs, beads and rods.

The thermistor is an extremely sensitive device because its resistance changes rapidly with temperature.

The resistance of conventional metal-oxide thermistors decreases in a very non-linear manner with an increase in temperature.

The change in resistance per degree change in temperature is considerably larger than that which occurs with metals.

The resistance-temperature relationship for a thermistor can be described by an equation of the form

R_t = Ke^β/t

Where R_t, is the resistance at temperature t, with K and β being constant. Thermistors have many advantages when compared with other temperature sensors.

The simple series circuit for measurement of temperature using a thermistor and the variation of resistance with temperature for a typical thermistor.

The thermistor is an extremely sensitive device because its resistance changes rapidly with temperature.

Thermocouples:

Thermocouples are based on the See back Effect.

The thermocouple temperature measurement is based on a creation of an electromotiveforce (emf).

"When two dissimilar metals are joined together an e.m.f will exist between the two points A and B, which is primarily a function of the junction temperature. The above said

to be principle is See back effect..

The thermocouple consist of one hot junction and one cold junction

Hot junction is inserted where temperature is measured

Cold junction is maintained at a constant reference temperature.