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Resistance temperature detector ( RTD)

When a metal wire is heated the resistance increases. So, a temperature can be measured using the resistance of a wire. RTD incorporates pure metals or certain alloys that increases resistance as temperature increases and it conversely decreases resistance as temperature decreases. RTDs act similar to an electrical transducer.and it converts changes the temperature to voltage signals by the measurement of resistance.

The metals that are best suited for use as RTD sensors are pure of uniform quality and stable within to given range of temperature and able to give reproducible resistance- temperature readings. Only a few metals have the properties necessary for use in RTD elements. ,

RTD elements are normally constructed of platinum, copper, nickel or nickel-iron alloys.

Working principle of RTD

Resistance thermometers or resistance temperature detector works on the principle of positive temperature coefficient of resistance i.e. as temperature increases, resistance offered by thermometer also increases.
The resistance element of platinum and iron metal wire is wrapped around an electrically insulating support of glass, ceramic or mic and from the outside, the protective sheath of metallic tube can be provided. The lead wires are taken out from the resistance elements which are joined to the circuitry.

RTD formula

The resistance thermometers which are alternatively known as RTD works on the principle that “ the resistance of a metal varies with a change in temperature” according to the relation as,

RT =R0[1+α(T-T0)]

RT : Resistance at temperature ( T)
R0 : Resistance at temperature ( 0˚C)
α : Temp. coeff.
T: temp( ˚C)
T0 : Initial temp.

To measure the change in the resistance bridge network is used.
The resistance thermometer is connected to one of the arms of the Wheatstone bridge circuit when resistance thermometer is subjected to temperature variation, the Wheatstone bridge gets unbalanced. The galvanometer deflection can be directly calibrated to give temperature.

RTD Diagram :

construction of a resistance temperature detector

Figure shows the construction of an RTD. It has a resistor element connected to a Wheatstone bridge. The element and the connection leads are insulated and protected by a sheath. A small amount of current is continuously passing through the coil. As the temperature changes the resistance of the coil changes which is detected at the Wheatstone bridge.

RTDs are used in the form of thin films, wire wound, or coil. They are generally made of metals such as platinum, nickel, or nickel-copper alloys. Platinum wire held by a high-temperature glass adhesive in a ceramic tube is used to measure the temperature in a metal furnace.

Construction of RTD :

Coiled Platinum element.
Platinum due to Linearity and Chemical inertness.
Platinum is coiled on ceramic mandrel.
Coiled platinum protected by s.s metal sheath.
Ceramic or mica powder insulates the leads.
The leads connected in Wheatstone bridge.
The lead wires are usually of higher diameter than the diameter of the sensor wire to reduce the lead wire resistance.

Working Of RTD :

Steel protective sheath detects the temperature and transfer it to the platinum filament.
The change in the resistance value of the Platinum coil is very small with respect to the temperature.
So, the RTD value is measured by using a bridge circuit.
Temperature is determined by converting the RTD resistance value using a calibration expression.
Dummy wire reduces impedance effect and so the error.