Measurement of temperature changes

using RTD

Platinum Resistance Thermometer (PRT)

The platinum wire is wound in the form of spirals on am insulating material such as mica or ceramic.

The platinum wire is enclosed in a protective tube.

Open wire PRT (RTD)

Platinum wire comes under direct contact with the gas or liquid whose temperature is to be measured .The gas or liquid used should not be corrosive or conductive (In open type PRT ).

The resistance of the platinum wire changes with changes in the temperature of the liquid or gas .

The maximum temperature measurable is 1000 0K i.e.727 0C.

This type of sensors have positive temperature coefficient of resistivity as they are made from metals .

These are also know as Resistance temperature detectors (RTD).

The other metals that can be used are nickel and copper.

Closed type liquid filled probe

RTD PROBE

This probe is used to measure the temperatures of the high speed ,electrically conductive or corrosive type of liquids.

If the temperature is below 80 k ,the tube is filled with helium to avoid conduction of air .

Measurement technique using RTD

The measurement is done by connecting the PRT in a simple WHEAT STONE’S bridge circuit.

The voltage at point A is constant. Va = R3 \R1+R3 • The voltage at B changes with temperature.• The differential voltage VBA is amplified by a differential amplifier .• The variations in the output Vo is proportional to changes in temperature.

Advantages of RTD:

o They can be used for measurement of small as well as large temperature differences .

o High accuracy.o High reproducibility.o Good dynamic response i.e. it responds to temperature changes quickly.o Wide temperature range (-200 0C to 650 0C).o No need of temperature compensations.o Stability of performance over along period of time.

Disadvantages of RTD:

o Large size .o Sophisticated instrumentation is necessary for protection of resistance wire.o Low resolution of the order of 7 ohms/0C.o External dc power source is required.o High cost.

Platinum Resistance Thermometers