Rtd

RTD

1. RESISTANCE TEMPERATURE DETECTOR OR RESISTANCE THERMOMETER

2. PRINCIPLE: When an electric conductor is subjected to a temperature change, the resistance of the conductor changes. The change in resistance gives the measure of change in temperature when calibrated. The material used mainly for RTD is platinum. Reasons causing change in resistance of electric conductor: Change in dimension of conductor(expansion or contraction) Change in current opposing properties of material Resistance of conductor increases with temperature & vice-versa.

3. CONSTRUCTION: A ceramic mandrel is placed within a glass or a metal tube . Resistance wire is wound on the ceramic mandrel and the lead wires of the resistance wire project out of the ceramic mandrel. This arrangement refers to resistance thermometer. The leads of the resistance thermometer is connected to the wheat stone bridge. The tube is evacuated or filled with inert gas to protect the resistance wire.

4. OPERATION: A known constant current is passed through the resistance wire of the thermometer & initial resistance of the wire is measured using the wheat stone bridge. When resistance thermometer is introduced to the medium of which the temperature is to be measured ,change in temperature causes the resistance wire to get heated due to which resistance of the wire increases. Change in resistance of the wire is measured using wheat stone bridge and hence, change in temperature can be measured.

5. APPLICATIONS: Usually used when temperature difference is to be done from a distance. This is done by separating the sensing element(thermometer) & indicating element (Wheatstone bridge) as per convenience by increasing the length of lead wires. Used in continuous monitoring situations. ADVANTAGES: Simple in construction Accurate measurement. Average temperature measurement is possible by connecting the temperature sensing elements suitably. Very easy to install & replace the thermometer. Accuracy of measuring circuit can be checked easily by replacing the thermometer with a standard resistance. Easy reproducibility.

6. LIMITATIONS: Slow in response as the sensing wire is covered by a protecting glass or metal tube. Current leakage might take place between thermometer & ground. Lead resistance compensation becomes essential Thermoelectric emf may be generated due to a junction of 2 dissimilar metals.

7. How Does WHEATSTONE BRIDGE Work? Uses ratio of 3 known resistors Measures fourth unknown resistance

8. Wheatstone Bridge

9. Analysis Identify the currents

10. Consider the bridge at “balance”, i g =0

11. Some Algebra

12. Use to Measure Resistance Ratio Arms

13. Wheatstone Bridge Note that if and then