RTD

1. RESISTANCE TEMPERATURE
DETECTOR OR RESISTANCE
THERMOMETER

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

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

10. Wheatstone Bridge

11. Analysis
 Identify the currents

12. Consider the bridge at “balance”, ig=0
1 3
2
3 3
1 1 2 2
x
x x
i i
i i
i R i R
i R i R




g
g
g

13. Some Algebra
1 3 2
1 1 2 2
3
1 2
2
3
1
x
x
x
i R i R
i R i R
R R
R R
R
R R
R





14. Use to Measure Resistance
2
3
1
x
R
R R
R

Ratio Arms

15. Wheatstone Bridge
1R 3R
2R 4R
DC
dcV 0V+ -
a b
1 2R R 3 4R RNote that if and then
0 0a bV V V  