RTDs measure temperature by detecting changes in electrical resistance of a wire as temperature varies. There are two types based on whether resistance increases or decreases with temperature. RTDs are used in bridge circuits where changes in resistance produce voltage changes proportional to temperature. Thermocouples use the Seebeck effect where different metals produce voltage when joined and subjected to a temperature gradient. Common types include J, K, B, S, T, and R which vary in sensitivity and measurable temperature range. Both RTDs and thermocouples require signal conditioning due to their small voltage outputs and are calibrated using a temperature indicator, controller, and oven.

1. By:
Ashutosh Katti (T3814)
Aditya Medhekar (T3827)
Mechatronix seminar 15 April 2012 1

2. What is an RTD?
 RTD stands for Resistance Temperature Detector.
 The change in temperature is detected by the
change in resistance of the wire.
 There are two types of RTD, viz. having positive
and negative thermal coefficients of resistivity
(resistance increases or decreases with the
increase in temperature respectively).
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3. • RTDs are used for temperature measurements by using them
in bridge circuits.
• The change in temperature causes considerable resistance
change which gives a voltage drop in accordance with the
thermal coefficient of resistance of the wire.
• This voltage is further amplified and the temperature is read
thus. This is how the RTDs are used in circuits assisting in
automatic control and measurement with high accuracy.
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4. Advantages
 Due to no fluid present absolute temperature is recorded.
 It is highly sensitive and gives accurate results.
 It has a good range of temperature measurement. It can thus
measure from very low to very high temperature.
 Due to electrical output (resistance change) it can be used with
PLCs and complete automation can be achieved.
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5. Applications of RTD
 It is widely used in furnaces for automatic temperature
measurement.
 Due to its compactness, it replaces conventional
thermometers as well as thermocouples thus eliminating the
use of lots of wires.
 Used in medical and chemical laboratories to detect very low
temperatures (like dry ice and liquid nitrogen).
 Due to electrical output it is used wherever feedback system
is required and corrective action is thus taken in an
automated system.
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6. What are thermocouples?
• Thermocouples are metal couples which work on the
Seebeck effect.
• In this effect, any conductor is subjected to a thermal
gradient, it will generate a voltage.
• Measuring this voltage necessarily involves connecting
another conductor to the "hot" end. This additional
conductor will then also experience the temperature
gradient, and develop a voltage of its own which will oppose
the original.
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7. Types of thermocouples
There are about 4 to 5 types of thermocouples which are
categorized based on sensitivity and range.
• J and K type thermocouples are noted for their high sensitivity.
These however, have a limited range. (about 41µV/˚C for K and
55µV/°C)
• Platinum Types: The B, S, T and R type of thermocouples are less
sensitive (about 10µV/˚C) but they have a very significant
advantage of high range. They can measure large temperatures
and are usually used in furnaces.
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9. Thermocouple sensors
• Sensitivity: It is the voltage or emf produced per degree Celsius
rise or fall in temperature at the junction.
• Construction: Simply welded or twisted between 2 metals.
• Range: Thermocouples have the greatest range in all types of
temperature measurement instrument domains.
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10. • Time Response: It is the time required by the thermocouple to
reach thermal equilibrium with the surrounding. Industrial TC
using thick wire have lower response of about 20s however,
TCs of small gauge wires have response time of about 20ms.
• Signal conditioning: the output voltage from the
thermocouples is very small usually less then 50 mV. Thus
considerable amplification is required so as to be used for
practical applications
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11. RTD and Thermocouple Calibration
Setup
RTD TC Temperature indicator
ON/OFF controller
OVEN 230V AC
50 Hz
Switch
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