Temperature measurement by electrical instruments.

1. ELECTRICAL
TEMPERATURE
MEASUREMENT

2. TEMPERATURE
• In a simple words we can defined temperature
as a ‘ Measurement of degree of hotness and
coldness of an object’.
• On basis of kinetic theory, the temperature of
the system is the measure of the mean kinetic
energy of the molecules of a substance.

3. ELECTRICAL TEMPERATURE
MEASURING INSTRUMENTS
• In electrical methods of measuring temperature, the
temperature signal is converted into electrical signal
either through a change in resistance or voltage,
leading to a change in current development of
electro-motive-force (emf). The following elements
are used to convert temperature into electrical
signals:-
• 1) Thermocouples and thermopiles
• 2) Resistance Temperature detectors
• 3) Thermistors

4. THERMOCOUPLES
• The word thermocouple is a combination of
thermo means heat and couple means to
make the junction.
• It consists of wires of two dissimilar metals
welded at ends to for two junctions.
• The junction ate higher temperature is called hot
or measuring junction while the junction at lower
temperature is called cold or reference junction.

5. WORKING PRINCIPLE OF
THERMOCOUPLE
• Thermocouples works on thermoelectric
effect.
• If two wires of different metals M1 and M2
are joined together to form a loop with one
junction at temp. T1 and other at temp. T2,
an e.m.f. is generated and if an ammeter is
connected, a current flows in the circuit. This
phenomenon is called see beck effect.

6. CONTINUED…
• The magnitude of emf generated depends
on the temperature T1 and T2 and the
materials M1 and M2. One of the junction is
usually maintained at some constant known
temperature. The output voltage of the
circuit then indicates the temp. difference
relative to the reference temp.

7. ADVANTAGES
Rugged and inexpensive construction.
Fast response compared to filled system
thermometers.
They have a wide temperature range i.e. -270⁰C to
2800⁰C.
Very convenient for measuring the temperature at
one particular point in a piece of apparatus.
Calibration is easy.
Simple to use than resistance thermometers.

8. DISADVANTAGES
The temperature – emf relationship may be
non linear.
It can not be used for precision
measurement.
The emf generation per degree temperature
is very less, hence it may not applicable.
It is required to maintain constant reference
junction temperature.

9. RESISTANCE TEMPERATURE
DETECTORS (RTD)
• The electrical resistance of various
substances changes with temperature
changes. Therefore, any change in
temperature of a metal can be measured in
terms of change in its electrical resistance.
• The temp. measuring device works based
on principle of the resistance variation with
temp. is called Resistance Temperature
Detector (RTD).
• Platinum, copper and nickel metals are
generally used in resistance thermometer.

10. CONSTRUCTION
• The RTD consist of (1) a resistance
element, (2) suitable electrical leads and (3)
an indicating, recording or resistance
measuring device. The temp. sensing
element is normally in the form of a coil of a
very fine platinum, copper or nickel wire
wound on to an insulating ceramic former.
• The resistance coil and ceramic former is
protected by metal sheath. It should be
ensured that the resistance wire is free from
mechanical stresses.

11. CONTINUED…
• A metal of resistance wire has been strained will
suffer a change in the resistance characteristics.
The metal of wire is therefore usually annealed at
a temperature higher than that at which it is to
operate.
• The ends of resistance wire (coil) are welded to
stiff copper leads that are taken out to be
connected in one of the arms of Wheatstone
bridge circuit. The Wheatstone bridge circuit
measures the changes in resistance with respect
to temp. change.

12. WORKING
• When instrument sensor is placed in a liquid or
a gas medium whose temperature is to be
measured, the metal sheath suddenly reaches
the temperature of the medium.
• This change in temperature causes the coil
inside the metal sheath to heat or cool,
resulting in proportional change in the coil
resistance Rw, the Wheatstone bridge becomes
unbalanced and hence galvanometer will give
deflection which is calibrated to show result in
terms of temperature.

13. ADVANTAGES
Small size, sensor easily installed and replaced.
Fast response and high accuracy of
measurement.
Wide temperature range (-200 to 650⁰C)
Use for remote sensing temperature
measurement.
Reference junction is not required.
Good reproducibility
Stable and accurate measurement during long life
span.

14. DISADVANTAGES
Expensive than other instruments.
Large bulb size than thermocouples.
It requires power supply and bridge circuit.
Due to current flowing through bridge circuit and
hence heating of sensing element. Therefore
accuracy is reduced.

15. THERMISTORS
• Thermistors is a semi-conductor material whose
electrical resistance decreases with increase in
temperature.
• The semi-conductor have high negative
temperature co-efficient of resistance . As
temperature increases, the resistance of
thermistor is decreased or temperature
decreases, the resistance increases.

16. CONTINUED…
• The mathematical relationship between
resistance and temperature of thermistor is given
by:-
• R = resistance at temperature T in ⁰K
• Ro = resistance at absolute temperature To in ⁰K
• B = constant, depends upon material of
thermistor.

17. WORKING
• The thermistor act as a temperature sensor and it
is placed on a body whose temperature is to be
measured. It is also connected to electrical
circuit.
• When the temperature of the body changes, the
resistance of the body also changes, which is
directly indicated by the circuit as the
temperature since the resistance is calibrated
against the temperature.

18. ADVANTAGES
Small size, fast response and less expensive.
High sensitivity due to negative temperature co-
efficient of resistance.
High sensitivity to temperature changes makes
the thermistors extremely for precision
temperature measurement, control and
compensation.
No reference junction is required.
Suitability for narrow spans.

19. DISADVANTAGES
It has non linear response.
Unstable at high temperature.
Problem due to interchangeability of individual
elements.
Not suitable for wide temperature span.

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