RTD (Resistance Temperature Detector)
The RTD stands for Resistance Temperature Detector. The
RTD is defined as the resistor which is used for measuring
the temperature.
The resistance of the conductor change with temperature
and this property of conductors is used in the RTD for
measuring the temperature. RTDs have positive
temperature coefficient.
The RTD is made of pure metals like platinum, nickel,
copper etc. The change of resistance concerning with the
conductor is explained by the equation shown below.
R=
Where, Ro= resistance at temperature T=0 and α1, α2,
α3... are constants. 3
Fig.1 Industrial platinum resistance thermometer
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The platinum is used in RTD because it can withstand high
temperature and also it has high stability. The metals have
the positive-temperature coefficient, i.e. their resistance
increases with the increases in temperature and this
property of metals are used in the RTD.
The requirements of a conductor material to be used in
these thermometers are :
1) The change in resistance of material per unit change in
temperature should be as large as possible.
2) The material should have a high value of resistivity so
that minimum volume of material is used for the
construction of RTD.
3) The resistance of the materials should have a
continuous and stable relationship with temperature.
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Metals Used for Resistance Thermometers
Metal Resistance temperature co- Temperature range °C Melting
efficient °C (per cent) point °C
Min. Max.
Platinum 0.39 -260 110 1773
Copper 0.39 0 180 1083
Nickel 0.62 -220 300 1455
T ungsten 0.45 -200 1000 3370
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The characteristics of various materials used for resistance
thermometers are plotted in Figure. The curves are nearly linear, and
for small temperature range, it is very evident.
R/R0
Fig.2 Characteristics of materials used for resistance thermometers
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Advantages.
1) Fast response
2) High degree of accuracy
3) Wide working range of temperature
4) No necessity of temperature compensation
Disadvantages of the RTD
1) High cost
2) Possibility of self heating
3) Complex circuitry
Applications
1) In heating ovens.
2) In process industries.
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Thermistors
The thermistor is a kind of resistor whose resistivity depends
on surrounding temperature. It is a temperature sensitive
device. The word thermistor is derived from the
word, thermally sensitive resistor. The thermistor is made of
the semiconductor material that means their resistance lies
between the conductor and the insulator.
The thermistor is used for measuring the small range of
temperature, which is nearly up to -60ºC – 15ºC. The range
of resistor in the thermistor is from 0.5 – 0.75 Ω. It is an
extremely sensitive device used for measuring the
temperature of home appliances.
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Types of Thermistors
The thermistor is classified into types. They are the negative
temperature coefficient and the positive temperature
coefficient thermistor.
1) Negative Temperature Coefficient Thermistor – In
this type of thermistor the temperature increases with
the decrease of the resistance. The resistance of the
negative temperature coefficient thermistor is very large
due to which it detects the small variation in
temperature.
2) Positive Temperature Coefficient Thermistor – The
resistance of the thermistor increases with the increases
in temperature. 11
Construction of Thermistors
The thermistor is made with the sintered mixture of
metallic oxides like manganese, cobalt, nickel, copper,
iron, uranium, etc. It is available in the form of the bead,
rod and disc. The different types of the thermistor are
shown in the figure below.
The bead form of the thermistor is smallest in shape, and it is
enclosed inside the solid glass rod to form probes.
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The disc shape is made by pressing material under high
pressure with diameter range from 2.5 mm to 25mm.
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Resistance Temperature Characteristic of Thermistor
The relation between the absolute temperature and the resistance of
the thermistor is mathematically expressed by the equation shown
below.
Where,
RT1 – Resistance of the thermistor at absolute temperature T1 in Kelvin.
RT2 – Resistance of the thermistor at absolute temperature T2 in Kelvin.
β – a constant depending on the material of thermistor.
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The graph below shows that the thermistor has a negative
temperature coefficient, i.e., the temperature is inversely proportional
to the resistance. The resistance of the thermistor changes from 105 to
10-2 at the temperature between -100C to 400C.
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Advantages of Thermistor
The following are the advantages of the thermistor.
1) The thermistor is compact, long durable and less expensive.
2) The properly aged thermistor has good stability.
3) The response time of the thermistor changes from seconds to
minutes. Their response time depends on the detecting mass and
the thermal capacity of the thermistor.
4) The self-heating of the thermistor is avoided by minimising the
current passes through it.
5) The thermistor is installed at the distance of the measuring circuit.
Thus the reading is free from the error caused by the resistance of
the lead.
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