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Names and different
methods to measure the
temperature
Resistance temperature
detectors (RTD)
Thermister NTC and
PTC
Thermocouples
Pyrometers
Smart temperature
sensors
Applications
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Thermocouples (T/C) are formed when two dissimilar
metals are joined together to form a junction.
Joining together the other ends of the dissimilar
metals to form a second junction completes an
electrical circuit.
A current will flow in the circuit if the two junctions
are at different temperatures.
The voltage difference between the two junctions is
measured, and this difference is proportional to the
temperature difference between the two junctions.
Three effects are associated with thermocouples.
Seebeck effect: It states that the voltage produced
in a thermocouple is proportional to the
temperature between the two junctions.
Peltier effect: It states that if a current flows
through thermocouple one junction is heated (puts
out energy) and the other junction is cooled
(absorbs energy
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The emf generated can be approximately
expressed by the relationship:
where T1 and T2 are hot and cold junction
temperatures in K. C1 and C2 are constants
depending upon the materials. For Copper/
Constantan thermocouple, C1=62.1 and
C2=0.045 .
Thermocouples are extensively used for
measurement of temperature in industrial
situations. The major reasons behind their
popularity are:
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(i) they are rugged and readings are consistent,
(ii) they can measure over a wide range of temperature,
and
(iii) their characteristics are almost linear with an accuracy
of about 0.05%.
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we have three laws of thermoelectric circuits
that provide us useful practical tips for
measurement of temperature .
law of homogeneous circuit (i): The first law
can be explained using figure 1. It says that
the net thermo-emf generated is dependent on
the materials and the temperatures of two
junctions only, not on any intermediate
temperature.
law of intermediate metals: If a third material is
introduced at any point (thus forming two
additional junctions) it will not have any effect,
if these two additional junctions remain at the
same temperatures . This law makes it possible
to insert a measuring device without altering
the thermo-emf.
law of intermediate temperatures :The third
law is related to the calibration of the
thermocouple. It says, if a thermocouple
produces emf e1, when its junctions are at T1
and T2, and e2 when its junctions are at T2 and
T3; then it will generate emf e1+e2 when the
junction temperatures are at T1 and T3
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In the process control of chemical reactions, temperature control is of
major importance, since chemical reactions are temperature dependent.
In an average household , at least a dozen temperature sensors can be
found in various places, raging from coffee machine to heating system
to a car.
Low cost
Small size
Robust
Wide range of operation
Reasonably stable
Accurate for large temperature changes
Provide fast response
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RTD’s are built from selected metals (typically
Platinum), which change resistance with
temperature change.
The resistance temperature detector (RTD)
measures the electrical conductivity as it varies
with temperature.
The electrical resistance generally increases
with temperature, and the device is defined as
having a positive temperature coefficient.
The magnitude of the temperature coefficient
determines the sensitivity of the RTD.
Apart from Platinum, other metals are used for
RTD’s such as Copper and Nickel.
Platinum is the most common and has the best
linear characteristics of the three, although
Nickel has a higher temperature coefficient
giving it greater sensitivity.
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The temperature coefficient defines
how much the resistance will
change for a change in
temperature, and has units of
ohms/oC.
The greater the temperature
coefficient, the more the resistance
will change for a given change in
temperature.
This ultimately defines how
sensitive the device is.
RTD’s are generally quite linear,
however the temperature
coefficient does vary over the range
of operation.
As an indication, the temperature
coefficient for Platinum is averaged
at 0.00385 over the range from
0oC to 100oC, but varies by about
2% over
this range.
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Advantages
- Good sensitivity
- Uses standard copper wire
- Copper RTD’s minimize
thermocouple effect
Disadvantages
- Bulky in size and fragile
- Slow thermal response time
due to bulk
- Self heating problems
- More susceptible to
electrical noise
- More expensive to test and
diagnose
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A thermistor is a semiconductor device
formed from metal oxides.
The principle of temperature
measurement with a thermistor is that
its resistance changes with
temperature.
Most thermistors differ from normal
resistors in that they have a negative
coefficient of resistance, this means
that the resistance decreases with an
increase in temperature.
Negative (NTC) thermistors are the
more common although positive (PTC)
are also available.
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Types of thermistors
vary in a number of
ways and one change is
their response to
temperature changes.
Thermistors are not
linear, and their
response curves vary for
the different types.
Some thermistors have
a near linear
temperature resistance
relationship, others are
available with a sharp
change in slope
(sensitivity) at a
particular characteristic
temperature.
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Advantages
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Disadvantages
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Small size
Fast response
Very high sensitivity (Select range)
No cold junction compensation
Inexpensive
Polarity insensitive
Wide selection of sensors
Not easily interchangeable
Non linear
Narrow span
Fragile
High resistance, noise problems
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Pyrometric methods of
temperature measurement use the
electromagnetic radiation that is
emitted from a material. The
emitted radiation is proportional to
the temperature.
Any object with a temperature
above absolute zero will radiate
electromagnetic energy.
Infrared pyrometers measure the
amount of energy radiated from an
object in order to determine its
temperature.
There are a number of different
types of infrared pyrometers:
- Total radiation
- Single wavelength
- Dual wavelength
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Advantages
- Non contact measurement
- High temperature sensing
- Remote sensing
- Fast response and can sense objects in motion
- Sense small or area targets
Disadvantages
- Expensive
- Non linear response
- Subject to emissivity of material
- Require wide range of operation