The measurement of temperature is very important in industrial application, so therefore continuous measurement of temperature is very much essential.
Temperature can be measured through various types of sensors; all of them run on the same basic principle that they give temperature in the output according to the changes produced in their physical characteristics in the input.
According to the type of application, measurement of temperature can be divided into following parts:
1. CONTACT METHOD : This method is used when the body (whose temperature is to be measured) and the sensor (which is measuring the temperature) can remain in contact with each other, in other words, we can say that if the body and the sensor can remain in contact with each other during the measurement of temperature than contact method is used.
Contact method uses three types of thermometers (temp. inst.)
1) Expansion Thermometers
2) Filled System Thermometers
3) Electrical temperature Instruments
1.1 EXPANSION THERMOMETERS:
In expansion thermometers bimetallic devices are used, in bimetallic devices there are two different materials whose rate of thermal expansion is also different. Therefore, in bimetallic devices there are strips of two metals which are bonded together. When heated, one side will expand more than the other, thus the resulting expansion is translated into temperature reading by a mechanical linkage to a pointer.
The advantage of this type of instrument is that they are portable and they do not need a power supply.
And the disadvantages of this type of instrument are that they are not as accurate as other devices and they do not readily lend themselves to temperature recording.
1.2 FILLED SYSTEM THERMOMETERS:
Filled system thermometer simply means that they are filled with any of the substitute. They generally come in two main classifications: the mercury type and the organic-liquid type. Since, mercury is considered an environmental hazard, so there are regulations governing the shipment of that type of devices that contain it. Now a day, there are filled system thermometers which employ gas instead of liquids.
The advantages of these types of devices are that they do not require any
electric power, they do not pose any explosion hazard and they are stable even after repeated cycling. And the disadvantage of these types of devices is that
they do not generate data that are easily recorded or can be transmitted and
they do not make spot or point measurements.
1.3 ELECTRICAL TEMPERATURE INSTRUMENTS:
As the name implies
these types of instruments sense the temperature in the terms of electrical
quantities like voltage, resistance etc. Therefore, we can say that these types of
instruments are not directing indicating thermometers like mercury in glass
devices.
In the majority of industrial and laboratory processes the measurement point is
usually far from the indicating or controlling instrument. This may be due to
necessity (e.g. an adverse environment) or convenience (e.g. centralized data
acquisition).Therefore; Devices are required which convert temperature in to
another form of signal, usually electrical quantities.
The most common devices used in these types of temperature instruments are
(a) thermocouples, (b) resistance thermometers and (c) thermistors, the similarity
is, all of them require some form of contact with the body (of whose temperature
is to be measured), the mode of contact could be immersed or it could be
surface, depending on the construction of the sensor and the application where it
is used.
1.3(a) THERMOCOUPLES:
Thermocouples essentially comprises of a thermo
element (which is a junction of two dissimilar metals) and an appropriate two
wire extension lead. A thermocouple operates on the basis of the junction
located in the process producing a small voltage, which increases with
temperature. It does so on a reasonably stable and repeatable basis.
1.3(b) RESISTANCE THERMOMETER :
Resistance thermometer utilizes a
precision resistor, the resistance (Ohms) value of which increases with temperature.
RTD has had positive temperature coefficient. Such variations are very stable and
precisely repeatable.
1.3(c) THERMISTORS :
Thermistor is a semiconductor used as a temperature
sensor. It is manufactured from a mixture of metal oxides pressed into a bead,
wafer or other shape. The bead is heated under the pressure at high
temperatures and then encapsulated with epoxy or glass. Beads can be very
small, less than 1 mm in some cases.
The result of all these is a temperature sensing devices that displays a very
distinct non linear resistance versus temperature relationship. The resistance of
thermistor decreases with increase in the temperature; this is called as
negative temperature coefficient of thermistor.
Thermistor exhibits a very large resistance changes for a small temperature change.
This can be as large as 3 to 5% per °C. This makes them very sensitive to small temperature changes. They can detect temperature change as low as 0.1 °C or
smaller. A thermistor element is significant smaller in size compared to RTDs.
The sensitivity of thermistors to temperature change and their small size make it
ideal for use in medical equipment.
DISADVANTAGE OF THERMISTORS:
Because they are semiconductors,
thermistors are more susceptible to permanent decalibration at high
temperatures than are RTD's or
thermocouple. The use of thermistors is
generally limited to a few hundred degree
Celsius and manufacturers warn that
extended exposures even well below
maximum operating limits will cause the
thermistor to drift out of its specified
tolerance.
Thermistors can be made very small which
means they will respond quickly to temperature changes. It also means that
their small thermal mass makes them especially susceptible to self-heating
errors. Thermistors are good deal more fragile than RTD's or thermocouple and
they must be carefully mounted to avoid crushing or bond separation.
2. NON-CONTACT METHOD : This method is used when the body
(whose temperature is to be measured) and the sensor (which is measuring the
temperature) are not allowed to remain in contact with each other, in other
words, we can say that if the body and the sensor are not allowed to remain in
contact with each other during the measurement of temperature then non
contact method is used.
Most common thermometers (temperature instrument) using Non-Contact
method are:
a) Infra red sensors and Pyrometers and Thermal Imagers
(a) INFRARED SENSORS & PYROMETERS :
Infra red sensors & Pyrometer,
now a day is the most common non contact temperature instrument in the
industrial applications as it is easy to operate and use if the working principle is
known to the user. Infra red sensor & Pyrometer measures the temperature of the
object without being in the contact of the body but how is it possible? The answer
is here, every object whose temperature is above absolute zero (-273.15K) emits
radiation. This emission is heat radiation and its wavelength/frequency depends
upon the temperature. So, this property of emission is used when the temperature
is to be measured via non contact method.
The term infra red radiation is also in use because the wavelength of the majority
of this radiation lies in the electromagnetic spectrum above the visible red light
which is in the infra red region.
Similar to the radio broadcasting where emitted energy from a transmitter is
captured by a receiver via an antenna and then transformed into sound waves,
the emitted heat radiation of an object is received by detecting devices and
transformed into electric signals, and in this way the temperature of an object is
measured through non contact temperature measuring instruments.
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