What is a Thermocouple?
Thermocouple is the devise used extensively for measurement of the
temperature of the body. Temperature is the fundamental property just like the
mass and time and is frequently measured quantity. Thermocouple comprises of at
least two metals joint together to form two junctions. One is connected to the
body whose temperature is to be measured; this junction is called as hot or
measuring junction. The other junction is connected to the body of known
temperature; this is called as cold or reference junction. Thus the
thermocouple enables measuring the unknown temperature of the body with
reference to the known temperature of the other body.
Principle of Working of Thermocouple
The working principle of thermocouple is based on three effects, discovered
by Seebeck, Peltier and Thomson. All these have been described in brief below.
1)
Seebeck effect: The Seebeck effect
states that when two different or unlike metals are joined together at two
junctions, an electromotive force (emf) is generated at the two junctions. The
amount of emf generated is different for different combinations of the metals.
2) Peltier effect: As per the Peltier effect when two
dissimilar metals are joined together to form two junctions, the emf is
generated within the circuit due to different temperatures of the two junctions
of the circuit.
3) Thomson effect: As per Thomson effect, when two unlike
metals are joined together forming two junctions, the potential exists within
the circuit due to temperature gradient along the entire length of the
conductors within the circuit.
In most of the cases the emf suggested by Thomson effect is very small and
it can be neglected by making proper selection of the metals. The Peltier
effect play prominent role in the working principle of the thermocouple.
2) Peltier effect: As per the Peltier effect when two
dissimilar metals are joined together to form two junctions, the emf is
generated within the circuit due to different temperatures of the two junctions
of the circuit.
3) Thomson effect: As per Thomson effect, when two unlike
metals are joined together forming two junctions, the potential exists within
the circuit due to temperature gradient along the entire length of the
conductors within the circuit.
In most of the cases the emf suggested by Thomson effect is very small and
it can be neglected by making proper selection of the metals. The Peltier
effect play prominent role in the working principle of the thermocouple.
Construction
A thermocouple is constructed of
two dissimilar metal wires joined
at one end. When one end of each wire is connected to a measuring
instrument, the thermocouple becomes a
sensitive and highly accurate measuring device.
Thermocouples may be constructed of
several different combinations of
materials. The performance of a thermocouple material is generally determined by using that
material with platinum. The most important factor to be considered when selecting a pair of materials is
the "thermoelectric difference" between the two materials. A significant difference between
the two materials will result
in better thermocouple performance.
Figure 4 illustrates the characteristics of the more commonly used
materials when used with platinum. Other
materials may be used in addition to those shown in Figure 4. For
example: Chromel- Constantan is excellent
for temperatures up to 2000°F; Nickel/Nickel-Molybdenum sometimes replaces Chromel-Alumel; and Tungsten-Rhenium is used for
temperatures up to 5000°F. Some combinations used
for specialized applications are Chromel-White Gold, Molybdenum-Tungsten,
Tungsten-Iridium, and Iridium/Iridium-Rhodium.
Basic Differences Between A Thermocouple And A Thermistor
A Thermocouple and a thermistor are both temperature
measurement devices used in process manufacturing. They offer different
benefits, and both are widely used in the pharmaceutical industry, food
industry, beverage industry, processing industry, and other industries.
Understanding the difference between thermocouples and thermistors will help
you know which instrument is best suited for your needs.
A thermocouple, based on the Seebeck effect, is a type of
thermometer that reads the temperature by measuring the voltage created by
placing one end of two different metal wires twisted together in a substance,
and keeping the other ends at a constant temperature. This creates an electric
voltage, and by measuring this voltage scientists in the nineteenth century
were first able to accurately measure temperatures over 500 degrees Fahrenheit.
Thermocouples are excellent process temperature measurement devices, extremely
well suited for harsh and high temperature environments (with some
thermocouples operating perfectly well up to 4200 degrees Fahrenheit).
Thermocouples are used in dozens of engineering, chemical, manufacturing,
industrial, waste treatment, plastic production, energy production, and food
processing facilities.
Thermocouples are made in many different types. Each type is a standardized
junction of two particular metal wires (such as a junction of iron and
constantan, which is known as type J). Each type of thermocouple has a certain
range of temperatures that it is suited for. Types T, E, K, and J are the most
common and versatile junction types for thermocouples. Other types are helpful
for more extreme environments where high velocity, extreme temperatures, high
pressure, corrosive temperatures, and high moisture oxidation risks are
present. Thermocouples can be customized with the right coated wires and
adjustments to be fire proof, explosion proof, and long lasting even in harsh
environments. They are very accurate within their temperature range. Most offer
very rapid response times.
Thermistors also measure temperatures, but operate differently from
thermocouples. Instead of producing an electric voltage like a thermocouple, a
thermistor (also known as a thermal resistor) has an electrical resistance that
varies with the temperature. There are two basic types (or families) of
thermistors. Positive temperature coefficient thermistors (also known as PTCs)
have an electrical resistance that is in direct proportion to the temperature.
Negative temperature Coefficient thermistors (also known as NTCs) have an
electrical charge that changes in inverse proportion to the temperature.
Thermistors are small, rugged, and accurate, and have a fast response time.
They are frequently used in laser diode applications due to their low price.
They are more cumbersome and difficult to use and decipher than a thermocouple
is, due to the fact of thermistors' non-linearity.
Chart
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Temperature
Measurement Comparison Chart
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Criteria
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Thermocouple
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RTD
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Thermistor
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|
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-267°C to 2316°C
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-240°C to 649°C
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-100°C to 500°C
|
|
Accuracy
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Good
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Best
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Good
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Linearity
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Better
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Best
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Good
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Sensitivity
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Good
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Better
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Best
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Cost
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Best
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Good
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Better
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