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thermocouple

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									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.




(Thermocouple Construction Figure 4     Thermocouple Material Characteristics When
Used with Platinum)

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

              Temperature Measurement Comparison Chart
Criteria      Thermocouple       RTD               Thermistor
Temp Range    -267°C to 2316°C   -240°C to 649°C   -100°C to 500°C
Accuracy      Good               Best              Good
Linearity     Better             Best              Good
Sensitivity   Good               Better            Best
Cost          Best               Good              Better

								
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