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Friday, February 22, 2013

Thermocouple



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