THERMOCOUPLE THEORYWHAT’S A THERMOCOUPLE?A thermocouple is a device consisting of two different conductors (usually metal alloys) that produce a voltage, proportional to a temperature difference, between either end of the two conductors.A thermocouple circuit is formed when two dissimilar metals are joined at both ends and there is a difference in temperature between the two ends. This difference in temperature creates a small current and is called the Seebeck effect after Thomas Seebeck who discovered this phenomenon in 1821.

When there is a difference in temperature between the two ends of this circuit, a small voltage is formed within the circuit. This voltage or EMF (electro motive force) is usually measured in the 1/1000th of a volt (millivolt). Most people’s body produces more voltage than that! The higher the difference in temperature, the higher the voltage. If the right pairs of materials are used, these thermocouple circuits can be used to measure temperature.

The junction that is put into the process in which temperature is being measured is called the HOT JUNCTION. The other junction which is at the last point of thermocouple material and which is almost always at some kind of measuring instrument, is called the COLD JUNCTION.

COLD JUNCTION COMPENSATION

In the above example, one end of the thermocouple is @ 1000° and the other end is @100° so the difference is 900°. If we wanted to measure the temperature in a furnace, wecould use a thermocouple to do so. If the above example were used, the temperature

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inside the furnace is 1000° and the temperature outside is 100°, the thermocouple wouldindicate a difference in temperature between the inside and outside of 900°.The only problem with the example above is that we want to know the temperature insidethe furnace, not the difference between the outside and the inside. To do this with athermocouple, we need to apply “Cold Junction Compensation”. To apply this coldjunction compensation, all we need to know is the temperature of the cold junction.

The measuring instrument normally does this cold junction compensation. Theinstrument measures the temperature at the point where the thermocouple attaches andadds that temperature back in to the equation as per the above example. The instrumentthen displays the result of this equation.It is important to maintain thermocouple material throughout the circuit as in the case of asensor that is located some distance from the measuring instrument. Specially codedextension wire is normally used.

In the above example, thermocouple extension wire was not used in the circuit and so anerror has occurred due to incorrect cold junction compensation.

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

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Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

THERMOCOUPLE REFERENCE TABLES

Tables have been established worldwide that show temperature vs. millivolt outputfigures for the various accepted thermocouple combinations or “types”.These reference tables are all based on a reference or cold junction temperature of 32°F(0°C), which is the freezing point of pure water. All manufacturers follow these referencetables, which are published in ASTM document E-230.

THERMOCOUPLE TYPES

There are several different recognized thermocouple types available. Each type has different useful temperature ranges as well as different recommended applications. ASTM, which is recognized in the United States as the authority for temperature measurement, has established guidelines for the different thermocouple types. These guidelines cover composition, color codes, and manufacturing specifications.

BASE METAL THERMOCOUPLES

Base metal thermocouple types are composed of common, inexpensive metals such as nickel, iron and copper. The thermocouple types E, J, K, N and T are among this group and are the most commonly used type of thermocouple.Each leg of these different thermocouples is composed of a special alloy, which is usually referred to by their common names.

Type E – The type E thermocouple is composed of a positive leg of chromel (nickel/10% chromium) and a negative leg of constantan (nickel/45% copper). The temperature range for this thermocouple is –330 to 1600°F (-200 to 900°C). The type E thermocouple has the highest millivolt (EMF) output of all established thermocouple types. Type E sensors can be used in sub-zero, oxidizing or inert applications but should not be used in sulfurous, vacuum or low oxygen atmospheres.The color code for type E is purple for positive and red for negative.

Type J – Type J thermocouples have an iron positive leg and a constantan negative leg. Type J thermocouples have a useful temperature range of 32 to 1400°F (0 to 750°C) and can be used in vacuum, oxidizing, reducing and inert atmospheres. Due to the oxidation (rusting) problems associated with the iron leg, care must be used when choosing this type for use in oxidizing environments above 1000°F.The color code for type J is white for positive and red for negative.

Type K – The type K thermocouple has a Chromel positive leg and an Alumel (nickel/ 5% aluminum and silicon) negative leg. The temperature range for type K alloys is –328 to 2282°F (-200 to 1250°C).

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Type K sensors are recommended for use in oxidizing or completely inert environments. Type K and type E should not be used in sulfurous environments. Because type K has better oxidation resistance then types E, J and T, its main area of usage is at temperatures above 1000°F but vacuum and low oxygen conditions should be avoided.

Type N – Type N thermocouples are made with a Nicrosil (nickel – 14% chromium – 1.5 % silicon) positive leg and a Nisil (nickel – 4.5% silicon - .1% magnesium) negative leg. The temperature range for Type N is –450 to 2372°F (-270 to 1300°C) and the color code is orange for positive and red for negative. Type N is very similar to Type K except that it is less susceptible to selective oxidation. Type N should not be used in vacuum and or reducing environments in an unsheathed design.

Type T – Type T thermocouples are made with a copper positive leg and a constantan negative leg. The temperature range for type T is –328 – 662°F (-200 to 350°C) and the color code is blue for positive and red for negative. Type T sensors can be used in oxidizing (below 700°F), reducing or inert applications.

Rooman 208Seebeck effect

Diagram of the circuit on which Seebeck discovered the Seebeck effect. A and B are two different metals.

The Seebeck effect is the conversion of temperature differences directly into electricity and is named for German physicist Thomas Johann Seebeck, who, in 1821 discovered that a compass needle would be deflected by a closed loop formed by two metals joined in two places, with a temperature difference between the junctions. This was because the metals responded differently to the temperature difference, creating a current loop and a magnetic field. Seebeck did not recognize there was an electric current involved, so he called the phenomenon the thermomagnetic effect. Danish physicist Hans Christian Ørsted rectified the mistake and coined the term "thermoelectricity". The voltage created by this effect is on the order of several microvolts per kelvin difference. One such combination, copper-constantan, has a Seebeck coefficient of 41 microvolts per kelvin at room temperature.[2]

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The voltage V developed can be derived from:

where SA and SB are the thermopowers (Seebeck coefficient) of metals A and B as a function of temperature and T1 and T2 are the temperatures of the two junctions. The Seebeck coefficients are non-linear as a function of temperature, and depend on the conductors' absolute temperature, material, and molecular structure. If the Seebeck coefficients are effectively constant for the measured temperature range, the above formula can be approximated as:

The Seebeck effect is used in the thermocouple to measure a temperature difference; absolute temperature may be found by setting one end to a known temperature. A metal of unknown composition can be classified by its thermoelectric effect if a metallic probe of known composition, kept at a constant temperature, is held in contact with it. Industrial quality control instruments use this as thermoelectric alloy sorting to identify metal alloys. Thermocouples in series form a thermopile, sometimes constructed in order to increase the output voltage, since the voltage induced over each individual couple is small. Thermoelectric generators are used for creating power from heat differentials and exploit this effect.

Peltier effect

The Peltier effect is the presence of heat at an electrified junction of two different metals and is named for French physicist Jean-Charles Peltier, who discovered it in 1834. When a current is made to flow through a junction composed of materials A and B, heat is generated at the upper junction at T2, and absorbed at the lower junction at T1. The Peltier

heat absorbed by the lower junction per unit time is equal to

where ΠAB is the Peltier coefficient for the thermocouple composed of materials A and B and ΠA (ΠB) is the Peltier coefficient of material A (B). Π varies with the material's temperature and its specific composition: p-type silicon typically has a positive Peltier coefficient below ~550 K, but n-type silicon is typically negative.

The Peltier coefficients represent how much heat current is carried per unit charge through a given material. Since charge current must be continuous across a junction, the associated heat flow will develop a discontinuity if ΠA and ΠB are different. Depending on the magnitude of the current, heat must accumulate or deplete at the junction due to a non-zero divergence there caused by the carriers attempting to return to the equilibrium that existed before the current was applied by transferring energy from one connector to another. Individual couples can be connected in series to enhance the effect.

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Thermoelectric heat pumps exploit this phenomenon, as do thermoelectric cooling devices found in refrigerators.

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Ali 207APPLICATION OF THERMO COUPLES:Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a temperature gradient into electricity.

Steel industry:Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process. Disposable, immersible, type S thermocouples are regularly used in the electric arc furnace process to accurately measure the temperature of steel before tapping

Heating appliance safety:Many gas-fed heating appliances such as ovens and water heaters make use of a pilot flame to ignite the main gas burner when required. If it goes out, gas may be released, which is a fire risk and a health hazard. To prevent this, some appliances use a thermocouple in a fail-safe circuit to sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame, generating a voltage which operates the supply valve which feeds gas to the pilot. So long as the pilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. If the pilot light goes out, the thermocouple temperature falls, causing the voltage across the thermocouple to drop and the valve to close

ADVANTAGES: They are inexpensive. Interchangeable. They are supplied with standard connectors. They can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are

self powered and require no external form of excitation.

LIMITATION:The main limitation with thermocouples is accuracy and system errors of less than one degree Celsius (C) can be difficult to achieve.

Accuracy of temperature sensors is referred to as limits of error and apply only to brand new, un-used temperature sensors. Once a sensor is exposed to elevated temperatures, there is no guaranteed accuracy. All manufacturers adhere to these limits, which are establish by ASTM and are covered under their publication ASTM E –230. The Limits of Error tables appear in the SensorTec catalog as well as many competing manufactures catalogs.

STANDARDS ABOUT THERMOCOUPLES

♦ According to ASTM color code guidelines, which apply to most North American sensor

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manufacturers, the Red leg is always negative.

♦ 2 types of thermocouples (types J and K) have one leg, which is magnetic. With these 2 types, you can

use a magnet to determine polarity.

♦ The hot junction of a thermocouple can be made by any means possible as long as there is good,

constant contact between the two wires.

♦ Special limits of error thermocouple sensors do not have to have special limits of error extension

wire.

♦ Non-thermocouple materials can be used in thermocouple circuits under the right conditions. Non-

thermocouple connectors, terminals and slices can be used as long as there is no temperature gradient

present at the areas where these items are used.

♦ Extension wire does not have to be a large gauge to work in an application where the sensor is placed a

long way from the measuring instrument. Most modern temperature monitoring instruments are current

based so lead wire resistance is not critical.

♦ It is possible to get an average temperature reading using multiple thermocouples as long as the

sensors are wired in parallel and the resistance of these different sensors is the same.

The relation between seebeck and peltier co efficent:

Πab = T(L or H) Sab= T(L or H) (Sa-Sb)

=- Πba

Where T(L or H) is either the absolute temperature of cold junction TL or hot junction TH

whereas seebeck coefficient is a strong function of temperature , defined as

S=dEsdT

And peltier coefficient is defined as

Πab =−QIab where -Q is the heat transfer rate from the junction in watts, Iab is

the direct current flowing in the generator, in amps

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MaterialS,VK

MaterialS,VK

Aluminium PlatinumConstantan GemaniumCopper siliconIron

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