Thermocouple Calibiration

7/29/2019 Thermocouple Calibiration

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Abstract

There are various kinds of temperature sensors used in the chemical industries such as thermistors,

RTDs, pyrometers and many more. All of them have a unique temperature range and sensitivity.

Economic considerations are always very important in installing these sensors. Their accuracy,

repeatability, reproducibility and ability to eliminate errors should also be analysed closely.

The following assignment will give u an idea about thermocouples and its calibration mainly. As

thermocouples is a very vast topic so I have included the important topics such as the three important

laws of thermocouples which are Law of intermediate materials, Law of homogeneous material and

Law of successive or intermediate temperatures. It also includes types of thermocouples such as

refractory metal thermocouples, base metal thermocouples and noble metal thermocouples and their

sub types too. Its applications and different temperature ranges for the given types. The pros and cons

of using a thermocouple are also included. Then comes the calibration procedure of a thermocouple.

Basically in a thermocouple we have a hot junction and a cold junction so ways to calibrate it are

shown and illustrated by diagrams. Thermocouples have various applications in heating appliance

safety, steel industries, thermopile radiation sensors, process plants manufacturing, Radioisotope

thermoelectric generators and many more.

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Table of Contents

1.Thermocouple Theory 42.Laws of Thermocouple 4

Law of intermediate materials 4

Law of homogeneous materials 4

Law of intermediate temperatures 5

3.Types of Thermocouple 5Base metal Thermocouples 5

Noble Metal Thermocouples 6

Refractory Metal Thermocouples 7

4.Facts About Thermocouple 85.Advantages and Disadvantages 86.Aging of Thermocouple 97.Calibration procedure 10

Controlled Temperature Sources 10

Reference Junction 11

Reference Thermometers 13

8.Applications 14



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THERMOCOUPLE

A thermocouple is formed when two dissimilar metals are joined and a temperature difference occurs

between the two ends. This temperature difference creates a current and is called the SeeBeck Effect.

This was named after the scientist Thomas Seebeck who discovered this in 1821.

A temperature difference between the two ends of the circuit, voltage is generated within the circuit.

The voltage is measured in the 0.001 th of a volt. Higher the temperature difference creates higher

voltage. Right pair of metals should be used to measure the temperature. The junction put into the

process in which temperature is measured is known as the Hot Junction. The other junction which is

always at some kind of measuring instrument is called the Cold Junction.

For example if we want to measure the temperature inside a furnace; inside the furnace its 1000C and

outside temperature is measured to be 100C. Thermocouple would thus indicate a difference of

900C.

The problem with the thermocouple is if we want to know the temperature inside the furnace and not

the temperature difference between inside and outside. For this we apply the Cold Junction

Compensation. For this, we need to know the cold junction temperature and normally the measuring

instrument normally does it. It measures the point at which the thermocouple attaches then adds the

temperature back into the equation. The result is then displayed. Thermocouple material should be

used throughout the circuit. Specially coded extension wire is normally used. If the extension wire is

not used in the circuit an error may occur due to incorrect cold junction compensation. (1)

LAWS OF THERMOCOUPLES

Law of intermediate materials

Algebraic sum of the thermoelectric forces in a circuit comprising of any number of dissimilar metals

is zero if all of the junctions are at a uniform temperature. If a third metal is inserted and if the twonew junctions are at the same temperature, there will be no net voltage generated by the new metal.

Law of homogeneous material

A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the

application of heat alone, regardless of how it might vary in cross section. In other words, temperature

changes in the wiring between the input and output do not affect the output voltage, provided all wires

are made of the same materials as the thermocouple.



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Law of successive or intermediate temperatures

If two dissimilar homogeneous materials produce thermal emf(A) when the junctions are at T1 and T2

and produce thermal emf(B) when the junctions are at T2 and T3 , the emf generated when the

junctions are at T1 and T3 will be emf(A) + emf(B) . (2)

BASIC THERMOCOUPLE TYPES

There are several different kinds of thermocouples covering different ranges for them and different

applications. ASTM has all guidelines in them covering from its composition to color codes and other

specifications.

Base Metal Thermocouples

Base metal thermocouples are composed of inexpensive metals like

nickel, iron and copper. The thermocouple types E, J, K, N and T are among this group. They 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 JType J thermocouples have an iron positive leg and a constantan negative leg.

They have a temperature range of 32 to 1400F (0 to 750C) 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 1000F.

The color code for type J is white for positive and red for negative.

Type EThe 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 is330 to 1600F (-200 to 900C). The type E

thermocouple has the highest millivolt output. 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.



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Type KThe type K thermocouple has a Chromel positive leg and an Alumel (nickel/

5% aluminum and silicon) negative leg. Temperature range for type K alloys is328

to 2282F (-200 to 1250C). Type K sensors are recommended for use in oxidizing or inert

environments. Type K should also not be used in sulfurous

environments. Because type K has better oxidation resistance than types E, J and T, its

main area of usage is at temperatures above 1000F but vacuum and low oxygen

conditions should be avoided.

Type TType T thermocouples are made with a copper positive leg and a constantan

negative leg. The temperature range for type T is328662F (-200 to 350C) and

color code is blue for positive and red for negative. Type T sensors can be used in

oxidizing (below 700F), reducing or inert applications.

Type NType N thermocouples are made with a Nicrosil (nickel14% chromium

1.5 % silicon) positive leg and a Nisil (nickel4.5% silicon - .1% magnesium) negative

leg. The temperature range for Type N is450 to 2372F (-270 to 1300C) 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.

Noble Metal Thermocouples

They are manufactured with a wire that is made with precious metals like Platinum and Rhodium.

They are used in oxidizing or inert applications and must be used with a ceramic protection tube

surrounding the thermocouple element. They are usually fragile and must not be used in applications

that are reducing or in applications that contain metallic vapors.

Type BType B thermocouples are made with a platinum/30% rhodium positive leg and a platinum/6%

Rhodium negative leg. The temperature range for this type is 323092F (0 to 1700C) and the color

code is gray for positive and red for negative.

Type RType R thermocouples are made with a platinum/13% rhodium positive leg



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and a pure platinum negative leg. The temperature range for type R is 322642F (0 to

1450C) and the color code is black for positive and red for negative.

Type S

Type S thermocouples are made with a platinum/10% rhodium positive legand a pure platinum negative leg. The temperature range for type S is 322642F (0 to

1450C) and the color code is black for positive and red for negative.

Refractory Metal Thermocouples

They are manufactured with a wire made from the exotic metals tungsten and Rhenium. They are

expensive, difficult to manufacture and wire made with these metals are brittle. These thermocouples

are expected for use in vacuum furnaces at extremely high temperatures and must never be used in the

presence of oxygen at temperatures above 500F. There are several different combinations of alloys

but only one generally used at this time.

Type CType C thermocouple is made with a tungsten/5% rhenium positive leg and tungsten 26% rhenium

negative leg and has a temperature range of 324208F (02320C). The color code for this type is

white with red for positive leg and red for the negative leg. (3)

THERMOCOUPLE REFERENCE TABLES

Tables have been established internationally that show temperature vs. millivolt output

for the various accepted thermocouple combinations.

These reference tables are based on a reference or cold junction temperature of 32F

(0C); which is the freezing point of pure water. They are published in ASTM document E-

230.

ERROR LIMITS

Accuracy of temperature sensors is referred to as limits of error and apply only to brand

new, temperature sensors. Once a sensor is exposed to elevated temperatures,

there is no guaranteed accuracy.



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FACTS

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

sensor manufacturers, the Red leg is always negative.

The hot junction of a thermocouple requires a good, constant contact between the two wires. Material

may be any.

Two types of thermocouples (types J and K) have one leg, which is magnetic. With these

two types, you can use a magnet to determine polarity.

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

conditions. Non-thermocouple connectors, terminals and slices can be used where

there is no temperature gradient present at the areas where these items are used.

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

error extension wire.

We can get an average temperature reading using multiple thermocouples as

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

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. (4)

ADVANTAGES AND DISADVANTAGES

Thermocouple is used to measure extreme temperatures. The properties of thermocouples were

originally discovered in 1822 by Thomas Seebeck. Temperature differences that the thermocouple

measures result from thermoelectric voltage differences produced by the two dissimilar metals which

make a circuit.

Thermocouple temperature range is from 200 to 2600 degrees Celsius. Therefore, thermocouples are

used in a variety of circumstances in industries. Their large temperature range also allows them to be

used at high temperatures where most temperature sensors stop working.



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Thermocouples have the ability to be brought into direct contact with the material they are measuring.

Since they use voltage readings to measure temperatures, the only requirement when they are brought

in direct contact with the material is that the thermocouple is safely grounded.

Thermocouples are made up of two dissimilar metals so they are vulnerable to corrosion, which can

sometimes be difficult to detect. It can result in misreading by the thermocouple; therefore, proper

care and maintenance of thermocouples is necessary.

Thermocouples are complex and have many sources of error. This is mostly due to their operation.

Thermocouples use two dissimilar metals that have two different voltage readings produced from a

change in temperatures on alternative ends of the metals. They also have a cold junction, which

consists of copper wires to allow the metal to meet the instrumentation. They use various types of

electronic compensation at the cold junction to allow for more accurate readings; this account for

marginal errors, depending upon the type of thermocouple used

The correct calibration of thermocouples can be tedious and difficult. It is always essential

thermocouple calibration be done next to another--already calibrated--thermocouple. During the

calibration bath, the output is not reproduced exactly as it was, making correct calibration essential.

The relationship between the process temperature and the thermocouple signal (millivolt) is not linear.

(5)

AGING OF THERMOCOUPLES

Thermocouples are often used at high temperatures and in reactive furnace atmospheres. The practical

lifetime is determined by aging. The thermoelectric coefficients of the wires in the area of high

temperature change with time and the measurement voltage drops. The simple relationship between

the temperature difference of the joints and the measurement voltage is only correct if each wire is

homogeneous, unlike the aged thermocouple. Relevant for the generation of the measurement voltage

are the properties of the metals at a temperature gradient. If an aged thermocouple is pulled partly out

of the furnace, the aged parts from the region previously at high temperature enter the area of

temperature gradient and the measurement error is significantly increased.

However an aged thermocouple that is pushed deeper into a surface gives a more accurate reading.

(6)



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THERMOCOUPLE CALIBRATION PROCEDURE

Branom Instrument calibrates thermocouple and thermocouple wire according with one of the

following American Society for Testing and Material (ASTM) Standards: E207-88, standard method

of Thermal EMF Test of single thermoelement materials by comparison with a secondary standard of

similar EMF temperature properties. E220-86, standard method by comparison techniques for

calibration of thermocouples.

These standards describe the type of temperature source, measuring equipment, standards, and

procedures needed to accurately perform a calibration. We will look at each of the elements in detail

below.

CONTROLLED TEMPERATURE SOURCE:

The temperature source used for calibrating should be stable enough to provide a constant temperature

(+/-.2 F) for a short length of time (approx.20 mm.) at any temperature at which the temperature bath

or other source is to be used. The temperature source must have a zone of uniform temperature into

which the thermocouple measuring junction is inserted. The length of the temperature source must be

enough to permit a depth of immersion sufficient to assure that the measuring junction temperature is

independent of temperature gradient along the thermocouple wires.

CONTROLLED TEMPERATURE SOURCES:

Fixed Point: For highly accurate measurements, fixed point cells are used. It consists of ametal sample inside a graphite crucible with a graphite thermometer sufficiently submerged

into the metal sample. When the metal sample is heated to the freezing point, it will produce a

stable and constant temperature. For better comprehension of the operation of fixed point

cells, the following terms are useful.

Fixed Point: A reproducible temperature of equilibrium between different phases of amaterial.

Freezing Point: The fixed point between the solid and liquid phases of a material.

REFERENCE JUNCTIONS:

A thermocouple's output is based on the difference in temperature between the measuring junction

(hot junction) and the reference junction (cold junction). See Figure A.

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REFERENCE JUNCTION TEMPERATURE:

A controlled temperature must be provided in which the reference junction is maintained at a constant

temperature as chosen. The reference junction temperature should be controlled to a better accuracy

than that expected from the thermocouple calibration. The most commonly used reference

temperature is 32 F, others may be used too.

ICE BATH:

One of the most common reference junctions is the ice bath. The ice bath is made up of a mixture of

melting shaved ice and water. The ice bath is a convenient and inexpensive way to achieve an ice

point, it can be easily reproduced and with high accuracy. Junctions formed between the

thermocouple materials and instrument leads can be simply immersed into the slush mixture, or

alternatively glass "U" tubes containing a quantity of mercury approximately 3/4" to 1" depth can be

placed into the slush mixture. Quick electrical connection can then be made between thermocouple

and instrument leads through the mercury. Figure B



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Note: An improperly used ice bath can result in serious errors. The largest error arises due to melting

of the ice at the bottom of the bath until the reference junctions are below the ice level and surrounded

by water alone. This water may be as much as 7F above the ice point.



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AUTOMATIC ICE POINT:

The automatic ice point is an electrical refrigerated device in which equilibrium between ice and

water is constantly maintained. Heat transfer is controlled by the change of volume of water in

freezing. The user may insert reference junctions formed from their own calibrated wire into somecommercially available devices which provide wells. Others are provided with many reference

junction pairs brought out to terminals which the user may connect into his system.

ELECTRONIC COMPENSATION:

This method employs a compensation circuit containing a source of current and a combination of

fixed resistors and a temperature sensitive resistor (TSR). This device can be designed to produce

similar EMF to that of the thermocouple being calibrated. The Electronic Compensator will make

EMF compensations to the thermocouple circuit based in the difference in EMF from 32 to ambient

temperature.

MEASURING INSTRUMENTS:

The choice of a specific instrument for measuring the thermocouple output will depend on the

accuracy required of the calibration being performed. Fluke 702 calibrator or Altek 422 is sufficient

for most thermocouple calibrations.

REFERENCE THERMOMETERS:

The reference thermometer used for the comparison calibration of a thermocouple depends on the

temperature range covered, the accuracy desired, the capabilities, or the preference of the calibration

laboratory. The following are different examples of reference thermometers.

LIQUID-IN-GLASS THERMOMETERS:

Liquid-in-glass thermometers cover the range from -300 to 950Fahrenheit with an accuracy of 1- 3

Fahrenheit depending on the thermometer type and the width of the range covered. They are

inexpensive but fragile, and if the highest degree of accuracy is to be achieved, an individual

thermometer must cover a very narrow temperature range so that the graduation intervals can be as

large as possible. A disadvantage of the liquid-in-glass thermometer is distinct reading errors because

of fine graduations. Taylor Instruments offers Superior Grade Certified Secondary Reference

Thermometers individually or in matched Celsius or Fahrenheit sets, which Branom stocks.



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PLATINUM RESISTANCE THERMOMETERS:

A standard platinum resistance thermometer (SPRT) is the most accurate standard available, however,

it is the most expensive standard, and other standards depending upon the temperature range covered,

are acceptable alternatives in addition to the accuracy desired, the capabilities, or the preference of thecalibration laboratory.

TEST ASSEMBLY PLACEMENT IN THE FURNACE:

For accurate calibration results depth of immersion is the most important consideration. The depth of

immersion must be sufficient to eradicate the effects of heat transfer away from the junction. Since

heat transfer characteristics are dependent on the mass of material being put into the temperature

source, it is impossible to establish a minimum depth of immersion that would be useable under all

circumstances.

WIRING CONNECTION FROM TEST ASSEMBLY TO READOUT

INSTRUMENT.

The actual wiring necessary to connect the test assembly, reference junction and readout instrument

will depend on the quantity of thermoelements in the test assembly, the type of reference junction

used and whether or not a switching device is used, but the basic requirements are the same.

Thermocouple extension wire is used to connect thermoelements to the reference junction. Copperwires are used between the reference junction and readout instrument.(7)

APPLICATIONS

Thermocouples are suitable for measuring a large temperature range, up to 2300 C. They are less

suitable for applications where smaller temperature differences need to be measured with high

accuracy, for example the range 0100 C with 0.1 C accuracy. Thermistors and resistance

temperature detectors are more suitable for these applications. Applications include temperature

measurement forkilns, gas turbine exhaust, diesel engines, and other industrial processes.

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. For prevention 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. As 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

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thermocouple temperature falls, causing the voltage across the thermocouple to drop and the valve to

close.

Some systems, known as millivolt control systems, extend this concept to the main gas valve as well.

Not only does the voltage created by the pilot thermocouple activate the pilot gas valve, it is also

routed through a thermostat to power the main gas valve as well. Here, a larger voltage is needed than

in a pilot flame safety system described above, and a thermopile is used rather than a single

thermocouple. Such a system requires no external source of electricity for its operation and so can

operate during a power failure, provided all the related system components allow for this. Note that

this excludes common forced air furnacesbecause external power is required to operate the blower

motor, but this feature is especially useful for un-powered convection heaters. (8)

Steel industry

Steel and Iron industries make extensive use of type B, S, R and K thermocouples for temperature

monitoring and chemistry of the steel making process. In an electric arc furnace, disposable,

immersible, type S thermocouples are regularly used to accurately measure the temperature of steel

before tapping. The cooling curve of a small steel sample can be analyzed and used to estimate the

carbon content of molten steel.

Thermopile radiation sensors

Thermopiles are used for measuring the intensity of incident radiation, visible or infrared light, which

heats the hot junctions; the cold junctions are on a heat sink. It is possible to measure radiative

intensities of only a few W/cm2

with commercially available thermopile sensors.

Manufacturing

Thermocouples can be used in the testing of prototype electrical and mechanical apparatus. For

example, switchgearunder test for its current carrying capacity may have thermocouples installed and

monitored during a heat run test, to confirm that the temperature rise at rated current does not exceed

limits.

Radioisotope thermoelectric generators

Thermopiles can be applied to generate electricity in radioisotope thermoelectric generators.

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Process plants

Chemical production and petroleum refineries employ computers for logging and limit testing various

temperatures associated with a process. For such cases a number of thermocouple leads will be

brought to a common reference block (a large block of copper) containing the second thermocouple of

each circuit. The temperature of the block is then measured by a thermistor. Simple computations are

used to determine the temperature at each measured location.(9)

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References

1. http://www.sensortecinc.com/docs/technical_resources/Thermocouple_Theory.pdf(Retrieved on

12/10/10)

2. http://en.wikipedia.org/wiki/Thermocouple#Laws_for_thermocouples(Retrieved on 12/10/10)

3. Daniel D. Pollock: The Theory and Properties of Thermocouple Elements; American Society for

Testing and Materials; Hardcover; Pg15-20; 1971.

4. David R Keyser., Naval Ship Engineering Center Philadelphia PA Philadelphia Div; Computer

Optimization Of Thermocouple Calibration; Defense Technical Information Center ; Pg 50-59; 1970

5. http://www.engineeringtoolbox.com/thermocouples-d_496.html (Retreived on 13/10/10)

6. http://www.branom.com/literature/thermocouple.html (Retreived on 13/10/10)

7. http://en.wikipedia.org/wiki/Thermocouple#Applications (Retreived on 14/10/10)

8. John G. Webster: The measurement, instrumentation, and sensors handbook; Springer; Volume 1;

Pg 23-36; 1999

9. Thomas D. McGee, Thomas Donald McGee: Principles and methods of temperature measurement;

Wiley-IEEE; Hardcover; Pg 278-285; 1988

http://www.sensortecinc.com/docs/technical_resources/Thermocouple_Theory.pdfhttp://en.wikipedia.org/wiki/Thermocouple#Laws_for_thermocoupleshttp://en.wikipedia.org/wiki/Thermocouple#Laws_for_thermocoupleshttp://www.engineeringtoolbox.com/thermocouples-d_496.htmlhttp://www.branom.com/literature/thermocouple.htmlhttp://en.wikipedia.org/wiki/Thermocouple#Applicationshttp://www.google.com.pk/search?tbs=bks:1&tbo=p&q=+inauthor:%22Thomas+D.+McGee%22http://www.google.com.pk/search?tbs=bks:1&tbo=p&q=+inauthor:%22Thomas+Donald+McGee%22http://www.google.com.pk/search?tbs=bks:1&tbo=p&q=+inauthor:%22Thomas+Donald+McGee%22http://www.google.com.pk/search?tbs=bks:1&tbo=p&q=+inauthor:%22Thomas+D.+McGee%22http://en.wikipedia.org/wiki/Thermocouple#Applicationshttp://www.branom.com/literature/thermocouple.htmlhttp://www.engineeringtoolbox.com/thermocouples-d_496.htmlhttp://en.wikipedia.org/wiki/Thermocouple#Laws_for_thermocoupleshttp://www.sensortecinc.com/docs/technical_resources/Thermocouple_Theory.pdf


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Thermocouple Calibiration