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Thermocouples and their Calibration

To obtain a full version, download a PDF copy of this report: Click here, Report 2, Report 3 (Reports on the same topic but from a different author)

 

The Thermocouple:

A thermocouple or thermocouple thermometer is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert heat into electric power. They are inexpensive and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures.

A thermocouple circuit has at least two junctions: the measurement junction and a reference junction. Typically, the reference junction is created where the two wires connect to the measuring device. This second junction it is really two junctions: one for each of the two wires, but because they are assumed to be at the same temperature (isothermal) they are considered as one (thermal) junction. It is the point where the metals change - from the thermocouple metals to what ever metals are used in the measuring device - typically copper.

Working principle:

 The working principle of thermocouples depends on the see beck effect.

See beck effect:

 The Seebeck effect states when two dissimilar metal wire are connected with each other in a loop to form two junctions, maintained at two different temperatures, a voltage potential or electromotive force (E=emf) will be generated and the current will flow through the loop circuit.

 

 The current will be proportional to the difference in temperature between the junctions and the metals used. The higher the temperature difference, the higher is the electromotive force (emf) and the current flow in the loop.

Thermocouple Types:

Thermocouple types vary in the

1.      combinations of metals used and the

2.      calibration used.

 

About 13 'standard' thermocouple types are commonly used. Eight have been given internationally recognized letter type designators. The letter type designator refers to the emf table, not the composition of the metals - so any thermocouple that matches the emf table within the defined tolerances may receive that table's letter designator.

 

The four most common calibrations are J, K, T and E. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. [1]

 

There are four "classes" of thermocouples (based on the metals used):

 

                     The home body class (called base metal),

                     The upper crust class (called rare metal or precious metal),

                     The rarified class (refractory metals) and,

                     The exotic class (standards and developmental devices).

 

Base metals - up to 1000˚C

Type J, Type E, Type T, Type K

Noble metals – up to 2000˚C

Type R, Type S, Type B

Refractory metals – up to 2600˚C

Type C, Type D, Type G

Selection of Thermocouple:

The most important question that has to be solved when installing a thermocouple is which one to use? The following criteria are used in selecting a thermocouple:

         Temperature range (Industry generally prefers K and N types because of their suitability to high temperatures).

          Chemical resistance of the thermocouple or sheath material (including corrosion resistance).

         Abrasion and vibration resistance.

          Installation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter) i.e. the ease of use is important.

         Sensitivity of thermocouple (the T type is usually favored by industries on this basis).

         Cost.

Advantages with thermocouples:

They:

1.      can be very rugged

2.       are immune to shock and vibration

3.      are useful over a wide temperature range (Capable of being used to directly measure temperatures up to 2600C)

4.      are simple to manufacture,

5.       require no excitation power,

6.      there is no self heating and

7.      they can be made very small.

  1. The thermocouple junction may be grounded and brought into direct contact with the material being measured.

 

Disadvantages with thermocouples:

 

1.                   Temperature measurement with a thermocouple requires two temperatures be measured, the junction at the work end (the hot junction) and the junction where wires meet the instrumentation copper wires (cold junction). To avoid error the cold junction temperature is in general compensated in the electronic instruments by measuring the temperature at the terminal block using with a semiconductor, thermistor, or RTD. This is called “Compensation”.

2.                   The output signal produced is relatively low.

3.                   Thermocouple operation is relatively complex with potential sources of error. The materials of which thermocouple wires are made are not inert and the thermoelectric voltage developed along the length of the thermocouple wire may be influenced by corrosion etc.

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

5.                   The calibration of the thermocouple should be carried out while it is in use by comparing it to a nearby comparison thermocouple. If the thermocouple is removed and placed in a calibration bath, the output integrated over the length is not reproduced exactly.

6.                   The main limitation is accuracy: system errors of less than one kelvin (K) can be difficult to achieve. [2]

 

USES:

Thermocouples are widely used in science and industry; applications include temperature measurement for:

1.      Kilns.

2.      Gas turbine exhaust.

3.      Diesel engines.

Some other major applications in industry includes

 

1. 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. The cooling curve of a small steel sample can be analyzed and used to estimate the carbon content of molten steel.

 

2. Thermopile radiation sensors

Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heats the hot junctions, while the cold junctions are on a heat sink. It is possible to measure radiative intensities of only a few μW/cm2 with commercially available thermopile sensors. For example, some laser power meters are based on such sensors.

 

3. Manufacturing

 Thermocouples can generally be used in the testing of prototype electrical and mechanical apparatus. For example, switchgear under 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 designed limits.

 

4. Radioisotope thermoelectric generators

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

 

5. Process plants

Chemical production and petroleum refineries will usually employ computers for logging and limit testing the many temperatures associated with a process, typically numbering in the hundreds. 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 in turn measured by a thermistor. Simple computations are used to determine the temperature at each measured location. [3]

 

Calibration:

 

A calibration is a matter of qualifying the sensor-under-test. Only by knowing the limitations of the sensor it is possible to thrust the measurements and optimize the process control.

Instrument calibration is the process of adjusting the instruments output signal to match a known range of variables.

 

Importance of calibration:

 

Calibration is important because all instruments tend to drift from their last setting. This is because spring stretch, electronic component undergo slight changes on the atomic level, and other working parts bend or lose their elasticity. [4]

 

Process instrument calibration:

 

Calibration consists of comparing the output of the process instrument being calibrated against the output of a standard instrument of known accuracy, when the same input is applied to both instruments. During this calibration process the instrument is tested over its whole range by repeating the comparison procedure for a range of inputs.

 

The instrument used as a standard for this procedure must be one which is kept solely for calibration duties. It must never be used for other purposes. Most particularly, it must not be regarded as a spare instrument which can be used for process measurements if the instrument normally used for that purpose breaks down. Proper provision for process instrument failures must be made by keeping a spare set of process instruments. Standard calibration instruments must be totally separate. [5]

 

 

Calibration of thermocouples

 

A thermocouple is calibrated by comparing its response with a standard thermometer at the same temperature. The standard thermometer may be another thermocouple, a platinum resistance thermometer or a liquid in glass thermometer.

The thermocouples are calibrated by one or more of three general methods, depending on

  • the type of thermocouple,
  • the temperature range,
  • the accuracy required.

In the first method, thermocouples are calibrated by comparison with a reference thermocouple.

In the second method, thermocouples are calibrated against a standard platinum resistance thermometer.

 In the third method, thermocouples are calibrated at four defining temperatures, the freezing points of zinc, aluminum, silver, and gold.

Calibration problem areas are immediately apparent. There must be available:

  • Means for measuring the output of the temperature sensor
  • Satisfactory temperature standard
  • Controlled temperature environment [6]

 

 

Thermocouple calibration procedure:

 

     The following information gives the detail of equipment requirements and proper techniques needed to accurately calibrate thermocouples and thermocouple materials.

 

Controlled temperature source:

 

The temperature source used in the process of calibrating should be stable enough to provide a constant temperature for a short length of time at any temperature at which the temperature bath or other source is to be used. The temperature source should have a zone of uniform temperature into which the thermocouple measuring junction may be inserted. The length of the temperature source must be adequate so that the measuring junction temperature is not affected by a temperature gradient along the thermocouple wires.

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) [7]

Reference junction temperature:

A controlled temperature must be provided in which the reference junction is maintained at a constant chosen temperature. 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 degrees F., but other temperatures may be used if desired.

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 reproduced with ease and with exceptional accuracy. Junctions formed between the thermocouple materials and instrument leads can be simply immersed into the slush mixture, or alternatively glass "U" tubes containing mercury into the slush mixture. Quick electrical connection can then be made between thermocouple and instrument leads through the mercury. [7]

 

Electronic compensation:

This method employs a compensation circuit containing a source of current and a combination of fixed resistors and a temperature sensitive resistor. This device can be designed to produce similar EMF to that of the thermocouple being calibrated.

Measuring instruments:

The choice of a specific instrument for measuring the thermocouple output will depend on the accuracy required of the calibration being performed.

Reference thermometers:

The reference thermometer to be used for the comparison calibration of a thermocouple will depend upon

         the temperature range covered,

         the accuracy desired,

         the capabilities,

         the preference of the calibration laboratory.

The following are different examples of reference thermometers.

Platinum resistance thermometers:

A standard platinum resistance thermometer is the most accurate standard available, however, it is the most expensive standard, and other standards are acceptable alternatives depending upon the temperature range covered.

Liquid- in- glass thermometers:

Liquid-in-glass thermometers are available to cover the range from -300 to 950 degrees Fahrenheit. with an accuracy of from .1 to 3 Fahrenheit depending on the type of thermometer and the width of the range covered. They are relatively inexpensive but they are fragile, and if the highest degree of accuracy of which they are capable 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 further disadvantage of the liquid-in-glass thermometer is that because of their fine graduations reading errors are a distinct possibility.

Test assembly in furnace:

Depth of immersion is the most important consideration if accurate calibration results are to be obtained. The depth of immersion must be sufficient to eliminate the effects of heat transfer away from the junction. It is impossible to establish a minimum depth of immersion that would be useable under all circumstances since heat transfer characteristics are dependent on the mass of material being put into the temperature source.

Wiring connection from test assembly to read out instrument:

The actual wiring necessary to connect the test assembly, reference junction and readout instrument will depend on the quantity of thermo elements in the test assembly, the type of reference junction used and whether or not a switching device is used. Thermocouple extension wire is used to connect the thermo elements to the reference junction. Copper wires are used between the reference junction and readout instrument.

Thermocouple wire, wiring procedure:

Ideally, the samples of the thermocouple material to be calibrated and the standard thermocouple element should be cut long enough so that they reach directly from the temperature source to the reference junction without the need for extension wires. If this is not possible extension wires may be used, but they must be securely connected directly to the test assembly conductors. If extension wires must be used, remove any oxide layer that may be on the surface of the test assembly conductors and attach an extension wire of the same material to each conductor by laying the extension wire alongside the conductors and joining them securely by means of an alligator clip.

Thermocouple calibration wiring procedure:

When calibrating thermocouples, it is faster and more convenient to use a thermocouple switching box. The extension wires from the thermocouples are placed into one side of the reference junction. Multiple pairs of copper lead wire will exit the reference junction and will be connected to the switch box. One pair of copper lead wires will run from the readout instrument to the thermocouple switch box.

Junction location:

One of the primary advantages of calibrating thermocouple materials against a base-metal standard of similar EMF output is that the sample to be calibrated are welded to the base-metal standard forming a common junction thus achieving good isothermal conditions between the test thermo element and the standard. Furthermore, because the test thermo element and the standard produce nominally the same EMF vs. platinum the EMF output changes little over a fairly broad temperature range, thereby reducing the need for precise temperature source control.[7]

 

Measurement:

Set your controlled temperature source to the specified temperature and allow it to adequately stabilize. Immerse the test assembly into the test temperature medium and provide sufficient time for the test assembly to stabilize. Once the test assembly is stable the EMF generated between the test specimen and the reference standard can be recorded. Avoid soaking the test assembly at temperature for a prolonged period of time, as it can cause permanent changes to occur in the thermo elements.

Once the reading is taken, raise the test temperature to the next higher temperature, first removing the test assembly from the temperature source, or advance the test assembly to the next temperature source. Allow the temperature source and the test assembly to stabilize as before, and take a second set of readings at the new temperature. In all cases take the reading in sequence from the lowest to the highest temperature. A base metal reference standard shall be used for one series of temperature changes only.

Thermocouple calibration:

The Test thermocouple junction should be located so that it is in intimate contact with the junction of the standard. Without making a radiograph of the thermocouple it is impossible to know exactly where the junction is located. A few generalizations can be made which enables junctions to be located quite closely. First, the cap weld on a metal sheathed thermocouple is normally about as thick as one-half the sheathed diameter. Second, a "U" junction is normally about one-half the sheathed diameter. The thermocouple standard should be tied to the thermocouple with a fine gauge wire. The junction of the standard should be bent so that it is in contact or at least very close to the point where it has been calculated that the junction is located.

Thermocouple calibration chart:

The calibration curve for a thermocouple is often constructed by comparing thermocouple output to relatively precise thermometer data. Then, when a new temperature is measured with the thermocouple, the voltage is converted to temperature terms by plugging the observed voltage into the regression equation and solving for temperature. [8]

 

 

References:

[1] http://www.capgo.com/Resources/Temperature/Thermocouple/Thermocouple.html [retrieved on November 9, 2010]

 

[2] http://www.engineeringtoolbox.com/thermocouples-d_496.html [retrieved on November 9, 2010]

 

[3] http://en.wikipedia.org/wiki/Thermocouple [retrieved on November 9, 2010]

 

[4] S.K Singh;”Industrial Instrumentation and Control” McGraw Hill publishing company limited, New Delhi; page no (139-140)

 

[5] Alan S. Moris; “Principles of Measurement and Instrumentation”; second edition page no (70-71)

 

[6] Fundamentals of Temperature, pressure and Flow measurements; “Robert P. Benedict”; John Wiley and Sons publisher; third edition page no (146)

 

[7] http://www.branom.com/literature/thermocouple.html [retrieved on November 9, 2010]

 

[8] http://www.itl.nist.gov/div898/handbook/pmd/section1/pmd133.html [retrieved on November 9, 2010]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
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