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Wednesday, February 2, 2011

Thermocouples








               One of the most common industrial thermometer is the thermocouple. It was discovered by Thomas Seebeck's in 1822. He noted that a voltage difference appeared when the wire was heated at one end. Regardless of temperature, if both ends were at the same temperature there was no voltage difference. If the circuit were made with wire of the same material there was no current flow.


A thermocouple consists of two dissimilar metals, joined together at one end, and produce a small unique voltage at a given temperature. This voltage is measured and interpreted by a thermocouple thermometer.


The thermoelectric voltage resulting from the temperature difference from one end of the wire to the other is actually the sum of all the voltage differences along the wire from end to end


Thermocouples can be made from a variety of metals and cover a temperature range 200 oC to 2,600 oC. Comparing thermocouples to other types of sensors should be made in terms of the tolerance given in ASTM E 230.



Base metal thermocouples


Thermocouple
Maximum Temperature (oC)
Continuous
Spot
Copper-Constantan
400
500
Iron-Constantan
850
1,100
Chromel-Constantan
700
1,000
Chromel-Alumel
1,100
1,300
Nicrosil-Nisil
1,250
-
Tungsten-Molybdenum*
2,600
2,650

* Not used below 1250 oC.





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.

How Thermocouple Works?

The general circuit for the working of thermocouple is shown in the figure 1 above. It comprises to two dissimilar metals A and B. These are joined together to form two junctions, p and q, which are maintained at the temperatures T1 and T2 respectively. Remember that the thermocouple cannot be formed if there are no two junctions. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions.

If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero. If the junctions are maintained at different temperature, the emf’s will not become zero and there will be net current flowing through the circuit. The total emf flowing through this circuit depends on the metals used within the circuit as well the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable devise.

For measurement of the temperature of the body, one junction of the thermocouple is connected to the body whose temperature is to be measured. This junction is called as hot junction or the measuring junction. The other junction is connected to the body whose temperature is known. This junction is called as cold or reference junction.

Within the circuit of the thermocouple the devise for measuring the current or emf flowing the circuit is connected. It measures the amount of emf flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures. In the figure 2 above the two junctions of the thermocouple and the devise used for measurement of emf (potentiometer) are shown.

Now, the temperature of the reference junctions is already known, while the temperature of measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly.

Devices Used for Measuring emf within the Thermocouple Circuit

The amount of emf developed within the thermocouple circuit is very small, which is usually in millivolts, hence some highly sensitive instruments should be used for measuring the emf generated in the thermocouple circuit. The two devices used commonly for measuring emf within the thermocouple circuit are ordinary galvanometer and voltage balancing potentiometer. The manually or automatically balancing potentiometer is used more commonly.

The figure 2 above shows potentiometer connected in the thermocouple circuit. The junction p is connected to the body whose temperature is to be measured. The junction q is the reference junction, whose temperature can be measured by the thermometer. In some cases the reference junctions can also be maintained at the ice temperature by connecting it to the ice bath (see figure 3). This devise can be calibrated in terms of the input temperature so that its scale can give the value directly in terms of temperature.


Advantages with thermocouples

  • Capable of being used to directly measure temperatures up to 2600 oC.
  • The thermocouple junction may be grounded and brought into direct contact with the material being measured.

Disadvantages with thermocouples

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

Thermocouple Types

Thermocouples are available in different combinations of metals or calibrations. 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.


Some of the thermocouple types have standardized with calibration tables, color codes and assigned letter-designations. The ASTM Standard E230 provides all the specifications for most of the common industrial grades, including letter designation, color codes (USA only), suggested use limits and the complete voltage versus temperature tables for cold junctions maintained at 32 oF and 0 oC.


There are four "classes" of thermocouples:
  • 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).
The home bodies are the Types E, J, K, N and T. The upper crust are types B, S, and R, platinum all to varying percentages. The exotic class includes several tungsten alloy thermocouples usually designated as Type W (something).



Instrument
Temperature
Range
Accuracy
Recommended (oF)
Maximum (oF)
Type J probes
32 to 1336
-310 to 1832
1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater
Type K probes
32 to 2300
-418 to 2507
1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater
Type T probes
-299 to 700
-418 to752
0.9 to 3.6oF or 0.4% of reading above 32oF, whichever is greater
Type E probes
32 to 1600
32 to 1650
1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater
Type R probes
32 to 2700
32 to 3210
2.5oF or 0.25% of reading, whichever is greater
Type S probes
32 to 2700
32 to 3210
2.5oF or 0.25% of reading, whichever is greater

Aricle Source: www.engineeringtoolbox.com


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