Question

Q3/ a) What are the main differences that specified Thermocouples or why we use a certain thermocouple from another one? (4 Marks) b) In your opinion, who can we make the experiment of Thermocouple clear of mistakes? (4 Marks) c) Talk about how you performed the thermocouple experiment and how you collected data in your own words, not from the sheet. (4 Marks)

Answer 1

a) What are the main differences that specified Thermocouples or why we use a certain thermocouple from another one? (4 Marks)

Below are 4 thermo couples types

Type T, Copper (+) vs. Constantan (nickel-45%copper) (−).

This type is moisture resistant, very stable, and useful to 370°C (700°F) in air, a temperature limit imposed primarily by oxidation of the copper element. When used in vacuum or in reducing or inert atmospheres, operation at somewhat higher temperatures may be possible. It is also suitable for subzero use down to -200°C (-370°F), but stock materials are not normally pretested in this range, so special selection and additional calibration of materials is usually required for such use. Neither wire is magnetic,but visual identification by metal color is easy.

Type J

Iron (+) vs. Constantan (nickel-45%copper) (−).

These thermocouples are suitable for use in vacuum, air, reducing, or oxidizing atmospheres to 760°C (1400°F) in the heavier gage sizes. Rapid oxidation of the iron wire at temperatures above 540°C (1000°F) limits the expected service life of the finer sized wires. Type J wires of any size should not be used in sulfurous atmospheres above 540°C (1000°F). Sub zero use of this type is limited because of rusting and embrittlement of the iron conductor. The positive (iron) wire is strongly magnetic and the negative one is non-magnetic.

Type E

Chromel (nickel-10%chromium) (+) vs. Constantan (nickel-45%copper) (−).

Type E is recommended for use to 900°C (1600°F) in oxidizing or inert atmospheres. This type is also quite suitable for low temperature work down to about -230°C (-380°F), and develops the highest output emf of any standardized type. For subzero work, special selection and testing is usually required.

Type K

Chromel (nickel-10%chromium) (+) vs. Alumel (nickel-5%aluminum and silicon) (−).
Thermocouples of this type are suitable for use in oxidizing or inert atmospheres at temperatures up to 1260°C (2300F), These thermocouples are the traditional base-metal choice for hightemperature work. Type K is quite vulnerable to sulfur attack and should not be exposed to sulfurcontaining atmospheres. These thermocouples perform best in clean oxidizing atmospheres and, except for short time periods, are not recommended for use under partially oxidizing conditions, in vacuum, or when subjected to alternating cycles of oxidation and reduction. Identification of these wires is usually made by magnetic response. The positive leg is non-magnetic, while the negative one shows a moderately magnetic response.

b) In your opinion, who can we make the experiment of Thermocouple clear of mistakes? (4 Marks)

Few things one who ever experimenting with thermocouples should account are

The errors that could have come while doing experiments.

1. Human error

a) Error in noting down readings

b) error in measurement of lengths

2. machine error

1. Extension lead wires
2. Reference junctions
3. Data acquisition system
4. Calibrations
5. Decalibration
6. Thermal shunting
7. Electrical shunting
8. Electrical leakage
9. Electrical noise

c) Talk about how you performed the thermocouple experiment and how you collected data in your own words, not from the sheet. (4 Marks)

An example of procedure is given below.

The thermocouple is prepared to meassure the temperature by joining them together electrically. then the free ends of the thermocouple is connected to a volt meter with a scale set of 10³. (milli volt). placed the joint to the surface which the measurement to be made. Noted down the indicated voltage . After noting down few repeated readings, the emf is found from the standard chart given. Converted the emf to Temperature from the given chart.

Average temperature will give the resultant temperature.

Thanks

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