Experiment O2 (icl).docx

INSTRUMENTATION & CONTROL LAB 2

Submitted To Engr.Usman

INSTRUMENTATION & CONTROL LAB CONTENTS            

Experiment Title Objectives Introduction Working Principle Types of Thermocouples Type J thermocouple Experimentation Observations Advantages & Disadvantages Applications Results References

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INSTRUMENTATION & CONTROL LAB EXPERIMENT : 02 Determine the time constant of a typical Iron constantan thermocouple. Objective:  

Introduction to thermocouples. To determine the time constant of a typical iron constantan thermocouple.

Introduction: When two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit. Thomas Seebeck made this discovery in 1821.

If this circuit is broken at the center, the net open circuit voltage (the Seebeck voltage) is a function of the junction temperature and the composition of the two metals. The pair of metals forming the circuit is called a thermocouple. The effect is due to conversion of thermal energy to electrical energy.

All dissimilar metals exhibit this effect. The most common combinations of two metals are listed in Appendix B of this application note, along with their important characteristics. For small changes in temperature the Seebeck voltage is linearly proportional to temperature: ∆eAB = α∆T

Where α, the Seebeck coefficient, (mV/K) is the constant of proportionality. Thermocouple are the most widely use temperature sensors in the process industry and plastics manufacturing equipment like injection molding and extrusion lines.

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INSTRUMENTATION & CONTROL LAB

Seebeck Coefficient(α)

Materials

19

Iron

-35

Constantan Table : 01

Working Principle: The thermocouple principle mainly depends on the three effects namely Seebeck, Peltier and Thompson. See beck-effect This type of effect occurs among two dissimilar metals. When the heat offers to any one of the metal wire, then the flow of electrons supplies from hot metal wire to cold metal wire. Therefore, direct current stimulates in the circuit. Peltier-effect This Peltier effect is opposite to the Seebeck effect. This effect states that the difference of the temperature can be formed among any two dissimilar conductors by applying the potential variation among them. Thompson-effect This effect states that as two disparate metals fix together & if they form two joints then the voltage induces the total conductor’s length due to the gradient of temperature. This is a physical word which demonstrates the change in rate and direction of temperature at an exact position.

Working of Thermocouple The thermocouple schematic diagram is shown in the below figure. This circuit can be built with two different metals, and that are coupled together by generating two junctions. The two metals are surrounded to the connection through welding. When the temperature of the junction is dissimilar from each other, then the electromagnetic force generates in the circuit.

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INSTRUMENTATION & CONTROL LAB

Types Of Thermocouples: Based on different metals used and range of temperature sensing the thermocouples are classified into many types.

Type B E J K N R S T

Temperature Positive leg 0-1700°C -100- 1000°C 0- 750°C -100-1300°C -230-1300°C 0-1600°C 0-1600°C -200-350°C

Pt-30% Rd Chromel Iron Chromel Nicrosil Pt- 10% Rd Pt- 10% Rd Copper Table : 02

Negative leg

Color Code

Pt-6% Rd Constantan Constantan Alumel Nisil Platinum Platinum Constantan

Grey& Red Purple Red White& Red Yellow& Red Orange&Red Black & Red Black & Red Blue & Red

Type J Thermocouple: The type J is also very common. It has a smaller temperature range and a shorter lifespan at higher temperatures than the Type K. It is equivalent to the Type K in terms of expense and reliability.

Type J Temp Range:  

Thermocouple grade wire, -346 to 1,400F (-210 to 760C) Extension wire, 32 to 392F (0 to 200C)

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INSTRUMENTATION & CONTROL LAB Experimentation: Apparatus Required:     

Thermocouple Heater Digital Voltmeter Stop watch Hookup wires

Procedure:         

Join the one end of the hookup wires with thermocouple and the other with the digital voltmeter. Note the reading on the voltmeter. Switch on the heater. As thermocouple receives heat, digital voltmeter reading changes. Note the reading after every minute. After sometime, the measured voltage shall settle on a particular time constant from the plot. Draw a graph of emf vs time and establish the time constant from the plot. Calculate the Temperature at all measured emf using the thermocouple tables of J type thermocouple. Draw the graph of emf vs temperature and the time vs temperature also to study the relationship between them.

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INSTRUMENTATION & CONTROL LAB Observed Values: Observations 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Time(min) Voltage/ emf 0 5.0 1 5.4 2 5.8 3 6.1 4 6.5 5 6.8 6 7.1 7 7.4 8 7.7 9 7.9 10 8.1 11 8.2 12 8.3 13 8.4 14 8.5 15 8.6 16 8.7 17 8.8 18 8.9 19 9.0 20 9.1 21 9.1 22 9.2 23 9.3 24 9.4 25 9.4 26 9.4 Table : 03

Temperature(C) 1250 1350 1450 1525 1625 1700 1775 1850 1925 1975 2025 2050 2075 2100 2125 2150 2175 2200 2225 2250 2275 2275 2300 2325 2350 2350 2350

Results From Graphs: Graph showing the result between time and voltage reading observed during experimentation.

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INSTRUMENTATION & CONTROL LAB 10 9 8

Voltage(mV)

7 6 5 4 3 2 1 0 0

5

10

15

20

25

30

Time (min)

Graph : 01 Graph showing the result between time and voltage reading observed during experimentation.

2500

Temperature (C)

2000

1500

1000

500

0 0

5

10

15

Time (min)

Graph : 02 Time Constant Calculation by emf:

20

25

30

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INSTRUMENTATION & CONTROL LAB Max Reading on Voltmeter = 9.4 m V 63.2 % Of Max

= 5.9408 m V

Time Constant

= 2 min

Time Constant Calculation by temperature: Max Reading on temperature = 2350 ° C 63.2 % Of Max

= 1485.2 ° C

Time Constant

= 2.7min.

Advantages & Disadvantages of Thermocouple The advantages include the following.      

Accuracy is high It is Robust and can be used in environments like harsh as well as high vibration. Thermal reaction is fast The operating range of temperature is wide. Wide operating temperature range Cost is low and extremely consistent

The disadvantages include the following.  

It has low-accuracy. The thermocouple recalibration is hard

Thermocouple Applications Some of the applications of thermocouple include the following.     

These are used as the temperature sensors thermostats in offices, homes, offices & businesses. These are used in industries for monitoring temperatures of metals in iron, aluminum, and metal. These are used in the food industry for cryogenic and Low-temperature applications. Thermocouples are used as a heat pump for performing thermoelectric cooling. These are used to test temperature in the chemical plants, petroleum plants. These are used in gas machines for detecting the pilot flame.

Conclusion:

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INSTRUMENTATION & CONTROL LAB From the above results and graphs we can conclude that there is a direct relationship between the temperature difference and emf /voltage. As the temperature increases emf also increase and vice versa. Also, the emf or voltage productions depends on:  Types of materials used.  Temperature difference between the measuring junction and reference junction. There is also a direct relationship between time and temperature. References:  http://www.omega.com  https://www.sciencedirect.com/topics/engineering/iron-constantan  https://www.elprocus.com/what-is-a-thermocouple-definition-working-principlediagram-applications/  https://www.thermocoupleinfo.com/

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