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THERMOCOUPLE

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shivam kumar

on 10 November 2013

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Transcript of THERMOCOUPLE

THERMOCOUPLE
SUBJECT-PROCESS INSTRUMENTATION
MADE BY- SHIVAM KUMAR & RISHABH KANWAR
SUBMITTED TO-Mr RAJEEV SHARMA
designed by Péter Puklus for Prezi
WHAT IS THERMOCOUPLE?
Thermocouples operate under the principle that a circuit made by connecting two dissimilar metals produces a measurable voltage (emf-electromotive force) when a temperature gradient is imposed between one end and the other.
One at a reference temperature (like 0 oC) and the other junction at the temperature to be measured. A temperature difference will cause a voltage to be developed that is temperature dependent. (That voltage is caused by something called the Seebeck effect.)

Principle of Thermocouple
Laws for theromocouple operation
TOPIC-THERMOCOUPLE
SYSTEMATIC THERMOCOUPLE DIAGRAM
What type of materials should be used in Thermocouples?
Depends on requirements:
-Temperature range
-Required accuracy
-Chemical resistance issues
-Abrasion or vibration resistance
-Installation requirements (size of wire)
-Thermal conduction requirements

THERMOCOUPLE
ADVANTAGES
• Temperature range: Most practical temperature ranges,
from cryogenics to jet-engine exhaust, can be served using
thermocouples. Depending on the metal wires used, a
thermocouple is capable of measuring temperature in the range
–200°C to +2500°C.
• Robust: Thermocouples are rugged devices that are immune
to shock and vibration and are suitable for use in hazardous
environments.
• Rapid response: Because they are small and have low thermal
capacity, thermocouples respond rapidly to temperature
changes, especially if the sensing junction is exposed. They
can respond to rapidly changing temperatures within a few
hundred milliseconds.
• No self heating: Because thermocouples require no
excitation power, they are not prone to self heating and are
intrinsically safe.
THANK YOU
ANY QUESTIONS ?
1. The Law of Homogeneous Circuits
An electric current cannot be sustained in a circuit of a single homogeneous metal
2. The Law of Intermediate Metals
If two dissimilar metals performs a thermocouple, and a third dissimilar metal is introduced to the circuit, as long as the temperature along the entire length of the third metal is kept uniform, the output voltage will NOT be affected
3. The Law of Intermediate Temperatures
If a thermocouple with 2 junctions with temperatures T1 and T2 produces a voltage difference V1, and voltage difference of V2 in temperatures T2 and T3, then voltage generated when the temperatures are T1 and T3 will be V1+V2.

Types of Thermocouples
DISADVANTAGES
• 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.

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, emf is generated within the circuit due to the different temperatures of the two junctions of the circuit.

3) Thomson effect: As per the 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 the Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect plays a prominent role in the working principle of the thermocouple

EMF VS TEMPERATURE
APPLICATIONS
REFRENCES
High temperature furnaces
Rocket engines
Amunitions
http://en.wikipedia.org/wiki/Thermocouple
www.omega.com/prodinfo/thermocouples.html‎
www.tempsens.com/thermocouples.html‎
http://www.geminidataloggers.com/info/thermocouple-data-logger-typical-applications
Full transcript