Thermocouple

 

Thermocouple

Thermocouple Working principle


Thermocouple working principle based on see back effect.

See back effect

 This states that an electric current flows in a closed circuit of two dissimilar metals, having temperature differences at both junctions.

In thermocouple two dissimilar metals have joined on one end, This is called hot junction and it is exposed to the temperature we want to measure, the other junction which is called cold junction or reference junction is usually made ar transmitter.

 One junction is at a reference temperature (like 0 oC) and the other junction at the temperature to be measured.

 A temperature difference will cause the voltage to be developed at the reference junction and produced voltage is directly proportional to the temperature. 

 





A thermocouple produces mv (DC).


External Reference Junction Techniques

Reference junction technology is usually considered one of the main problems of any thermocouple installation. Individual instruments with thermocouples are generally provided with automatic ‘cjc' (cold junction compensation). These devices sense the temperature at the point where the thermocouple is joined to the copper wiring of the instruments, and apply a corrective signal. Scanning devices such as data loggers are increasingly using this method.

A Thermocouple measures the difference between the hot and cold junctions, suppose we want to measure a furnace temperature 1000 degree temperature but our reference junction is 100-degree temperature then,

the formula would be: 

1000 (hot junction) - 100 (cold junction) = 900 deg. F. 

 There seems to be a problem since we said that the furnace was at 1000 deg. F. So here is an error of 100 deg. F.

This brings us to COLD JUNCTION COMPENSATION.


COLD JUNCTION COMPENSATION 

CJC or cold junction compensation is usually done automatically by the measuring instrument. 

The instrument measures the temperature at the cold junction and adds it back to the equation. 

1000 (hot junction) - 100 (cold junction) = 900 deg. F + 100 deg. F (cold junction temp) = 1000 deg F 

 This way the instrument indicates the actual temperature of the hot junction, and error because of the cold junction at some higher temperature is compensated. 

This COLD JUNCTION compensator is usually located at the terminals on the back of the indicating instrument and you must maintain the T/C material of the cabling up to this point at the indicator. 

 Proper measurement by thermocouple required that there is not used of any other metals between the hot junction and the cold junction of compensation the thermocouple. If the wire is needed to connect the T/C to the indicating instrument, the lead wire must be made of the same material as the T/C. 

 

Thermocouple  Assembly Types

Three Basic Types Of Thermocouple Assembly

 Ground junction

 Ungrounded junction

 Exposed junction

Ground junction


For the measurement of corrosive gases and liquids, the grounded junction thermocouple is used. Grounded junction thermocouples are also used for high-pressure applications.

Ungrounded junction

 For the measurement of corrosive gases and liquids temperatures in critical electrical applications, ungrounded junction thermocouples are used.


Exposed junction

The exposed junction is mostly used for the measurement of static or flowing non-corrosive gas temperatures where the response time must be minimal.



Types of Thermocouple

There are many types of thermocouple used in temperature measurement,
Following are the types of thermocouple

B Type

 Measuring range 

From -200 to 1800 °C

 material 

 Platinum -30%Rhodium+ Platinum -6% Rhodium

E Type

Zero to 1000 °C

 chromel + contantan

J Type

Zero to 750 °C ,Iron + Contantan(cu Ni)

K Type

From -200 to 1200 °C 

chromel (NiCr) +  alumel( Ni Al)

R Type

Zero to 1800 °C 

Platinum + Rhodium

S Type

Zero to 1800 °C 

Platinum + Rhodium-Nickel

T Type

From -200 to 350 °C

 copper + constantan

 

Thermowells

 Thermowells are used to protect the detector and so that the detector can be changed without interrupting the process. One downside of using a thermowell is the time delay it introduces into the measurement system due to thermal lag.

 Thermowell should be installed where a good representative sample of the process fluid temperature can be measured.

 The optimum immersion length of a thermowell depends on the application. If the well is installed perpendicular to the line, the tip of the well should be between one-half and one-third of the pipe diameter.

 If the well is installed in an elbow, the tip should point towards the flow.

 The speed of response of a sensor in a thermowell will be slower than that of an unprotected bulb. Keeping the clearance between bulb and pocket down to an absolute minimum and filling the space with oil or glycol (antifreeze) can reduce this effect.

 

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