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# Thermocouple Amplifier

Home made thermocouple and an analog cold junction compensator which gives output voltage directly proportional to temperature in °C

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A device to process thermocouple voltage.
The output can easily be seen on any mili-voltmeter (multimeter) precisely.

Why i built this:
the main motive of making this project is to deeply learn -
> What are thermocouples
> How to make home made thermocouple
> How to measure absolute temperature using them
> What is Cold junction compensation and methods to solve that problem.
> Designing analog electronic circuit to process thermocouple signal.

Here I am using OP-amps signal conditioning to process the miniscule thermocouple voltage.

For the cold junction compensation, a home made iso-thermal block is made and a Celsius sensor LM35 is used to monitor cold junction temperature.

First i would like to give a glimpse of the project here and full detailed "how to" and "problems" are explained in the "Build instructions section"

The idea is to make an analog electronic circuit using OP-amps to read specific type of thermocouple  output voltage { in this project type-T } and get absolute temperature measured precisely in terms of ( 1mV/°C ) output.

Few tests will be performed to benchmark various characteristics, see how precise the circuit is performing and working of the final project.

I always love to deep dive into things which i find fascinating so in this project i have covered detailed theory and practical implementation.

## Thermocouple amplifier :

The yellow wire coming out off the box is the home made thermocouple.

The red box contains the whole analog circuit inside which processes the thermocouple's miniscule voltage and gives absolute temperature reading in terms of     { 1mV/ °C } output which can be viewed on a multimeter  /milivoltmeter

## How to connect:

+  connects to positive probe of the multimeter and
-  connects to negetive probe of the multimeter.

The multimeter needs to be set in milli-volt range to display correct temperature

## Schematic :

Following is the schematic of the complete circuit designed after experimenting on breadboard.

## Iso-thermal Block :

This is the part which monitors temperature of the cold junction which is necessary factor to compensate for the cold junction.

This block couples the cold junctions THERMALLY  but isolates ELECTRICALLY.

(detailed explanation is in the theory and instructions section)

## Home made Thermocouple (T - type) :

For the thermocouple i chose to make T- type thermocouple because material for making it were easy to find.

T- type thermocouple uses Constantan and Copper combination.

(detailed explanation is in the instructions section)

The measuring accuracy is ±1 °C. This also fits in the actual accuracy of the industrial T type thermocouple. and measuring range is -200°C to +400°C .

# Theory :

I won’t go very deep in detail theory, but will stick more with practical implementation

A thermocouple consists of  two wires (say metal A and metal B) which are joined at one end the HOT end that is the end from which temperature is being measured and the other end is called the COLD end. When these two junctions are in temperature difference, potential difference builds between the open ends as shown in the following figure.

Thermocouples work on the principle of Seebeck effect.

Thus thermocouple acts like a transducer and can be used as temperature sensor.

The output voltage is differential  i.e. ,  V out  ∝  { T hot  -  T cold }
Voltage due to T cold  needs to be eliminated. This is called compensation since we don't want voltage due to T cold.
Output voltage without any amplifier will be:
Thot = Temperature of the hot junction
Tcold =
Temperature of the cold junction
V out  =  { T hot  -  T cold }
x  { Seebeck coefficient }

## Types of Thermocouples:

There are various types of Thermocouples with different ranges and dedicated applications

## Simplified arrangement of thermocouple:

Typically In domestic and industrial applications a slight variation of the thermocouple is used. Instead of two separate junctions as shown in previous figures, connecting wires are treated as cold (reference junction) which are isothermally coupled.

This type of arrangement is much more practical, easier and eliminates unwanted junctions ( junctions which behave as unwanted thermocouples ). which causes error in the output voltage.

we know :  V out  ∝  { Thot  -  Tcold }

it applies here also, V out  ∝  { Tu1  -  T(u2 +u3) }

we need to compensate for the voltage...

• 1 × LM 35 - To-92 Celsius temperature analog sensor
• 2 × OP07 CP -DIP 8 Precision OP-amp
• 1 × μA741 - DIP 8 Operational Amplifiers, general purpose
• 1 × 6458D - DIP 8 Dual Op amp, general purpose
• 2 × 10K ohm ±1% 1/4W Resistor
• ### Some more details about the Isothermal Block

To make the Isothermal Block I used LED strip's aluminum plate

It perfectly serves the purpose of an Isothermal block which is to thermally couple the cold junctions but isolate them electrically.

# Video contents :

00:00  -  Project Overview
01:31   -   High temperature test demonstration
03:42  -  Medium temperature test
05:30  -  Low temperature test

• 1
Making the Thermocouple ( T- type)

Gathering materials:

I chose to make T- type thermocouple since thats the  thermocouple i found that was easy to make out off scrap and materials which are easy to find.

is made using some salvaged constantan wire off of a power resistor and enameled copper wire form old motor.

I'll explain why enameled copper  wire is used

in modern power resistors sometimes nichrome wire is used but after observing the performance of thermocouple and calculating the sensitivity of the thermocouple ,it was clear that the wire i got from that resistor is constantan and not nichrome ! (or maybe its nichrome, but its performing the same).

Making it permanent and durable:

The enameled copper wire is stripped from the end and twisted with the constantan wire and then with a makeshift spot welder ! made out of a 12 V transformer with carbon electrode from a battery I spot welded the twisted ends of thermocouple.

Now since the copper wire is enameled both wires are twisted to a length and then wrapped with plumbing Teflon tape (  the yellow tape in the pictures below )

To make it durable a small piece of aluminum sheet is crimped over the twisted end.
The aluminum strip is also there to increase the area of contact of the thermocouple tip.

There we have the T type thermocouple !

• 2
Rough Idea of building the circuit

First lets see whats the logic and working of cold junction compensation :

Thot = Temperature of the hot/measuring junction
Tcold = Temperature of the cold junctions
Cold junctions are kept in a isothermal block and the temperature of isothermal block is measured i.e. T in the picture below
Voltage Vout is without amplifier Vout = (Thot - Tcold) x S   ....... i.e V in the picture below

To compensate the effect of the cold juntions , we add the previously measured temperature of the isothermal block T(which is in terms of voltage) to V as shown above and by doing this we neutralize the effect of cold junctions and get the final absolute reading Vabs of the temperatue being measured at the measuring junction.

To understand this I took help from these lectures, they are very usefull . You all can check them for more insight: (not my channel)
> Hardware design lecture
> Industrial instrumentation lecture
> Casual talk on Thermocouples

example to understand:

lets say temperature at measuring junction is 180°C ( at U1 in above diagram) and lets say our circuit is operating at room temperature say:  28°C (that will be also temperature of the isothermal block)
S - > Seebeck coefficient ( sensitivity of thermocople )
We are using T type thermocouple so S = 42.79µV

A -> Amplifier gain
(assuming amplification is a such that output is in terms of  1mV/°C)
A= 23.3
( i'll explain how to calculate Seebeck coefficient and Gain in step 4 )

## Now WITHOUT COMPENSATION if we only use AMPLIFIER

output will be :

Vabs  =  { Thot  -  Tcold } x S x A
Vabs  =  { 180  -  28 } x 42.79µV x 23.3
Vabs  =  { 152 ] 42.79 x23.3
Vabs  =  6.504 mV x 23.3
Vabs = 151.54 mV   .............. [Wrong measurement ]

## Now WITH COMPENSATION and AMPLIFIER:

output will be :

Vabs  =  { Thot  -  Tcold  + Tcold  } x S x A    ................ [ Compensation ]
Vabs  =  { 180  -  28  + 28 } x 42.79µV x 23.3
Vabs  =  { 180 ] x 42.79µV x 23.3
Vabs  =  7.7022 mV x 23.3
Vabs  =  179.4mV ≈ 180mV  ................ [Correct measurement ]

## There's also a different way to do this and we'll use this method:

Above method is  correct, but is difficult to implement practically. Now ,here we add the compensation voltage after processing the thermocouple voltage.
This is much easier and since the lm 35 already has sufficient sensitivity no need to amplify it.

Vabs  =  { Thot  -  Tcold  } x S x A  + { Tcold x sensitivity of cold junction sensor }
( In this project sensitivity of lm35  = 10mV/°C )
Vabs  =  { 180  -  28  } x 42.79µV x 23.3    + { 28 x 10mV }
Vabs  =  { 152 x 42.79µV x 23.3 }  + 28mV
Vabs  =  151.54mV  +28mV
Vabs  =  179.4mV ≈ 180mV  ................ [Correct measurement ]

Thats Idea so lets start implementing it practically and design the circuit in parts and understand each part...

• 3
Split power supply

For powering the whole circuit , a standard 9V battery with a resistor divider and op-amp based split power supply is used ,providing + 4.5V, 0V and - 4.5V voltage rails.

An op-amp  (nothing specific about 6458D i just had one salvaged so i used it) is used to buffer the middle point of voltage divider thus giving a buffered ground reference i.e. 0 volts as shown above.

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