Thermocouple

Thermocouple:- KPC Principal

1. Thermocouple Diptarup Bandyopadhyay

2. Thermocouple : Content Thermo-Electric Laws What is Thermocouple? Types of Thermocouple Construction Laws of Thermocouple Cold Junction Compensation Calibration and Troubleshooting

3. Thermo-Electric Laws Seebeck Effect: If two dissimilar homogeneous metals are formed two junctions and a temperature difference is maintained between these two junctions , a thermo- emf will be produced in the circuit. This is Seebeck Effect . The Thermo- emf E α Temp difference ( T1 – T2)

4. Thermo-Electric Laws Peltier Effect: If two dissimilar homogeneous metals are formed two junctions and if a current is passed through this , the heat is evolved in one junction and heat is absorbed in other junction. It is opposite of Seebeck Effect. Peltier cooler is formed on this principle.

5. Thermo-Electric Laws Thomson Effect : When  electric current  passes through a circuit of non homogeneous single material , a temperature gradient will exist along its length.

6. Thermocouple What is ? A  thermocouple  is an electrical device consisting of two dissimilar homogeneous  electrical conductors  forming  two junctions and maintained at different  temperatures . It has two junctions – the measured junction is known is HOT junction ( THot ) while the cold junction is known as REFERENCE junction ( T Ref ). Measurement of EMF : By Potentiometric Circuit.

7. Thermocouple

8. Types of Thermocouple Type Positive Material Negative Material Range Sensitivity Application T Copper Constantan -200 to 350 o C 43 µV/°C Oxidizing and Reducing environment. Good moisture resistant. J Iron Constantan -40 to 750 o C 50 µV/°C Reducing environment. Most economical. K Chromel Alumel −200 °C to +1350 °C  41 µV/°C Oxidizing environment and most linear. Commonly used. E Chromel Constantan −50 °C to +740 °C 68 µV/°C Oxidizing environment. Highest Sensitivity. S Platinum-Rhodium (90%Pt/10%Rh) Platinum up to 1600 °C .003–.007 mV/ o F Oxidizing environment. Small size & Fast response. R Platinum-Rhodium (87%Pt/13%Rh) Platinum 0 to 1600 o C .003–.008 mV/ o F Oxidizing environment. Small size & Fast response. B Platinum-Rhodium (70%Pt/30%Rh) Platinum up to 1800 °C .0003–.006 mV/ o F Inert and Slow Oxidizing C Tungsten- Rhenium (95%W/5%Re) Tungsten-Rhenium (74%W/26%Re) 2329 ℃   Inert or vacuum . High Temp. But brittle. G Tungsten   Tungsten-Rhenium (74%W/26%Re) 2315 o C   Inert or vacuum . High Temp. But brittle. D Tungsten- Rhenium (97%W/3%Re) Tungsten-Rhenium (75%W/25%Re) 2300 o C   Inert or vacuum . High Temp. But brittle.

9. Construction of Thermocouple T/C s are made from two wires , twisted or wielded at one end. This is measuring junction or hot junction. To prevent the entangled of the wires among themselves to produce other points of couple , single hole or dual hole ceramic beads are used. The wires passes through it. To protect from mechanical vibration, these wires are protected by a s.s. tube ( which is known as sheath). The introduction of sheathing material introduces the time lag. To overcome that , either the junction is flashed with SS tube or bare junction T/C is used. The two thermocouple wires are connected to a porcelain base terminal block with screw fitted. The connecting terminal is marked with polarity ( i.e. positive and negative). A threaded metallic cover protects the connecting terminals. A connecting lead gland in terminal head allows the lead wire to connect T/C with measuring system.

10. Construction of Thermocouple

11. Thermocouple: EMF Calculation A T 1 T 2 B Either Clockwise or Anti Clockwise for each node For T 1 , EMF = E B-A /T1 For T 2 , EMF = E A-B /T2 Total EMF = E B-A /T1 + E A-B /T2 E B-A /T 1 = - E A-B /T 1

12. Measurements : Laws of T/C Law of Homogeneous Material: The e.m.f of a thermocouple depends only on the temperatures of the junctions and is independent of the temperatures of the wires connecting the junctions.  This means that the leads connecting the instrument can be exposed to temperature fluctuations without affecting the measurement.

13. Measurements : Laws of T/C Laws of Intermediate Materials: If any third metals are introduced into a couple, keeping the junction temperature of the third materials to the couple is same then no additional e.m.f is contributed for third material.

14. Measurements : Laws of T/C Laws of Intermediate Temperature: The law of intermediate temperature states that if a couple produces an output E 1 for the temperature difference of T 1 and T 3 , then it is equivalent to the summation of emf E 2 ( for temperature T 1 and T 2 ) and E 3 ( for temperature T 2 and T 3 ), where T 1 >T 2 >T 3 .

15. Cold Junction Compensation (CJC) The cold junction compensation compensate this variation of ambient temperature by adding an additional emf for ambient temperature to 0 o C ( E amb-0 ) along with the emf for hot junction to ambient junction ( E Hot-amb ). E Hot-0 = E Hot-amb + E amb-0 EMF due to Measured Temp. EMF due to ambient temp. Compensated emf

16. Cold Junction Compensation (CJC)

17. Calibration & Troubleshooting Sensor : The sensors are being checked against some known temperature by mv - Temp chart for particular couple. Measuring Instrument : The measuring instrument is basically a voltmeter. Calibrated against some standard voltage source.

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