thermpocouple application and block diagram.ppt

thermo couple

1. Practical Aspects of Thermo-couples
P M V Subbarao
Professor
Mechanical Engineering Department
Correct Use of Themocouples ….

2. Standard Calibration Setup
Ice-point
The result of the calibration is a table of EMF versus T values.
The integral is never directly evaluated.
Instead a polynomial curve fit to the calibration data gives:
n
jnjjjj
TbTbTbbTFE  .....
2
2100 

j
t
T
T
ABAB dTTE 

3. From the same calibration data a curve fit of the form
n
jnjjjj
EcEcEccEGT
0
2
020100
.....
This equation is directly useful for temperature measurements
with thermocouples.
The coefficients of the calibration equations for type J
thermocouples are given in Tables

6. Practical Thermocouple Circuits
•The equations given below are now applied to develop
thermocouple circuits that can be used in a laboratory.
•The following ideas are important.
•Compensation with a reference junction in an ice bath;
•Compensation with a reference junction at an arbitrary
temperature;
•Use of zone boxes for large numbers of thermocouples.
n
jnjjjj
EcEcEccEGT
0
2
020100
..... 
n
jnjjjj
TbTbTbbTFE  .....
2
2100

7. Practical Precautions
•The polynomial curve fit relies upon the thermocouple
wire being perfect.
•It must not become decalibrated during the act of making a
temperature measurement.
•Most measurement errors may be traced to one of these
seven primary sources:

8. Primary Sources of Errors
1. Poor junction connection
2. Decalibration of thermocouple wire
3. Shunt impedance and galvanic action
4. Thermal shunting
5. Noise and leakage currents
6. Thermocouple specifications
7. Documentation

9. Poor junction connection
•To reach a higher measurement temperature, the joint must
be welded.
•But welding is not a process to be taken lightly.
•Overheating can degrade the wire.
•The welding gas and the atmosphere in which the wire is
welded can both diffuse into the thermocouple metal,
changing its characteristics.
•The difficulty is compounded by the very different nature
of the two metals being joined.
•Commercial thermocouples are welded on expensive
machinery using a capacitive-discharge technique to insure
uniformity.

10. Decalibration

11. Leakage Resistance

12. Laws of Thermocouples

13. Law of Intermediate Metals
Insertion of an intermediate metal into
a thermocouple circuit will not affect
the emf voltage output so long as the
two junctions are at the same
temperature and the material is
homogeneous.
–Permits soldered and welded joints.

14. A Demonstration of the Law of Intermediate Metals
3
1
2 54
6
emf
Temperature
T
ref T
2 and 4
Measured
Emf
Fe
C
Fe (+)
C (-)
P (+)
T
candle
1
2
T
measured
3
5
4
6
Signs of the
materials used

15. Law of Intermediate Temperatures
If a thermocouple circuit develops a net
emf
1-2for measuring junction
temperatures T
1and T
2,
and a net emf
2-3for temperatures T
2and T
3,
then it will develop a net voltage of emf
1-3=
emf
1-2+ emf
2-3when the junctions are at
temperatures T
1and T
3.
emf
1-2+emf
2-3= emf
1-3
T3
T1
T3 T2
T2 T1

16. A Demonstration of the Law of Intermediate
Temperatures
emf
T
1 T
2
Fe
C
T
3
emf
23
emf
1-2+emf
2-3= emf
1-3
emf
13
emf
12

17. The integrity of a thermocouple system
•The integrity of a thermocouple system can be improved
by following these precautions:
•Use the largest wire possible that will not shunt heat away
from the measurement area.
•If small wire is required, use it only in the region of the
measurement and use extension wire for the region with no
temperature gradient.
•Avoid mechanical stress and vibration which could strain
the wires.
•When using long thermocouple wires, connect the wire
shield to the dvm guard terminal and use twisted pair
extension wire.

18. •Avoid steep temperature gradients.
•Try to use the thermocouple wire well within its
temperature rating.
•Use a guarded integrating A/D converter.
•Use the proper sheathing material in hostile environments
to protect the thermocouple wire.
•Use extension wire only at low temperatures and only in
regions of small gradients.
•Keep an event log and a continuous record of
thermocouple resistance.

19. Single and multiplexing

20. Use of a Zone Box for Multiple Thermocouples

21. A zone box with a thermistor to measure T
r.

23. Effect of Non-uniform Zone Box Temperature
•A zone box is designed to hold the temperature of all reference
junctions at a uniform and easily measured value.
•For convenience only a limited number of sensors (typically one)
are used to measure the temperature of the reference junctions.
•A special model is to be developed to assess the measurement
error introduced by non-uniformities in zone box temperature.

24. Common sheathed thermocouple probe

27. Thermopile

28. Temperature Measurement Errors
•Conduction
•Convection
•Radiation
•Response Time
•Noise
•Grounding issues and shorts, especially on metal surfaces