1. RAVISANKAR G EMS INSTRUMENTS
THERMOCOUPLES, RTD’S AND PYROMETERS
RAVISANKAR G EMS INSTRUMENTS

2. THEMOCOUPLE TYPES J - Fe-Cons 0-750 K - Cr -Al 0-1100
S- Pt-Pt Rh (10%)
R- Pt-Pt Rh(13%)
B- Pt-Pt Rh(30%)
E-Cr-Cons
T- Cu-Cons
N-Ni-NiCl

3. SELECTION OF THERMOCOUPLES
Cost
Resistance to corrosion
EMF output
Speed of response
Nature of medium

4. THERMOCOUPLE CONSTRUCTION
Mineral Insulated thermocouples are popular now.
Exposed Type (with /without seal)
Non grounded type
Grounded type

5. REFERENCE JUNCTION COMPENSATION
Temp versus EMF for a thermocouple is supplied by manufacturer.
Measurement is based on ref junction at 0. C
Ref Junction is at room temp in most cases.
So it is required to add the observed EMF with that EMF which T/C would develop if the ref junction were at 0. C. This is done using temp sensitive resistors.

6. THERMOCOUPLE LAWS
Law of homogeneous Circuit: No current flows in th ckt made of a simple metal by application of heat alone
Law of Intermediate metals: If between any two points a no. of junctions appear, but all at same temp, EMF between these two points is independent of intermediate mmetals.

7. THERMOCOUPLE LAWS
Law of Intermediate temp: EMF for a couple with junction temp t1 & t3 is sum of emfsof two couples of same metals having junction temp t1 & t2 and t2 & t3 resp.
Extension wire & Compensating cable
JPX, KPX, NPX, SPX, SNX etc

8. RTD PT: Accuracy: 0.01 .C; Costly, stable uniform & high resistance
Ni: Accuracy: C; Large change and high resistance. Not uniform
Cu : Accuracy: C; Uniform, stable, cheap, & low value of resistance.

9. RTD Thick and thin film RTD.
-- Wire wound and thin pattern of pt metal alloy on ceramic substrate.
Two wire system
Three wire system
Four wire system

10. RTD VERSUS THERMOCOUPLE
High accuracy
Narrow Range
No Compensation required.
Expensive
Up to C only
Self heating may be a problem
THERMOCOUPLE
Moderate accuracy
Stable & low cost
Less output signal
Calibration Drifts
Electrical interface required
Higher installation cost-extension wires

11. PYROMETERS Radiation type :
Temp of continuously moving sheet of material can be measured.
High temp can be measured.( C )
Stable, non contact output signal.
Thermopile, Photocell, Thermistor, LDR, Photodiode are common detectors.

12. Thank you