THERMOCOUPLES, RTD’S
AND PYROMETERS
RAVISANKAR G
EMS INSTRUMENTS
THEMOCOUPLE TYPES








J - Fe-Cons
K - Cr -Al
S- Pt-Pt Rh (10%)
R- Pt-Pt Rh(13%)
B- Pt-Pt Rh(30%)
E-Cr-Cons
T- Cu-Cons
N-Ni-NiCl
0-750
0-1100
0-1300
0-1350
0-1350
0-1100
0-750
0-1400
SELECTION OF THERMOCOUPLES
Cost
 Resistance to corrosion
 EMF output
 Speed of response
 Nature of medium

THERMOCOUPLE CONSTRUCTION
Mineral Insulated thermocouples are
popular now.
 Exposed Type (with /without seal)
 Non grounded type
 Grounded type

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.

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.

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

RTD
.
 PT: Accuracy: 0.01 C; Costly, stable
uniform & high resistance

Ni: Accuracy: 0.01 .C; Large change and
high resistance. Not uniform
.
 Cu : Accuracy: 0.01 C; Uniform, stable,
cheap, & low value of resistance.
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

RTD VERSUS THERMOCOUPLE
RTD
 High accuracy
 Narrow Range
 No Compensation
required.
 Expensive
.
 Up to 400 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
PYROMETERS
Radiation type :
 Temp of continuously moving sheet of
material can be measured.
.
 High temp can be measured.(700-3000 C )
 Stable, non contact output signal.
 Thermopile, Photocell, Thermistor, LDR,
Photodiode are common detectors.
Thank you