Thermocouple Circuit Thermocouple Thermometry

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NIMT
Thermocouple Thermometry
Theory
The principle of The Thermocouple (Thermo+couple) is based on the Seebeck effect
that Thomas Seebeck observed this effect in 1821 .
Seebeck effect: A temperature gradient
or A temperature difference between two
points in a metal results in voltage
difference between these.
The emf between the thermocouple wire
is given by
E
AB
T
= ∫ ( S
T
0
A
− S
B
) dt
T
= ∫ S
Where S AB = The Seebeck coefficient for
the thermocouple pair A-B
T
0
AB
dt
V
Thermocouple
Voltage,emf
The thermoelectric voltage developed (dV) per
unit temperature difference(dT) in a conductor
is call Seebeck coefficient. S = dV/dT
Metal A, S A
Metal B, S B
Reference junction/
Cold junction
Measuring junction/
Hot junction
Thermocouple Circuit

Inhomogeneity Error In Thermocouple
•TC As Use
“The variation of the thermocouple output along the used length.”
E=S 12 (T1-T2)=small
1 2 3 4
TC emf,E
E=S 23 (T2-T3)=largeE=S 34 (T3-T4)=small
Un-Aged
wire
Transition
wire
Aged
wire
1 2
3
4
•TC in calibration furnace
Causes of inhomogeneity in thermocouple wire:
-Cold work (bending, twisting,etc.)
-Chemical (environment and in-use)
-Heat treatment (above 250 o C for base-metal)
- Change in immersionMeaningless results.
Typical inhomogeneity for new Thermocouple;
Pt thermocouple S R B ≈ ± 0.02%of E
Base-metal thermocouple ≈ ± 0.1%of E
(test by Thermal Scan)

Thermocouple Types
TC In Use
Standard Type
(Letter-designed types)
Non Standard Type
(Special designed)
Noble (Rare) Metal
Type B, R ,S
Base Metal
Type E,J,K,N,T
Pt/Pd, Au/Pt,
W3-Re/W25-Re(Type C)
General Range use :
•Type R or Type S < 1300 o C , Type B > 1300 o C.
•Type T,E and K At low temp / Room temp.
•Type K (< 200 o C), Type N (300 – 600) o C.
• Type K, N > 600 o C.

NIMT Thermocouple Calibration System
ITS-90
- Calibration Method-
มว
NIMT
SPRT / HSPRT
(-38.8344 o C to 961.78 o C )
NIMT Fixed Point Cell
Triple point of Mercury,Triple point of Water,Melting point
of Galium, Freezing point of Tin,Freezing point of
Zinc,Freezing point of Aluminum,Freezing point of Silver
(-38.8344 o C to 961.78 o C )
Nobel Metal TC
NPL(UK), NMIA(Aus)
Reference Standard Thermocouple
(Pt/Pd ,Au/Pt,Type B,Type R ,Type S)
(0 o C to 1100 o C )
1.Comparison with Fixed Point
Range (0 o C-962 o C)
Uncertainty 0.4 o C – 0.5 o C
Calibration Laboratory
SPRT
IPRT
Thermocouple
(All Type)
DTM
LIG
USERS
NIMT Traceability Chart(2006)
2.Comparison with SPRT / Ref. TC
Range (0 o C-1100 o C)
Uncertainty 0.5 o C – 1.5 o C