Thermocouple Thermometry 2006

Thermocouple Thermometry

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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 thermoelectric voltage developed (dV) per unit temperature difference(dT) in a conductor is call Seebeck coefficient. S = dV/dT

The emf between the thermocouple wire is given by T

T

E = ∫ ( S − S )dt = ∫ S dt AB

A

T0

B

V

Metal B, SB

AB

T0

Where SAB = The Seebeck coefficient for the thermocouple pair A-B

Metal A, SA

Thermocouple Voltage,emf

Reference junction/ Cold junction

Measuring junction/ Hot junction

Thermocouple Circuit

Inhomogeneity Error In Thermocouple “The variation of the thermocouple output along the used length.” •TC As Use

1

TC emf,E 2

3

4

E=S12(T1-T2)=small E=S23(T2-T3)=largeE=S34(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 250oC for base-metal)

- Change in immersionÆMeaningless 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 Standard Type (Letter-designed types) Noble (Rare) Metal Type B, R ,S

TC In Use Non Standard Type (Special designed)

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 oC , Type B > 1300 oC. •Type T,E and K At low temp / Room temp. •Type K (< 200 oC), Type N (300 – 600) oC . • Type K, N > 600 oC.

NIMT Thermocouple Calibration System - Calibration MethodITS-90

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SPRT / HSPRT (-38.8344 oC to 961.78oC )

Nobel Metal TC

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 oC to 961.78oC )

NPL(UK), NMIA(Aus)

Reference Standard Thermocouple (Pt/Pd ,Au/Pt,Type B,Type R ,Type S) (0 oC to 1100 oC )

1.Comparison with Fixed Point Range (0oC-962oC) Uncertainty 0.4 oC – 0.5 oC

Calibration Laboratory

SPRT

IPRT

Thermocouple (All Type)

DTM

LIG

USERS

NIMT Traceability Chart(2006)

2.Comparison with SPRT / Ref. TC Range (0oC-1100oC) Uncertainty 0.5 oC – 1.5 oC