Self-Validating ThermocouplesClosing presentation

NPL: Claire Elliott, Jonathan Pearce, Graham Machin

ESA-estec: Christian Schwarz, Robert Lindner

13th July 2012

The UK’s national standards laboratory

Develop & disseminate UK’s measurement

standards, ensure they are internationally

accepted

� Founded in 1900

� 450+ specialists in Measurement Science

� State-of-the-art laboratory facilities

� World leading National Measurement Institute

About NPL

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Overview

� Introduction

• Novel high temperature fixed points

• Self-validating concept

� Measurement

• Test arrangement

• Software

� Performance of self-validating thermocouples, up to 2300 ˚C

• Thermal cycling & extended exposure

• Benefits & current limitations

� Multi-cell design & initial results

� Future & applications

� Summary & conclusions

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Availability & Range

� Thermocouples provide the best uncertainties at high temperatures (for contact thermometry)

� Standardised types for high temperatures:

Type Max exposure

Pt-Rh types (B, R, S) 1600 ˚C (1800 ˚C)

Lower temp (K, N) 1150 ˚C (1350 ˚C)

Pt/Pd 1500 ˚C

W-Re (C, D, G) 2300 ˚C (2600 ˚C)

� W-Re thermocouples in particular degrade very quickly

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Embrittlement & Drift

� W-Re thermocouples are commonly used

above 1500 ˚C

• Suffer from embrittlement

• Quickly exhibit thermoelectric drift –

typically 10 ˚C within 10 h of operation

• Recalibration often impossible

� To address this issue, NPL, in

cooperation with ESA-estec, have developed an innovative method of validating the performance of high

temperature thermocouples in-situ

Further information: Brixy et al. High Temperatures – High Pressures 12, 625-631 (1994)

[After Brixy]

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Concept of Self-validation

� The user is enabled to perform a suitable adjustment to the reference function, if required – increasing measurement confidence

� This could be automated with a suitable algorithm

� Position of thermocouple measuring junction crucial

• Customised design from Omega Engineering

• What reliable materials could we use?

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Position the HTFP onto the thermocouple in-situ

Observe thermocouple output through the transition temperature

Apply suitable correction algorithm to the output reference function

Choose a HTFP – with a transition temperature to match process

Fixed-points for Calibration

� The international temperature scale of 1990 ( ITS-90 ) defines temperature above 0.65 K, through a series of temperature fixed-points

from…

The lowest vapour pressure point of helium (-270.15 ˚C)

to…

The freezing point of copper (1084.62 ˚C)

� How to calibrate thermocouples at higher temperatures?

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Innovative Fixed-Points

� Novel high-temperature fixed points (HTFPs)

� Many eutectic metal-carbon alloys have been shown to be suitably stable as HTFPs, for example:

• MP Fe-C 1153 ˚C

• MP Co-C 1324 ˚C

• MP Pd-C 1492 ˚C

• MP Rh-C 1657 ˚C

• MP Pt-C 1738 ˚C

• MP Ru-C 1953 ˚C

• MP Ir-C 2292 ˚C

• MP Re-C 2474 ˚C

• …

� MP determined by radiation thermometry

� Repeatability of the MP, known to be better than ±0.05 ˚C (k = 2)

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Self-validating Thermocouple

� The design consists of two parts:

• Miniature HTFP (containing a eutectic

M-C ingot)

• High temperature thermocouple

(Type C, W5%Re-W26%Re)

� Self-validation is a technique to check the calibration of a thermocouple in-situ

� The thermovoltage can be verified each time the thermal environment passes the fixed-point transition temperature

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Measurement set-up

Test Arrangement (1)

� High temperature graphite furnace (RD Webb Co., Model: RD-G)

� Argon atmosphere

Delivered Breadboard

� Five self-validating thermocouples have been provided

• Five W-Re thermocouples (from Omega Engineering)

• Five HTFP cells

� … with the equipment necessary to measure them:

• W-Re extension cable

• Zero point block calibrator

• Keithley 2182A digital nanovoltmeter

• GPIB-USB connector

• Laptop with software

• “TC Scanner”

• “Best Fit”

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Test Arrangement (2)

� Open up the furnace…

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Test Arrangement (3)

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� Open up the furnace…

� … remove the felt insulation stack…

Test Arrangement (4)

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� … remove the lid of the hot zone…

� … load the HTFP cell into a graphite shield…

� This ensures the temperature gradient over the cell is as uniform as possible

Test Arrangement (5)

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Test Arrangement (6)

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� … position the HTFP cell on the Type C thermocouple…

Test Arrangement (7)

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� … move the control thermocouple into position…

� Note, the furnace is controlled by:

• Type C thermocouple (below 1500 ˚C)

• Pyrometer (above 1500 ˚C)

Test Arrangement (8)

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� … replace the lid, felt insulation and seal the furnace…

Test Arrangement (9)

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� … connect up the thermocouples…

Test Arrangement (10)

� … ensure the reference junction is ready…

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Software (1) – “TC scanner”

� … start measuring...

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Software (2) – “Best fit”

� … and fit a curve to the melt plateau.

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Results

Results (1) - Example

� Thermal cycling

• Ramp rate of 1 ˚C/min through melt and freeze

• Held at maximum temperature for 1 hour

• Presence of cell does not impede sensor function (under these conditions) 15/26

Temperature Assignment

1720

1730

1740

1750

Tem

pera

ture

/ °

C

Time

5 min

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� Melting temperature assigned by radiation thermometry

ISO17025

traceable to ITS-90

HTFP alloy Melting temperature, °C

Uncertainty (k = 2), °C

Co-C 1323.28 0.64

Pt-C 1737.52 0.94

Ru-C 1952.98 1.00

Ir-C 2289.70 1.56

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1323.28 ± 0.64 ˚C

Results (2)

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1737.52 ± 0.94 ˚C

Results (3)

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1952.98 ± 1.00 ˚C

Results (4)

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Results (5)

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1323.28 ± 0.64 ˚C 1737.52 ± 0.94 ˚C

1952.98 ± 1.00 ˚C 2289.70 ± 1.56 ˚C

Results (6) - Thermal Cycling

� At high temperatures, the thermocouple is clearly unreliable

� Ru-C drift between 1st and 2nd melt is: 20.9 µV (~1.7 ˚C)

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Results (7) - Endurance

� Extended exposure

• Maximum temperature maintained for 10 hours

• Thermovoltage drift over 5 h (at Ir-C) is 347 µV, equivalent to ~43 ˚C

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Benefits

� By correcting for every step in drift, the user gains confidence in the temperature reading

� The thermocouple is kept within calibration: extending its useful life

� The process eliminates the uncertainty due to changing thermoelectric homogeneity

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Current limitations

� Current limitations:

• One ingot (fixed point temperature) and therefore,

• Limited temperature range of correction validity

• Ingot size

• Thermocouple and HTFP cell are separate items, at present

• Powder filling takes many steps

� All of the above can be overcome with development

� Design and use can be tailored for specific requirements

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Multi-cell (1) - Design

� Multi-cells to allow dual validation (design also

developed during the ESA/ESTEC project)

� Multi-cell containing both:

• Pt-C

• Ru-C

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Multi-cell (2) - Results

� Each ingot is clearly observed, showing good thermal contact

� Also tried Co-C and Pt-C, but found that the Co-C compartment leaked 30000

31000

32000

33000

34000

Ru-C:

1953 °C

Pt-C and Ru-C multicell

em

f /

µV

Time

1 h

Pt-C: 1737 °C

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Future & Applications

� Looking to develop into a commercial device

• User requirements

• Develop correction algorithm

� Benefits to industry are clear

• Interest in the improved temperature measurement and reliability

• Casting / manufacturing – reduced costs and enhanced quality

� Nuclear industry – self-validation with low neutron capture cross-section materials e.g. Fe-C and Cu

• EMRP project “MetroFission”

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Summary

� Introduced novel eutectic M-C fixed-points & the principle of self-validation

� Test arrangement and results

• Clear melting and freezing plateaus for all four HTFPs – up to 2300 ˚C

• The need for such validation is shown (particularly at Ir-C point)

� Benefits and current limitations

� Multi-cell design and results

� Future and applications

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Conclusions

� The concept of self-validation has been shown to be viable

• Cell size provides suitable immersion

• The presence of the cell does not impede on the function of the sensor, under these conditions

� Application of in-situ self-validation will achieve:

• Assured temperature measurement confidence

• Extended useful life of the sensor

� Which opens up the possibility for:

• Improved temperature measurement /reliability

• Reduced costs and enhanced quality

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Email: claire.elliott@npl.co.uk

www.npl.co.uk/temperature-humidity/