7/29/2019 4 - Ongrai

1/1

QueensPrinterandControllerof

HMSO,2011.

9786/0911

www.npl.co.uk

The miniature Pd-C eutectic cell (1492 C) or thermocouple selcalibrationhas been studied to transorm the reliability o thermoelectric perormanceo the Type C thermocouple by acilitating in-situ calibration without needor removal sensor or recalibration.

1. Introduction

The accuracy of high temperature measurementis signicant in terms of: reducing product waste,minimizing energy consumption and promotinghealth and safety.

Thermocouples are the most widely used temperaturesensor for high temperature measurement.

Type C (W5%Re/W26%Re) refractory thermocouplesare almost universally used >1700 C.

2. Metrology Challenge

A new Type C thermocouple has a typical uncertaintyof 1% of temperature (20 C at 2000 C) and thiscan rapidly and substantially increase with use.

Achievable accuracy is severely limited byhomogeneity, drift, and hysteresis.

The removal of W/Re thermocouples after hightemperature use is not possible due to embrittlement.

Thermocouples application is always dierent fromthe calibration situation.

Hence, in-situ sel-validation would signifcantlyimprove the use o type C thermocouples.

3. Measurement Set Up

Construction o a Pd-C miniature cell

The crucible was constructed from pure graphite.

The pure Pd wire was repeatedly added and melted toll the crucible (~ 4 g).

Dissolved graphite from the crucible formed aPd-C eutectic.

Figure 1: (a) Cross-sectional drawing o the miniature graphite crucible. All

dimensions are in mm. (b) The miniature graphite crucible in an upside-down

orientation lled with small pieces o the palladium wire.

Type C thermocouples

Format: MineralInsulated Metal

Sheathed (MIMS)

Sheath: Molybdenum

Insulator material:Hafnia

Wire diameter 0.25 mm

Ungroundedmeasurement junction

Ice point reference

Integrated deviceFigure 2: The arrangement o the

sel-validating thermocouple.

The thermocouple was tted to

the miniature Pd-C cell to make a

sel-validating device.

Measurement FurnaceFurnace supplier:Elite Thermal

Systems Temperature range:0-1800 C

Work tube inner diameter:12 mm

Figure 3: Temperature prole (above) o a single zone vertical urnace (right) used

to evaluate the quality o the device in a severe temperature gradient such as that

in a typical industrial application. Here, uniormity over the central 50 mm is 4 C.

4. Results

Plateau behaviour

Figure 4: Repeated Pd-C melts and reezes o the miniature cell, measured with

sel-validating type C thermocouple.

Repeatability

The emf of the point of inection of the melting plateau,for a number of realisations of the Pd-C miniature cell.

Figure 5: Repeatability o melting em values o the miniature Pd-C xed point

cell. Error bars represent the type A uncertainty associated with each point (k = 1).

The repeatability was 2.3 V or 0.15 C.

Thermal environment eectsThe emf at the melting plateau of the miniature Pd-C cellwith varying furnace oset temperatures was studied toexamine the inuence of the ambient conditions.

Figure 6. The melting em values as a unction o the ofset temperatures o the

urnace. Error bars represent the type A uncertainty associated with each point

(k = 1). The slope o the dashed line is 4.0 V/ C or 0.25 C / C. At zero ofset, theem is 26320 V 14 V.

5. Uncertainty

In this case the thermoelectric inhomogeneity can beomitted from the uncertainty budget as the calibrationis performed in-situ

This causes a very large decrease in the calibrationuncertainty for the thermocouple

Table 1: Uncertainty budget o the miniature Pd-C xed point cell using a Type C

thermocouple.

6. Conclusions

A miniature Pd-C eutectic xed point cell was fabricatedto evaluate the stability at ~1500 C of typeC thermocouples by means ofin-situ calibration.

The results show that the implementation of hightemperature eutectic xed points for self-validating

W/Re thermocouples is possible. This study shows that a step change improvement

in high temperature thermometry is possibleby mitigating the eects of sensor drift due tocontamination and inhomogeneity.

Higher temperature eutectic xed points and multiplexed points in one crucible for thermocouple self-validation are both under development

Reerences

Miniature metal-carbon eutectic xed point cells forself-validating Type C thermocouples, Ongrai O, PearceJ V, Machin G and Sweeney S J, Measurement Science &Technology 22 (2011) 105103

Miniature Co-C xed point cells for self-validatingthermocouples, Ongrai O, Pearce J V, Machin G andSweeney S J, Measurement Science & Technology22 (2011)015104

Self-validating thermocouples based on high temperaturexed points, Pearce J V, Ongrai O, Machin G andSweeney S J, Metrologia 47 (2010) L1-L3

Pd-C eutectic fxed point cells or asel-validating Type C thermocouple

O. Ongrai1-3, J. V. Pearce1, G. Machin1, S. J. Sweeney21 National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK

2 Advanced Technology Institute, University of Surrey, UK3 National Institute of Metrology ( Thailand), Pathumtani, Thailand

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

1260

1280

1300

1320

1340

Temperature/

oC

Position / cm

0 10 20 30 40 50 60 70 80 90 100

26150

26200

26250

26300

26350

26400

26450

Thermocoupleemf/V

Time / minutes

5 K

0 1 2 3 4 5

26338

26340

26342

26344

26346

26348

26350

26352

26354

Thermocoupleemf/V

No. of measurements

avg.

0.2 K

0 1 2 3 4 5 6 7 8

26315

26320

26325

26330

26335

26340

26345

26350

26355

Thermocoupleemf/V

Furnace offset /oC

1K

Uncertainty components Value DistributionStandard

uncertainty

Repeatability emf / V 2.3 Normal 2.3

Plateau determination / V 1.0 Rectangular 0.6

Thermal environment / V 4.0 Rectangular 2.4

Temperature of Pd-C / V 5.8 Normal 5.8

Voltmeter calibration/ V 0.5 Rectangular 0.3Spurious emf / V 0.5 Rectangular 0.3

Ice point uncertainty / V 0.1 Rectangular 0.1

Combined uncertainty (k=1) / V 7.0

Expanded uncertainty (k=2) / V 14

Expanded uncertainty (k=2) / C 0.9