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The miniature Pd-C eutectic cell (1492 °C) for thermocouple self–calibration has been studied to transform the reliability of thermoelectric performance of the Type C thermocouple by facilitating in-situ calibration without need for removal sensor for recalibration.

1. Introduction• The accuracy of high temperature measurement

is significant in terms of: reducing product waste, minimizing energy consumption and promoting health and safety.

• Thermocouples are the most widely used temperature sensor for high temperature measurement.

• Type C (W5%Re/W26%Re) refractory thermocouples are almost universally used >1700 °C.

2. Metrology Challenge• A new Type C thermocouple has a typical uncertainty

of 1% of temperature (±20 °C at 2000 °C) and this can rapidly and substantially increase with use.

• Achievable accuracy is severely limited by homogeneity, drift, and hysteresis.

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

• Thermocouples application is always different from the calibration situation.

Hence, in-situ self-validation would significantly improve the use of type C thermocouples.

3. Measurement Set Up

Construction of a Pd-C miniature cell • The crucible was constructed from pure graphite.

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

• Dissolved graphite from the crucible formed a Pd-C eutectic.

Figure 1: (a) Cross-sectional drawing of the miniature graphite crucible. All dimensions are in mm. (b) The miniature graphite crucible in an upside-down orientation filled with small pieces of the palladium wire.

Type C thermocouples• Format: Mineral

Insulated Metal Sheathed (MIMS)

• Sheath: Molybdenum

• Insulator material: Hafnia

• Wire diameter 0.25 mm

• Ungrounded measurement junction

• Ice point reference

Integrated deviceFigure 2: The arrangement of the self-validating thermocouple. The thermocouple was fitted to the miniature Pd-C cell to make a self-validating device.

Measurement FurnaceFurnace supplier: Elite Thermal

Systems Temperature range: 0-1800 °C

Work tube inner diameter: 12 mm

Figure 3: Temperature profile (above) of a single zone vertical furnace (right) used to evaluate the quality of the device in a severe temperature gradient such as that in a typical industrial application. Here, uniformity over the central 50 mm is ± 4 °C.

4. ResultsPlateau behaviour

Figure 4: Repeated Pd-C melts and freezes of the miniature cell, measured with self-validating type C thermocouple.

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

Figure 5: Repeatability of melting emf values of the miniature Pd-C fixed point cell. Error bars represent the type A uncertainty associated with each point (k = 1). The repeatability was 2.3 μV or 0.15 °C.

Thermal environment effectsThe emf at the melting plateau of the miniature Pd-C cell with varying furnace offset temperatures was studied to examine the influence of the ambient conditions.

Figure 6. The melting emf values as a function of the offset temperatures of the furnace. Error bars represent the type A uncertainty associated with each point (k = 1). The slope of the dashed line is 4.0 μV/ °C or 0.25 °C / °C. At zero offset, the emf is 26320 μV ± 14 μV.

5. Uncertainty• In this case the thermoelectric inhomogeneity can be

omitted from the uncertainty budget as the calibration is performed in-situ

• This causes a very large decrease in the calibration uncertainty for the thermocouple

Table 1: Uncertainty budget of the miniature Pd-C fixed point cell using a Type C thermocouple.

6. Conclusions• A miniature Pd-C eutectic fixed point cell was fabricated

to evaluate the stability at ~1500 °C of type C thermocouples by means of in-situ calibration.

• The results show that the implementation of high temperature eutectic fixed points for self-validating W/Re thermocouples is possible.

• This study shows that a step change improvement in high temperature thermometry is possible by mitigating the effects of sensor drift due to contamination and inhomogeneity.

• Higher temperature eutectic fixed points and multiple fixed points in one crucible for thermocouple self-validation are both under development

References• Miniature metal-carbon eutectic fixed point cells for

self-validating Type C thermocouples, Ongrai O, Pearce J V, Machin G and Sweeney S J, Measurement Science & Technology 22 (2011) 105103

• Miniature Co-C fixed point cells for self-validating thermocouples, Ongrai O, Pearce J V, Machin G and Sweeney S J, Measurement Science & Technology 22 (2011) 015104

• Self-validating thermocouples based on high temperature fixed points, Pearce J V, Ongrai O, Machin G and Sweeney S J, Metrologia 47 (2010) L1-L3

Pd-C eutectic fixed point cells for a self-validating Type C thermocouple

O. Ongrai1-3, J. V. Pearce1, G. Machin1, S. J. Sweeney2

1 National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK2 Advanced Technology Institute, University of Surrey, UK

3 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

Tem

pera

ture

/ o C

Position / cm

0 10 20 30 40 50 60 70 80 90 100

26150

26200

26250

26300

26350

26400

26450

Ther

moc

oupl

e em

f / μ V

Time / minutes

5 K

0 1 2 3 4 526338

26340

26342

26344

26346

26348

26350

26352

26354

Ther

moc

oupl

e em

f / μ V

No. of measurements

avg.

0.2 K

0 1 2 3 4 5 6 7 826315

26320

26325

26330

26335

26340

26345

26350

26355

Ther

moc

oupl

e em

f / μ V

Furnace offset / oC

1K

Uncertainty components Value Distribution Standard uncertainty

Repeatability emf / μV 2.3 Normal 2.3

Plateau determination / μV 1.0 Rectangular 0.6

Thermal environment / μV 4.0 Rectangular 2.4

Temperature of Pd-C / μV 5.8 Normal 5.8

Voltmeter calibration/ μV 0.5 Rectangular 0.3

Spurious emf / μV 0.5 Rectangular 0.3

Ice point uncertainty / μV 0.1 Rectangular 0.1

Combined uncertainty (k=1) / μV 7.0

Expanded uncertainty (k=2) / μV 14

Expanded uncertainty (k=2) / °C 0.9