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Pd-C eutectic fixed point cells for a self-validating Type ... -  · PDF file• Format: Mineral ... fixed points, Pearce J V, Ongrai O, ... Metrologia 47 (2010) L1-L3 Pd-C eutectic

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    The miniature Pd-C eutectic cell (1492 C) for thermocouple selfcalibration 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 V 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 V/ C or 0.25 C / C. At zero offset, the emf is 26320 V 14 V.

    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. 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

    Tem

    pera

    ture

    / oC

    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 / 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 / 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 / V

    Furnace offset / oC

    1K

    Uncertainty components Value Distribution Standard 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 0.3

    Spurious 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

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