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Thermocouples: Capabilities & Challenges. Ajay V. Singh Graduate Student. Department of Fire Protection Engineering University of Maryland College Park, MD. Introduction.
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Thermocouples: Capabilities & Challenges Ajay V. Singh Graduate Student Department of Fire Protection Engineering University of Maryland College Park, MD
Introduction • The basis of thermocouples was established by Thomas Johann Seebeck in 1821 when he discovered that a conductor generates a voltage when subjected to a temperature gradient. • To measure this voltage, one must use a second conductor material which generates a different voltage under the same temperature gradient. The voltage difference generated by the two materials can then be measured and related to the corresponding temperature gradient. • It is thus clear that, based on Seebeck's principle, thermocouples can only measure temperature differences and need a known reference temperature to yield the absolute readings • Thermocouple is a relative not an absolute temperature sensor. In other words, a thermocouple requires a reference of known temperature The temperature at the probe tip can then be related to the voltage output as, The above formula is effective only if the reference temperature TRef in the experiment is kept the same as the reference temperature specified on the data sheet. Dynamics of Boundary Layer Flames June 25, 2014 2
How to make a thermocouple: Best Practice Very common design. However, errors become substantial due to wire extension in flames and other combusting environments  Probably, the best way to go Thermocouple wire extension in a flame 
Thermocouples: Data Acquisition 2D traverse mechanism NI cDAQ 9171 Stepper Motor Controller Mettler Toledo load cell NI 9214 has in-built signal conditioner and Cold-junction compensation (CJC) module NI-9214, 16 channel, 24 bit ADC
Thermocouple: Concepts • Support wire diameters should be at least 4 times the wire diameter, so that the resistance per unit length of the fine wires is significantly higher than that of support wires • Bead diameters and shape vary widely, depending on the technique and care used to join the wires • In practice, most thermocouples have bead diameters in the range • Formed junction is not a “sphere” and resembles a “truncated sphere” (left) Schematic of a typical thermocouple arrangement for combustion-system measurements. Relatively large lead wires are placed in a protective, ceramic rod and then bent outward in order to provide tension for the thermocouple wires themselves and to allow for sufficiently long thermocouple wire segments to reduce conductive heat transfer back to the leads . (right) Electron micrograph (180 X) of the Pt/Pt-13%Rh thermocouple bead. Dark patches are the remnants of soot. 
Thermocouples: Conduction loss Conduction loss Radiation loss Uncertainties due to catalytic effects • Heat conduction to support wires is assumed to have negligible effect on the temperature of the junction if , where L is the length of the fine wire . Can be quantified and is negligible for • It is reasonable to assume that there are no radial temperature gradients in the wire, since it can be shown that radial thermal diffusion is always at least one order of magnitude less than the time constant for the wire diameters of 50 . • Conduction losses can be reduced by placing the thermocouple along an isotherm. However, in flames it’s a difficult task to achieve (especially in turbulent flames)
Thermocouples: Catalytic effects • Errors in thermocouple measurements due to catalysis can be reduced by coating the wires with a thin layer of non-catalytic coating • Often, silicon dioxide (SiO2) or yttrium oxide (Y2O3, toxic in nature) coatings are used to prevent exothermic reactions on the possibly catalytic platinum surface  • Although coating reduces the catalytic effect, it increases the thermocouple diameter, the response time of the thermocouple, and the radiation losses. It also changes the convective and conductive heat transfer of the thermocouple. Since the coating changes the heat transfer properties of the thermocouple, it is difficult to quantify the effect of catalysis. • SiO2 coatings in a reducing atmosphere can lead to the formation of silicon solid solution in the legs of a thermocouple. This would shift the Fermi-energy levels of the two components of the junction and can de-calibrate the potential difference output . Thermocouple coating microscopic images 
Thermocouples: Radiation Loss Can be quantified precisely and accurately. An energy balance on the thermocouple takes the following form : 0 0 (1) For an unsteady system, it becomes (2) (3) where Emissivity of Pt can be represented as a function of absolute temperature  For a steady system, it becomes (4) Here ‘d’ represents the diameter of fine wire or thermocouple bead depending on whether Cylindrical or Spherical Nu assumption is used Appropriate Nusselt number correlation should be used to model the convective heat transfer about the thermocouple.
Spherical Nusselt number Correlations For spherical bead approximation, Ranz and Marshall (1952)  (1) For Reynolds number between 0 and 200. Properties evaluated at Whitaker (1972)  (2) Properties evaluated at . is the gas viscosity evaluated at the surface temperature (3)  For all Re and Pr numbers of interest in low flow velocities
Cylindrical Nusselt number Correlations For cylinders, many Nusselt number correlations have been introduced, For the low Reynolds numbers applicable for fine-wire thermocouple measurements in combustion systems, the Collis and Williams (1959) correlation is most commonly used (Cylindrical Nu number correlation preferably that of Collis and Williams should be used) , Another widely quoted correlation is that due to Kramers (1946)  Andrews et. al. (1972)  evaluated the following expression for , With the Reynolds number evaluated at the so-called “film temperature”, With the gas properties evaluated at Gas properties evaluated at
Conclusions • A cylindrical Nu correlation (preferably that of Collis and Williams) should be used, with estimation of the local flow velocity when it is known. • It is better not to coat the thermocouple wires with a non-catalytic coating as it increases uncertainties and errors that cannot be quantified. • Conduction losses can be avoided if fine wire thermocouples are used (50-75 ). • Radiation loss from thermocouples at high temperatures is a major source of error in thermocouple measurements. • In turbulent flames, unsteady term comprising the time constant cannot be neglected if we wish to capture temperature fluctuations at high frequency. • Emissivity of thermocouples varies with temperature and surface characteristics including roughness, coating of the metal. Variation of emissivity with absolute temperature can be estimated for certain thermocouples.
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Questions? Dynamics of Boundary Layer Flames June 25, 2014 14