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A sensor exposed to hot gases and surrounded by cold walls is subject to radiation, velocity, and conduction errors. Radiation error depends on the heat transfer coefficient which, for a given flow field, depends on sensor diameter. Data from a probe with two sensors of appropriately chosen diameters can be used to estimate the true temperature of a hot gas stream, independent of wall temperature, even in the presence of cold walls. Once an appropriate pair of sensor diameters has been identified, a three-sensor probe can be constructed that is direct reading.This form of passive correction for radiation error is most useful for surveillance instrumentation in steady-state systems at temperatures below 1000K. The predicted temperature from a dual-junction probe is closer to the real gas temperature than the reading of a bare probe, and dual junction probes are easier to use than aspirated probes. A ten-fold reduction in radiation error has been predicted for a probe with one junction of 0.25 mm and the other of 1.55 mm, used in a gas flow at 800 K with walls at 750K.Design equations and data are presented by which diameter combinations can be selected for various conditions.
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