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A thermocouple junction of spherical form is to be used to measure the temperature of the gas stream. The junction is at 20 degree Celsius and is placed in a gas stream which is at 200 degree Celsius. Make calculations for junction diameter needed for the thermocouple to have thermal time constant of one second. Assume the thermos-physical properties as given below k = 20 W/ m K h = 350 W/m2 K c = 0.4 k J/kg K p = 8000 kg/m3

A thermocouple junction of spherical form is to be used to measure the temperature of the gas stream. The junction is at 20 degree Celsius and is placed in a gas stream which is at 200 degree Celsius. Make calculations for junction diameter needed for the thermocouple to have thermal time constant of one second. Assume the thermos-physical properties as given below k = 20 W/ m K h = 350 W/m2 K c = 0.4 k J/kg K p = 8000 kg/m3 Correct Answer 0.656 mm

T = p V c/h A = p r c/3h. So, r = 3 h T/p c = 0.000328 m = 0.328 m.

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