A long constantan wire of 1-mm diameter is butt welded to the surface of a large copper block, forming a thermocouple junction. The wire behaves as a fin, permitting heat to flow from the surface, thereby depressing the sensing junction temperature Tj below that of the block. Required:
a. If the wire is in air at 25 C with a convection coefficient of 10W/m^2âK, estimate the measurement error (TjâTo) for the thermocouple when the block is at 125 C.
b. For convection coefficients of 5, 10, and 25W/m^2, plot the measurement error as a function of the thermal conductivity of the block material over the range 15 to 400W/mâK. Under what circumstances is it advantageous to use smaller diameter wire?