The temperature of a gas flowing through a pipe was measured by a mercury-in-glass thermometer, dipped in an oil-filled steel tube welded radially to the pipeline. The thermometer indicates a temperature lower than the gas temperature. How large is the error in the temperature measurement if the thermometer reads 85 0C and the temperature of the pipe wall is 40 0C? The steel tube is 125 mm long and has a 1.5 mm thick wall. The thermal conductivity of this tube material is 56 W/m-K and the local heat transfer coefficient between the gas and the tube is 23.5 W/m2-K.
In what way the thermometric error can be reduced?