A spherical thermocouple of 2.5 mm diameter is used to measure the temperature of air flowing in a pipe. Initially both the thermocouple and the air are at a temperature of 300C. The air is heated to a temperature of 2350C and maintained at this temperature. Find the time required for the thermocouple to reach 2000C. Also find out the time constant of the thermocouple and comment on the suitability of this thermocouple to measure unsteady state temperature. For thermocouple material take : density = 900 kg/m3, specific heat = 0.4 kJ/kg K and thermal conductivity = 30 W/mK. 30