Selection of thermocouples

1)  Type of furnace;  whether batch or continuous and the frequency of measurement.

2)  Furnace atmosphere: The furnace atmosphere may be oxidizing or reducing, inert or vacuum. Accordingly    thermocouples are selected. For example Pt, Pt-Rh can be used in oxidizing and inert atmospheres up to     . Chromel -alumel thermocouples may be used in reducing atmosphere but at low temperatures.

3)  Response of thermocouple to temperature difference is important. Normally thermocouples are inserted in a     ceramic sheath. During temperature measurement, the hot junction of the thermocouple is heated by the     transfer of heat from sheath. Also large diameter of wire requires sufficient time for heating.

Illustration of gas temperature correction

Consider the measurement of temperature of gas by a thermocouple. When a thermocouple in a protection tube is inserted in gas or liquid, the outer surface of the tube receives heat either by convection or by convection+radiation.

Consider a thermocouple with an emissivity 0.75 measures the temperature of gas flowing in a duct whose wall temperature is 593 K. A thermocouple indicates 833 K. The forced convective heat transfer coefficient between the gas and the thermocouple is   . What would be the actual temperature of gas?