Direct Reading Thermometer is the simplest form of thermometer used to measure Outside Air Temperature. In our earlier lesson we had learnt about the Units of Temperature Measurement.
BI METALIC STRIP
Direct Reading Thermometer measures temperature using the principle of unequal expansion of a Bi-Metallic Strip. If we heat a metal strip in which two metals with different coeffecient of expansion are attached, the strip would bend in the direction of metal with low coefficient of expansion. Normally, Invar and Brass are the alloys used in the Bi-Metallic Strip. This thermometer is then calibrated to read the temperature outside the aircraft.
Direct reading thermometer is simply fixed on the wind shields of low speed aircraft. In the case of high speed aircraft, when the air is suddenly brought to rest by the aircraft, the temperature tends to increase adiabatically. Therefore, this type of thermometer is unsuitable at higher speeds due to adiabatic heating caused by the high speed movement of aircraft.
Having understood about the Direct Reading Thermometer, let us discuss about the Remote Indicating Thermometer in our next lesson. Small and low speed trainer aircrafts use the Direct Reading Thermometer, while the higher speed aircrafts used by airliners use the Remote Indicating Thermometer.