Thermometers have some inherent errors due to the challenges in measurement from a moving aircraft. In our earlier post, we had looked into thedesign and construction of TAT Probe.
INSTRUMENT ERROR
Despite notable improvements in manufacturing techniques, all instruments would have a certain amount of manufacturing imperfections. Instrument errors are caused due to manufacturing imperfections which can be reduced by use of better manufacturing technology.
ENVIRONMENTAL ERROR
Environmental errors originate from environmental factors like solar heating of temperature sensor when aircraft is parked in direct sunlight. Heating error can be overcome by shielding the probe with suitable covers. Ice accretion is another form of environmental error which can be reduced by using heating element. The error due to heating element is taken care of during calibration.
KINETIC AND ADIABATIC HEATING ERRORS
Heating errors are caused due to movement of aircraft through the medium which is air. These errors results in indication of higher than prevailing temperature. Obviously, heating errors increases considerably at higher speeds. Heating errors are classified into kinetic and adiabatic heating errors. Kinetic heating error is caused due to friction between the temperature probe and air molecules.
Due to the movement of aircraft, the number of molecules which hit the probe every second is higher than that of stagnant air. This causes an over-reading of temperature outside the aircraft. The friction error is maximum in Direct Reading Temperature Probe.
On the other hand, Adiabatic Heating is caused due to compression of air due to the energy created by the movement of air being brought to rest rapidly by the probe. Adiabatic heating error is high in Remote Indicating Temperature probe.
In this post, we have learnt about the errors in temperature measurement and the methods to reduce these errors, In our next lesson, we shall understand about the terms like SAT, TAT and RAT. SAT stands for Static Air Temperature, RAT for Ram Air Temperature and TAT for Total Air Temperature.