Air Temperature Measurement — Page 32, Lesson 38

Right, let's pick this up with the heating error. As aircraft speed increases, the measured temperature exceeds that of the still air. This is caused by a combination of kinetic, or friction, heating and adiabatic, or compression, heating. Let's take kinetic heating first. Kinetic heating is a contributor to the temperature rise effect in the Direct Reading thermometer. As the speed increases, more and more molecules of air per second impact against the flat plate at the surfaces of the probe. This causes a temperature rise due to friction. Now adiabatic heating. Adiabatic heating is the main contributor to the temperature rise effect in the Remote Reading, or Total Head, thermometer. It's caused by a conversion of energy, not a direct application of heat. For example, boiling a kettle is a direct application of heat, whereas pumping a bicycle pump causes a rise in temperature without any input of heat because pressure energy is converted into temperature energy. In the Remote Reading temperature probe, the outside airflow, which may be several hundred knots, is brought virtually to rest in the platinum measurement chamber very rapidly. The energy of the moving air is released in the form of a temperature rise. There is a known relationship between the relative speed and the temperature rise. The laws of thermodynamics are such that the combination of kinetic and adiabatic heating will always add up to a figure known as the Total Ram Rise. Unfortunately, any measurement process always has leaks and inefficiencies, so we do not measure the full ram rise. The amount of ram rise actually sensed is called the Measured Ram Rise. If we could measure the Total Ram Rise, we would measure the Total Air Temperature. However, in practice, what we measure is the Ram Air Temperature, which is lower. Finally, we have Static Air Temperature, sometimes written as TS. SAT is the temperature of the undisturbed air through which the aircraft is about to fly.