The TAT sensor (Total Air Temperature sensor) is designed to operate aerodynamically in accordance with MIL-P-27723E. The TAT sensor has two platinum RTDs located inside the central flow channel inside the strut. The elements are physically separated in the flow channel such that a failure of one element cannot induce a failure in the other. The following is the resistance versus temperature relationship for the TAT sensor:
- RT/RO = 1+ α[T - δ(T/100-1)(T/100) - β(T/100 –1)(T/100) 3]
- T = temperature in °C
- RT = resistance at temperature "T"
- RO = resistance at 0 °C (50 ohms)
- α = .003925
- δ = 1.45
- β = 0.0 for T > 0.0° C and 0.1 for T<0.0 °C
The static calibration accuracy of the TAT sensor is: ±(.25 + .005 X |T|) where “T” is temperature in °C.
The TAT sensor can operate in high-speed airflow conditions – the following equations are the theoretical relationships governing compressible air flow:
- Equation 1: Mach = [5 X ((PT / PS).2857 – 1)]1/2
- Equation 2: TS = TT / (1 + .2 X Mach2)
Mach = Mach Number
- PT = Total Pressure
- PS = Static Pressure (or Outside Air Temperature)
- TT = Total Temperature in
- TS = Static Temperature in
(Kelvin = Celsius + 273.16)
NOTE: The static temperature (TS) is often referred to as Outside Air Temperature (OAT).
Equation 2 would be used to determine the value of TS based on TT. However, in the real world, there is a repeatable error associated with the TAT sensor measurement - therefore, the static temperature TS can be calculated using a correction per the following equation:
- Equation 3: TS = TM / (1 + ϒ X .2 X Mach 2)
- TM = Measured Temperature in
- ϒ = Recovery Factor
For TAT sensors, the AVERAGE value for recovery factor is ϒ = .975.
Substituting Γ = .975 into Equation 3 yields the following:
- Equation 4: TS = TM / (1 + .195 X Mach 2)
For best accuracy, use Equation 4 to solve for TS based on the measured value of from the TAT sensor
(TM). Further improvements can be realized by wind tunnel testing an actual sensor to quantify the exact calibration errors and recovery factor. For practical purposes, it can be assumed that the recovery factor variation from the nominal value above will be within the range '.960 < ϒ < .990.