EGT PROBES ARE IN APU EXHAUST DUCT.SO THEY ARE NOT CONCERNED ABOUT AMBIENT TEMP
But, the exhaust duct is open to ambient @ -56C also...
EGT is temp above ambient.... not above cabin temp. So the cold junction must be outside pressurised area or electronically compensated for ambient. T.C's work because of a difference in temp between the hot junction (in the turbine) and the cold junction (could be anywhere) a voltage is generated proportional to the difference and displayed in temp. So when both hot & cold junction are at the same or similar temp, there will be only fractions of a microvolt being generated. Induction from other A/C systems may now affect the circuit and give false readings.
