From 737NG SDS for aspirated probes: 30-30-00-035
ICE AND RAIN PROTECTION - PITOT AND STATIC - TAT PROBE -
FUNCTIONAL DESCRIPTION
Functional Description The total air temperature (TAT) probe anti-icing system uses electric power and resistance-type heating elements.
The system uses 115v ac and 28v dc power. The probe heating element uses 115v ac power. The current detection circuit uses 28v dc power.
On the ground, put the control switch to the ON position. This energizes relay K1 and stops 115v ac power through the current detection circuit to the probe heater. The amber TEMP PROBE light does not come on.
Push the TEST switch (with control switch in ON position) to deenergize relay K1. This lets 115v ac power through the current detection circuit to the probe heater. If the probe heater does not use current, the circuit causes the amber TEMP PROBE light to come on.
In flight, relay K1 remains deenergized.
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The Low voltage is for excitation and signal generation. The high voltage is for heat.
In an aspirated probe the bleed air is to ensure a continuous flow of air through the probe and across the sensor.
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Then there's SDS: 34-21-00-023
ADIRS - TOTAL AIR TEMPERATURE PROBE
General
The total air temperature (TAT) probe measures the air temperature outside the airplane. An opening in the front of the sensor lets air flow through and around the sensing elements and exit through ports in the rear of the probe.
Sensing Elements
There are two sense elements in the TAT probe. Each sense element is a resistive element. The value of resistance changes when the temperature of the air flow across the element changes.
Bleed Air
The TAT probe gets bleed air from the APU duct in the keel beam. See the bleed air chapter for more information. (CHAPTER 36) Bleed air into the probe makes a negative pressure inside the probe. The negative pressure pulls outside air across the sensing elements. This permits accurate temperature measurement when the airplane is on the ground or moving at low speed.
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I can't find a thing in any of the manuals avaliable to me that would satisfactorily explain the reading that generated the original question.....gotta love this business
