How does the aneroid wafer expands? (ASI) So, how does it expand? (aneroid wafer - intend to the airspeed indicator) I have two “theories” (from what I’ve learned) which are :


The ram air entering through the pitot tube physically hitting, pushing the aneroid wafer forcing it to expand - giving us the actual indication in the cockpit. faster airspeed—> more air hitting, pushing  the aneroid wafer at higher velocity —> greater  expansion?
The ram air rapidly filling the aneroid wafer making the air more dense —> more pressure, therefore forcing it to expand due the pressure differential between the aneroid wafer and the sealed case, which contain the outside atmosphere pressure. Faster airspeed —> more air —> increasing pressure inside the aneroid wafer —> larger pressure differential—> greater expansion?


Which one is true ? or both maybe ?
The aneroid wafer is just a metal balloon.  Add air into its interior, above ambient pressure, and it expands.

The outside of the wafer is exposed to ambient static pressure.  The inside of the wafer is plumbed to the pitot inlet.  Ram air into the pitot raises the pressure within the pitot plumbing to above ambient, which being connected to the interior of the wafer, inflates it.  The movement of the inflating wafer is geared to the airspeed indicator and voila, ya got yer speed indication.