In chapter 8, about altimeters:
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf
A stack of sealed aneroid wafers comprise the main component of the altimeter. An aneroid wafer is a sealed wafer that is evacuated to an internal pressure of 29.92 inches of mercury ("Hg). These wafers are free to expand and contract with changes to the static pressure.
“Evacuated to 29.92 inches of mercury”? (In units we understand, that would be 1013hPa – inside the aneroid, which literally means “without air”). Really? That makes no sense!
I thought aneroids in altimeters and barometers were evacuated to as close as a vacuum as you could get, and the air pressure measured by how much they oppose the spring that’s preventing them from collapsing. This article: http://aviationknowledge.wikidot.com/aviation:sensitive-altimeters would seem to agree with that.
alioth wrote:
I thought aneroids in altimeters and barometers were evacuated to as close as a vacuum as you could get, and the air pressure measured by how much they oppose the spring that’s preventing them from collapsing.
If everything except the ambient air pressure was constant, it would work just as well with any air pressure inside the aneroid. Of course everything else isn’t constant — in particular the temperature is not. Air inside the aneroid would expand or contract with changes in temperature. Obviously that won’t do in an altimeter, so I think you are right about aneroids being evacuated to as close to vacuum as possible
Isn’t the temperature inside the Aneroid basically the same as outside the aneroid, thus mostly avoiding issues due to temperature diffs?
I always thought inside the aneroid was standard pressure
If the chamber was fully evacuated, it would immediately collapse totally (“totally” being limited to how much the berylium copper structure can deform) under any real atmospheric pressure, and would be unable to sense the required pressure variations.
Hence I think it must contain some gas. Then, the structure will deform either side of a relatively neutral shape.
I know a guy who makes barometrically activated devices (e.g. for the various parts of ejector seats) and he should know, but he’s not on email and I am away on holiday 
The aneroid is held open by a leaf spring so it doesn’t collapse.
The thing is “aneroid” means without air, so it doesn’t make sense that it would have standard atmospheric pressure, and every other source I’ve found that describes an aneroid literally says it contains a vacuum. Plus the PHAK statement “evacuated to 29.92in Hg” literally doesn’t actually make sense – it’s contradictory: it can either be evacuated, or it can be at standard atmospheric pressure – not both!
Some other sources:
From the book “JAR Professional Pilot Studies”:
Inside a sensitive altimeter, are two aneroid capsules (vacuums) which are corrugated for strength and kept open with a large leaf spring
(If the aneroid really did have standard atmospheric pressure, there would be no need to keep it open with the leaf spring)
From britannica.com’s article on altimeters:
Two or more aneroid capsules—i.e., thin corrugated metallic bellows from which air has been exhausted—are positioned near the inlet. These capsules expand when the outside air pressure falls (as in climbing) and contract when the outside air pressure rises (as in descending). By a mechanical arrangement of sector gears, pinion, backlash spring, and crankshaft, the expansion or contraction of the aneroid capsules is converted to the movement of pointers on a dial.
From Flying magazine, December 1975, page 53:
The aneroid itself is made from two thin, corrugated discs, about two-and-one-half inches in diameter, which look like miniature cereal dishes. These are joined with an airtight seal and then evacuated. Some manufacturers join the discs by electron-beam welding, thus creating an aneroid capsule automatically, since the welding process requires that the work be done in a vacuum.
The last one even describes a manufacturing method that results automatically in the aneroid being at vacuum. The contradictory statement “evacuated to 29.92inHg”, plus all of this other stuff, leads me to believe that the FAA PHAK is talking nonsense on the subject. That and pretty much any generic description of an aneroid barometer mentions that the aneroid is at very low pressure or a vacuum.
Actually it must be a spring, because if you used a gas to inflate the chamber, or there was any gas inside it, the reading would be hugely sensitive to the temperature inside the instrument.
I now recall my altimeter maker friend telling me that it is the springness of the beryllium copper bellows itself which he uses in his instruments.
While in principle it makes no difference, it is more practical to have the aneroid capsules sealed at a given reference pressure (not necessarily vacuum) and the outside (ie the instruments casing) varying. Hence an altimeter must be airtight.
The corrugation on the suface of the aneroids is not so much for strength as to improve linearity.
