The unit you have is a 50,000 foot Kollsman altimeter. It could have easily been as old as the late 1940s. The mechanism itself is known as a 5760 and it is the only unit that I have worked on that has the slot on the link where it meets the rocking shaft. Other Kollsman units were pinned with just a hole in the link. The "struts" on the diaphragm assembly are part of a temperature compensation feature. The "U" shaped piece that goes around the diaphragm is actually a bi-metal and will expand and contract. The expansion and contraction will act on those "struts" and slightly move the diaphragm to and fro depending on temperature.

The counterweight is to compensate for tilting the instrument--like rolls, or rotating that entire housing to set the calibration The bellows are not mass-less and with this instrument being so sensitive you have to account for the bellows flexing under gravity.

Thanks for posting this. I never appreciated until seeing this how an altimeter is basically just a precision micrometer that measures changes in the length of the aneroid chamber. If the altimeter is sensitive to a change in altitude of 10 ft, then that roughly corresponds to a 10 micron change in the length of the chamber, which is the size of a blood cell. Impressive clockwork!

Minor nitpick, it's not really the air pressure, but the "altimeter setting" which is the corrected air pressure for that altitude/temperature etc. The ATIS info at an airport gives this value not the actual air pressure. For Denver even though it's at over a mile high, it's altimeter setting for a "standard atmosphere" would be 29.92" of mercury just as it would be at a sea level airport on a standard atmosphere, while it's air pressure would be much lower at something like 25".

That beard's lookin good on ya man. Digging the camera work and editing on this, the new setup seems nice.

The bellows are sealed, this is confirmed when you press down on the bellows and the needle goes backwards, indicating nearer the ground. Sealed bellows, sealed enclosure, more pressure in enclosure = higher pressure = lower to ground

This is my first use of B-Roll on my own :)

Would it be brass or bronze mix so it's self lubricating ?

Avionics generally refers to radios and navigation equipment, This is an Instrument, like airspeed,Turn and bank indicator. The slot in the arm would allow for the barometric pressure adjustment, I believe. The 3rd hand would indicate over 10,000 feet 20k, 30k etc. Brass is used for corrosion resistance.

Brass and bronze, my guess, at least in part, because they're non magnetic and won't mess with the compass.

I almost shit a chicken when I saw Square D.

Ok so I'm guessing that the whole assembly is geared to be zeroed, the material choices are for low mass and flexability?

The production quality has improved since the TGG days...

Please tell me that you checked to see if it's a radium dial before opening it up. Old luminous dials can be pretty hot, and shed little tiny radium particles which do unhappy things to your lungs.

Found this same (apparent) model on the Smithsonian's website and lists the company as Square D Company, Kollsman Instrument Division airandspace.si.edu/collection-objects/sensitive-altimeter-square-d-company-kollsman-instrument-division

Some swiss lathe operator was eating good on this one haha.

Dang Chris that Beard looks nice on you!