Thank you wikipedia, it led me to this:
http://www.enotes.com/how-products-encyclopedia/aneroid-barometer
and quoted from there:
Design
Product design for an aneroid barometer involves a careful analysis of the contracting and expanding properties of the aneroid capsule, design of the temperature compensation system, and mechanical design of the linkage between the aneroid capsule and the sweep indicator.
The aneroid capsule is very thin, hollow, and usually shaped like a bellows. Most of the air is removed from the capsule so that the contraction and expansion of the capsule is strictly a function of the elasticity of the capsule and any of its supporting springs. Leaving air in the capsule would induce non-linearity into the capsule response. As the capsule contracted, if there were any air left, the air pressure in the capsule would rise, which would make further compression of the capsule harder. The barometer designer calculates how much the aneroid capsule will expand or contract under the expected range of pressures the barometer will be subjected to. Based on these movements, the designer specifies the linkages that will translate the movement of the capsule into the movement of a sweep indicator on the barometer face.
and
The aneroid capsule
- 5 Thin sheets of copper/beryllium metal (around 0.002 in [0.05 mm] thick) are stamped into the two halves of the aneroid capsule. The stamping dies are designed to leave a knife edge mating surface where the two halves will be joined.
- 6 The individual aneroid components are electron-beam welded. Electron beam welding requires that a concentrated stream of electrons be generated and focused on the joint to be welded. As the electrons collide with the part, the kinetic energy of the collision creates heat resulting in the fusion or melting of the two pieces to be joined. Electron beam welding can only be performed in a vacuum (because the air molecules would intercept the electron beam) which is very convenient as the aneroid capsule must also be free of air. Electron beam welding is performed by automated robot welding machines because a human welder could not provide the degree of accuracy needed to join the parts without damaging them.
So, yes, it looks like a popped aneroid expands. And it also suggests both aneroids in the ALDA are under negative pressure.
And when one pops, the IP pin gets pushed down... and you get a dog.