Fzx in Action - Rotary Pressure Gauge

A friend gave me this gauge to disassemble.

With some finger pressure from back to front, the bezel and plastic window were removed.  The two screws from the back were then unscrewed to expose the surprisingly complex inner workings.

The gray goo sealing the top of the air shaft  is apparently flexible enough to allow the copper ring to pivot.  I tested it with the workings exposed and the air pressure pushing on the enclosed portion of the copper ring turns the ring, pulling in the direction of the pointing screwdriver, and, finally, rotating the needle.

One millimeter of motion at the left end of the ring is approximately equal to 30 psi so it's a distance multiplier.  The tiny motion at the end of the air shaft is magnified by the copper ring lever (3rd class) which then pulls on another lever attached to a pivoting gear which then turns the gear on the needle shaft, resulting in rotation of the needle.  The wound spring returns the needle to zero.

There is no pressure relief valve so you have to hold the gauge on the valve while reading it.  Factory calibration probably consists of bending something to get a good reading - I see no easy way to do it.

Now we know.  And knowing, unfortunately, is far less than half the battle.

EDIT - On closer inspection, that round copper tube may be air filled and pressure actuated like one of these party whistles.  With pressure, it unwinds.

May God always grant me the courage to admit that I'm often wrong.