Karl Denninger wrote:

The reason I raise this in relationship to pressure gauges is that there
are "psig" gauges that are indeed sealed. Scuba pressure gauges are an
example - if they were open to ambient then salt water would corrode the
bejzeezus out of them, so they aren't, yet they still read in "psig".


Karl, those gauges are not "sealed" in the sense of having the bourdon
tube within a pressure capsule. They are sealed and oil filled only for
the purposes of keeping water out of the mechanism. They are subject to
ambient pressure through the oil filling. If they were accurate enough
and had a wide enough scale you would see that they do respond to sea
pressure as they display the differential between tank pressure and sea
pressure.

On our manned deep diving submersibles we placed pressure gauges inside
the pressure hull to read the contents off oxygen and air tanks located
outside the pressure hull. Otherwise they pressure displayed would drop
with increasing depth as any bourdon tube gauge only reads a
differential across the tube.

Rick