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On 12 Dec 2003 01:36:58 GMT, (Karl Denninger)
wrote:

In article ,
Steven Shelikoff wrote:
On 11 Dec 2003 16:10:49 GMT, (Karl Denninger)
wrote:

Those would read psia and the calibration of the scale would depend on
what the atmospheric pressure was when they were sealed.

some are referenced to absolute vacuum.

The latter are pretty rare; you certianly don't seem them in your local
welding or hardware store.

Referencing it to absolute vacuum is the same thing as a sealed unit
with a known absolute pressure sealed in. You only have to change the
scale to whatever you want. You could make is read 0 at whatever
absolute pressure you want, either 0 psi (absolute vacuum) or 14.7 psi
(to simulate psig even though it isn't really psig) or whatever.

That's only true if the scale is calibrated or even extends to the right
place (e.g. there is no "pin" at zero in the case of a gauge that reads
psig as opposed to psia)

Well, sure. The point is that if the gauge has a sealed reference, it's
reading an absolute pressure no matter what it's calibrated to. If the
calibration is off, the reading will be wrong. But it's still reading a
wrong "absolute" pressure. That is, the reading (correct or not) will
not change as the atmospheric pressure changes.

If the reference it open to the atmosphere, it's reading a gauge
pressure, or relative pressure, no matter what it's calibrated to. If
the calibration is off, the reading will be wrong. But it's still
reading a wrong "gauge" pressure. That is, the reading (correct or not)
will change as the atmospheric pressure changes. Unless, of course, the
needle is at the stop and has to go further in that direction.

Steve