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On 11 Dec 2003 16:10:49 GMT, (Karl Denninger)
wrote:

In article ,
Steven Shelikoff wrote:
On 11 Dec 2003 05:36:39 GMT, (Karl Denninger)
wrote:

In article ,
Steven Shelikoff wrote:
On 11 Dec 2003 03:04:00 GMT, (Karl Denninger)
wrote:

In article . net,
Rick wrote:
Karl Denninger wrote:

(It will pull a near-zero-zero vacuum..... as will the fuel pump,
in fact.
I've seen the fuel restriction gauge showing effectively a zero
vacuum when
I foolishly started the engine without remembering to turn on the fuel
valves first....)

Cringe ... zero vacuum is one atmosphere, one Bar, or around 14.7 pounds
per square inch absolute, it is no vacuum at all.

The most complete vacuum you can produce will still only create a
pressure differential of 14.7 pounds per square inch at sea level, or
one atmoshphere.

Rick

"zero-zero" in my view means "absolute vacuum", or -14.7 psig.....

It would probably be better for you to define an absolute vacuum as 0
psi instead of -14.7 psig. Your way depends on your altitude, the
barometric pressure.

Steve

That only works if you have a gauge calibrated in psia instead of psig

Most gauges are calibrated in psig...

Then they're not giving an accurate absolute pressure reading if the
pressure of the atmosphere is not 14.7 psi. So you can't use one to
tell you whether you have an absolute vacuum unless you know exactly
what the atmospheric pressure is. That's why you can't define an
absolute vacuum in terms of psig unless you know what the reference is.

In other, more simple words... A reading of -14.7 psig only tells you
that the pressure the gauge is reading is 14.7 psi less than the
atmospheric pressure. It tells you nothing about the absolute pressure
you are reading. To know whether you have an absolute vacuum, you have
know the absolute pressure.

If you've defined an absolute vacuum as -14.7 psig and you're reading
that measurement at an atmospheric pressure of 14.71 psi, you still have
some gas in there so it's not an absolute vacuum and your definition is
wrong.

Steve

True.

SOME gauges are referenced to atmospheric, some are sealed units

Those would read psig.

(referenced to whatever atmospheric was when they were manufacturered) and

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.

Steve