William R. Watt wrote:


still the vacuum on the suction side of a vacuum pump is not limited by
atmospheric pressure, even when connected to a vacuum bag set up for resin
curing. all it means is the pump is creating a vacuum greater than
atmospheric pressure and could be run at a lower speed. imagine a vacuum
pump strong enough to suck the resin, hull, and all right into the pump.
can't do that at atmopheric prssure.

That is why a lot of carbon spars are vacuum bagged inside a
pressureized and heated autoclave. Supprising how much you can squish a
carbon fiber layup at 100 psi. :-)

--
Glenn Ashmore

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William R. Watt wrote:

still the vacuum on the suction side of a vacuum pump is not limited by
atmospheric pressure, even when connected to a vacuum bag set up for resin
curing.

Well, actually it *is* limited by atmospheric pressure unless you
have the bag in a pressurized environment. Evacuate the bag to 0
psia/bara/mm hg/pa/torr/microns (whatever absolute units you want
to use) and the pressure differential between the bag interior and
exterior is simply the ambient pressure. In open air, that's
atmospheric pressure.

all it means is the pump is creating a vacuum greater than
atmospheric pressure and could be run at a lower speed. imagine a vacuum
pump strong enough to suck the resin, hull, and all right into the pump.

Imagine is all you *can* do unless you find an alternate motive
force besides the atmospheric pressure. Sans such motive force
(e.g. pressurized chamber), 14.7 psia is all you have to work
with, on a good day (well, you *do* have bag mass and acceleration
due to gravity, but that works for you on the top side, and
against you on the "bottom" side).

can't do that at atmopheric prssure.

Exactly.

Keith Hughes

"William R. Watt" wrote:

James Johnson ) writes:

Maybe he means 25 inches of vacuum, now is it inches of water or mercury (big
difference between the two)? Since atmospheric pressure at sea level is 14.7
psi there is know way he is pulling 25 psi vacuum unless he has it in a tank
pressurized to 10 psi above atmospheric.

yes, that would 25 inches of mercury. that is what is measured by the car
engine vacuum guage. if you car engine vacuum guage is 14.7 you have some
expensive engine work in your immediate future.

still the vacuum on the suction side of a vacuum pump is not limited by
atmospheric pressure, even when connected to a vacuum bag set up for resin
curing. all it means is the pump is creating a vacuum greater than
atmospheric pressure and could be run at a lower speed. imagine a vacuum
pump strong enough to suck the resin, hull, and all right into the pump.
can't do that at atmopheric prssure.

William R Watt

Can't do that with even a perfect vacuum. All you get is one
atmosphere of 'suction', which cannot ever actually exist in a
pneumatic system. Now, an hydraulic system, where the pump
evacuates a liquid, can pull harder than an air pump, up to the
point where the liquid boils, providing there is not one tiny bit
of gas in the system, which would cause the vacuum 'suction' to
max out at less than one atmosphere. No such system is possible.

In a mercury tube barometer, a little dissolved gas released by
the vacuum from it's imprisonment as a solute in the mercury, if
there is any fills the 31" space at the top of the tube at some
low pressure, low enough that the barometeric pressure outside
the tube presses hard enough to suspend the mercury.

So, pressure forming is used in applications where more than one
atmosphere of 'squish' is required. Explosive pressure forming
can provide hundreds of atmospheres of 'squish.'

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