I see that mistake so often that I just automatically think inches Hg.
Any decent vacuum pump should be able to reach 25 to 28" Hg in a really
well sealed bag but a perfect seal is very hard to get. While
laminating plywood requires as much vacuum as you can get most glass
bagging is done below 15" so pump volume is usually a lot more important
than ultimate pressure.

hugh wrote:

"William R. Watt" wrote in message
...

There is a website describing how to use a discarded 'fridge compressor
for vacuum bagging. The fellow who tried it used a vaccum gauge from an
automobile in his configuration and claims he got 25 psi vacuum for his
project. He just used standard plumbing fittings to connect the vacuum

bag.



hahaha.. good one. he claims he got 25 psi vacuum.....

do not pass go, do not collect $200. go to jail.

hugh

p.s. my old engineering profs would fail you the whole year for saying
somthing like that in any individual exam....



--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Glenn Ashmore" wrote in message
newsMQkb.80431$sp2.38130@lakeread04...
I see that mistake so often that I just automatically think inches Hg.
Any decent vacuum pump should be able to reach 25 to 28" Hg in a really
well sealed bag but a perfect seal is very hard to get. While
laminating plywood requires as much vacuum as you can get most glass
bagging is done below 15" so pump volume is usually a lot more important
than ultimate pressure.


true. the mercury thing can confuse people. i'm living in SI-land so a
unit like psi is already wierd enough. if you started talking about inches
of mercury here people would start looking up the address and phone number
of the nearest loony-bin. 'twould be like using a unit like a furlong per
fortnight or a slug/acre or something.... just unbelievably arcane and
difficult. ah well. us SI people owe the french for something....

still, every time i see that mistake i laugh because our engineering profs
did have a habit of trying it out on students and if you went for it you
were in BIG trouble.

hugh

p.s. i also always wonder if maybe people are vacuum bagging composites on
venus or half way down the atmosphere of jupiter.

Hugh,

How about laughing to yourself next time and explain to the guys that
didn't go to engineering school why it's wrong. No need to be rude.

Regards, Ron

Standard atmospheric pressure is a bit over 14 PSI so that is as much
pressure as a vacuum can produce. The first proven vacuum was produced
by Evalgelista Torricelli, Galileo's assistant, by filling a glass tube
with mercury and turning it upside down. The mercury dropped until the
weight of the mercury in the tube equalled the pressure of the air
outside. That was about 30". Torricelli almost got in a lot of trouble
about this because the Pope had decreed that there was no such thing as
a vacuum so he sent the idea to a Frenchman named Pascal to work on. For
many years we have happily used the height of the mercury to describe a
vacuum. We and the English used inches and everyone else used
milimeters. Then the French and Itallians conspired to change
everything at an international standards conference and we ended up with
Torr as the official measure. Being Itallian the unit had to be totally
irrational so a Torr is 1/760th of an atmosphere.

To add to the confusion the "Bar", (short for barometer) was used to
describe one standard unit of atmospheric pressure so vacuums are
measured in fractions of a bar called millibars. Also the French did
not like for an Itallian to get any credit so in an evil pact with the
British, they came up with the Pascal to describe one atmosphere of
pressure. To get the British to go along the Pascal was based on a unit
named after an Englishman, the Newton, but to keep the English in their
proper place they made the Pascal equal to a million Newtons.

The thing is, the measurement of "standard Atomspheric pressure" that
they used to set the actual values was off a bit so all of these units
are not quite accurate. That is why I still use inches of Mercury. :-)


Ron Thornton wrote:

Hugh,

How about laughing to yourself next time and explain to the guys that
didn't go to engineering school why it's wrong. No need to be rude.

Regards, Ron


--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

let's see. vacuum is an absence of atmosphere, or a presence of nothing but
room for there to be something. the earth's atmosphere applies a pressure
of approx 14.5 psi at sea level, with the pressure varying a little
depending on the weather. thus, an absolute vacuum has approx 14.5psi less
pressure than one feels at the surface of the earth. a vacuum bag with zero
pressure inside is 14.5psi lower than outside. thus it is impossible - in
earth terms at least - to talk of 25psi of vacuum. if one lived on jupiter,
one could talk about 25psi of vacuum and if the guy with the claim lives on
jupiter and is making composites there then i'll stop laughing and be
amazed.

as for laughing, the reason the engineering school profs got so upset with
this mistake is that they regarded it as something fundamental that should
have been learned at high school. so do i. it's kinda like wandering
around the world without knowing that light travels faster than sound or
that shakespeare was english. you know - basic stuff.

hugh

p.s. and i wasn't rude. i was laughing.



"Ron Thornton" wrote in message
...
Hugh,

How about laughing to yourself next time and explain to the guys that
didn't go to engineering school why it's wrong. No need to be rude.

Regards, Ron

William R. Watt ) writes:

claims he got 25 psi vacuum for his

right. change "psi" to "lb".
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Check out Harbor Freight. They sell a vacuum pump driven off
an air compressor for about $10. It is probably listed on
the web site as an automotive A/C part. I was in one of
their stores, it is packaged as an A/C evacuation pump.


John

"William R. Watt" wrote in
message ...
William R. Watt ) writes:

claims he got 25 psi vacuum for his

right. change "psi" to "lb".
--
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--------------------
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free community network
homepage: www.ncf.ca/~ag384/top.htm
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or it's returned

Those will work on a small bag for a few short sessions but they are
hard on a small air compressor when you need to hold a vacuum for
several hours. You would need to add a resin trap, tank, check valve,
solenoid and vacuum switch to save the compressor.

jcassara wrote:
Check out Harbor Freight. They sell a vacuum pump driven off
an air compressor for about $10. It is probably listed on
the web site as an automotive A/C part. I was in one of
their stores, it is packaged as an A/C evacuation pump.


John

"William R. Watt" wrote in
message ...

William R. Watt ) writes:


claims he got 25 psi vacuum for his

right. change "psi" to "lb".
--
----------------------------------------------------------

--------------------

William R Watt National Capital FreeNet Ottawa's

free community network

homepage: www.ncf.ca/~ag384/top.htm
warning: non-freenet email must have "notspam" in subject

or it's returned



--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

I just use 2 shop vacuums. Actually, one is a vacuum cleaner for
cleaning out pellet stoves. I've done small things with just one
regular shop vac I bought at Wal-Mart. The thing is to get a good
seal from your bag. You need lots of duct tape.

Ed.













(William R. Watt) wrote in message ...
There is a website describing how to use a discarded 'fridge compressor
for vacuum bagging. The fellow who tried it used a vaccum gauge from an
automobile in his configuration and claims he got 25 psi vacuum for his
project. He just used standard plumbing fittings to connect the vacuum bag.

I do not have the website address. I was browsing on a computer at the
public library at the time and did not write it down. However you can
find it by going to www.boat-links.com, clicking on "The Mother of All
Boat Links", clicking on the first box "amateur boatbuildign and repair"
and scrolling down untill you find the link about vacuum bagging.

Nowadays all discarded 'fridges go to a recyling depot where they "vacuum
bag" the freon gas to protect the ozone layer of the atmosphere. It should
be easy to get a compressor by telephoning the miniciapl garbage
department. Since compressors do wear out it would be a good idea to look
for ones off later model 'fridges (the date of manufacure should be on a
label on the 'fridge) bring home 2 or 3 to try.

The motors of most shop vacuums are cooled by the air that flows through the vacuum parts. If there's no
flow of air through the hose, there's none through the motor and the motors burn out quickly. A very few
shop vacuums (Fein and ?) have 'bypass' fans which cool the motor even if there's no flow through the hose.

Ed Askew wrote:

I just use 2 shop vacuums. Actually, one is a vacuum cleaner for
cleaning out pellet stoves. I've done small things with just one
regular shop vac I bought at Wal-Mart. The thing is to get a good
seal from your bag. You need lots of duct tape.

Ed.

(William R. Watt) wrote in message ...
There is a website describing how to use a discarded 'fridge compressor
for vacuum bagging. The fellow who tried it used a vaccum gauge from an
automobile in his configuration and claims he got 25 psi vacuum for his
project. He just used standard plumbing fittings to connect the vacuum bag.

I do not have the website address. I was browsing on a computer at the
public library at the time and did not write it down. However you can
find it by going to www.boat-links.com, clicking on "The Mother of All
Boat Links", clicking on the first box "amateur boatbuildign and repair"
and scrolling down untill you find the link about vacuum bagging.

Nowadays all discarded 'fridges go to a recyling depot where they "vacuum
bag" the freon gas to protect the ozone layer of the atmosphere. It should
be easy to get a compressor by telephoning the miniciapl garbage
department. Since compressors do wear out it would be a good idea to look
for ones off later model 'fridges (the date of manufacure should be on a
label on the 'fridge) bring home 2 or 3 to try.