cheap vacuum bagging
 

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

cheap vacuum bagging
 

Thanks Glenn,

I already knew, but obviously others did not.

I have never ever seen Glenn intentionally post anything in the least bit
"rude." Unfortunately others would most likely not say the same for me :-)

Fred

"Glenn Ashmore" wrote in message
news:zeSkb.80436$sp2.58104@lakeread04...
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

cheap vacuum bagging
 

hugh wrote:
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.

I would think that post curing would be a lot easier on Jupiter. :-)

--
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

cheap vacuum bagging
 

From what GA wrote it would be better to look for an older unit than a
later one as I assumed. It could be tested with a car vacuum guage. A
vaccum guage costs about $15 at Canadian Tire. I found one at a garage
sale last summer for $4 which included a timing light and a remote starter
switch.

As for burnout maybe the compressor doesn't have to run at full power.
Perhaps a light dimmer switch could be used to set it at the speed needed
to maintain the vacuum for the particular application. Or 2-3 compressors
could be hooked up together and run at lower power off the same dimmer switch.



--
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homepage: www.ncf.ca/~ag384/top.htm
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cheap vacuum bagging
 

The problem is lubrication. Refrigerators circulate oil with the
refrigerant to keep the compressor lubricated. When you use it as a
vacuum pump there is no oil. You can extend the life a little by
squirting a little WD40 in the intake before each session but that is of
limited value when the compressor runs 4 or 5 hours at a time.

I suspect that the older compressors last longer because they are built
"looser". As refrigerators got more energy efficient the compressors
have been built to closer and closer tolerances so constant lubrication
became more critical.

I made a high vacuum system back in science fair days out of an old
pre-WWII Kenvinator compressor and used it for several projects. When I
started doing woodwork I used it to bag veneers for several years. When
it finally died I tried several newer compressors but none lasted more
than a couple of sessions. They would run for 10 or 15 minutes and then
the thermal switch would kick them off. Ruined a lot of expensive
veneer trying to figure a way to keep them running.

There are at least 50 vacuum pumps on eBay right now that would be ideal
for vacuum bagging. All under $100 and many under $50. Not a bad price
considering that if a refrigerator compressor craps out in the middle of
a session you could ruin that much material or more.

Here are a few that would work.
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=11 773
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=26 236
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=26 236
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=46 547
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=46 548
Here is a perfect setup complete with flow guages and tank:
http://cgi.ebay.com/ws/eBayISAPI.dll...ategory=73 21



William R. Watt wrote:
From what GA wrote it would be better to look for an older unit than a
later one as I assumed. It could be tested with a car vacuum guage. A
vaccum guage costs about $15 at Canadian Tire. I found one at a garage
sale last summer for $4 which included a timing light and a remote starter
switch.

As for burnout maybe the compressor doesn't have to run at full power.
Perhaps a light dimmer switch could be used to set it at the speed needed
to maintain the vacuum for the particular application. Or 2-3 compressors
could be hooked up together and run at lower power off the same dimmer switch.



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

cheap vacuum bagging
 

The other problem is cooling. The freon and oil charge carry heat away
from the compressor which is probably why Glenn's tripped the thermal
switch. Before I got a "real" vacuum pump I used refrigeration
compressors successfully by cooling with a fan and constantly
lubricating thru a bleeder on the vacuum side with one of those air line
oilers made for lubing air tools. WD40 is not a very good lubricant for
this, I used light oil and sometimes even thinned motor oil with
kerosine. I also had an old airbrake compressor off a bus that I drove
with a 1 hp electric motor. I spite of the physics, I believe I could
have pulled 25 psi with it (maybe thru a worm hole from Jupiter). On
the output side she would put out 300 psi. All of this was back when
vacuum pumps were a lot more than $50 on Ebay.

Regards, Ron

PS. Fred, re-read my earlier post please. You will find it was
not directed at Glenn.

cheap vacuum bagging
 

Ron Thornton ) writes:

... I spite of the physics, I believe I could
have pulled 25 psi with it (maybe thru a worm hole from Jupiter). On
the output side she would put out 300 psi. All of this was back when
vacuum pumps were a lot more than $50 on Ebay.

another PSI supporter, "Pressure Scale Inches" (of mercury).
--
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cheap vacuum bagging
 

William R. Watt wrote:

Ron Thornton ) writes:


... I spite of the physics, I believe I could
have pulled 25 psi with it (maybe thru a worm hole from Jupiter). On
the output side she would put out 300 psi. All of this was back when
vacuum pumps were a lot more than $50 on Ebay.


another PSI supporter, "Pressure Scale Inches" (of mercury).

Oh, NO! Not another measure! We have enough already! ;-)

--
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

cheap vacuum bagging
 

"Glenn Ashmore" wrote in message
news:rr0lb.80765$sp2.32669@lakeread04...


Oh, NO! Not another measure! We have enough already! ;-)

Yes, but how many barnyard atmospheres would that be?

(both are recognized as official measurements)

cheap vacuum bagging
 

"Ron Thornton" wrote in message
...
I spite of the physics, I believe I could
have pulled 25 psi with it (maybe thru a worm hole from Jupiter).

no ron, you couldn't pull 25psi with it, no matter what you believe. the
amount it put out on the high pressure side won't help you there.

hugh

p.s. having worked on a bunch of different projects over the years i'm
surprised to see people going to so much trouble trying to make - as they
say - a silk purse out of a sow's ear. experience has told me many times
(and keeps doing so whenever i try to cheat it) that the cheapest way of
doing something is to buy the right equipment for the job rather than trying
to "make do" with something else. as glenn already said, it's also a good
way to ruin a lot of expensive materials.