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#11
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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 |
#12
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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 |
#13
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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 |
#14
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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. -- ------------------------------------------------------------------------------ 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 |
#15
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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 |
#16
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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. |
#17
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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). -- ------------------------------------------------------------------------------ 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 |
#18
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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 $50 on Ebay. another PSI supporter, "Pressure Scale Inches" (of mercury). Oh, NO! Not another measure! We have enough already! ;-) -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com |
#19
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cheap vacuum bagging
"Glenn Ashmore" wrote in message news:rr0lb.80765$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) |
#20
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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. |
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