Sam, sorry about the delay. Been busy, and we're off again in a few minuts.
The bad news is that your system will leak. Plan on it, and expect it. Its like painting a room, and trying not to get any paint on yourself. Just slap the brush on yourself, and stop worrying about it. 4 mil is somewhat thin for the plastic, and will develop leaks even faster. The leaks don't have to be very large to have a major impact on the system. A grain of sand on the concrete under the plastic, and you walk/set something on it, and there's a pinhole. You can still draw a good vacuum, but the idea of turning the pump off is probably not going to work. We don't need anywhere near high vacuum, so its not a problem. You can always try getting a large round
tank, and use it as a resevoir. Just remember they are designed as a pressure vessel, not vacuum, so care in use/choosing it is important.
My setup used two pumps. I tried out a nice high vacuum pump, but I found out it only has 1cfm flow. I then hooked up our ejector/pump setup that I made for a project several years ago. It worked fairly well, except for not catching the leaks during the run. The ejector pump system draws around 20cfm at can do 28.?" of vacuum. Its a very big brother to the water aspirator that I posted earlier. I can post pictures when we get back, if you're interested.
I really like Andrew's sealing material. Now to find some. Once a vacuum is developed, the plastic will seal to itself, so going overboard on edge sealing isn't necessary. My main leak was the vacuum tube inlet. Duct tape didn't provide an adequate seal.
You're correct in the carpet breathing layer. I was going for cheap, and readily available. I would like to come up with a list of parts/materials that would let anyone do this with minimal cost/hassle/knowledge. I would use this method rather than the platten method. With a flexible breathing layer, you are applying pressure to the plywood, getting it to conform to the foam/glue combination. As long as it on a flat surface, you end up with a flat product. With the plywood platten, you have to bend that, as well as your structural plywood to get a good glue joint. Which brings up anther point, that I've been thinking about...
...Pressure required to get a good glue joint goes down with the thickness of the plywood. Earlier in this thread, someone mentioned full vacuum is needed, but that is probalby with 1/2" plywood. If you go all the way down to 1/8" plywood, I think you would be supprised at how little pressure/vacuum you need. Your 2.5psi shop vac should be great.
Try the ball valve, and let me know if the system can maintain a vacuum. The lower the vacuum (uhmm less inches/lower psi?) the less problems you'll have with leaks. If your good/lucky, your system will maintain the vacuum for a useable amount of time.
A good industrial shop vac should be able to run continously for the entire run, as long as it stays cool. The motors in these things use the air they're drawing to provide cooling, so if you draw a vacuum for too long, they can burn up. You can use your ball valve to allow some bypass air into the vac to provide cooling. Its a trade off between max vacuum, and vac temperature. Even at 1psi, your taling about 144 lbs of pressure on the plywood. More than enough for 1/8", and probably enough for 1/4" plywood.
I hear pacing downstairs, and Anns going to kill me if we don't leave NOW. I'll follow up this weekend some time.
I have never seen a comercial setup, and am coming to this from the "get it done in the driveway for minimal cost" point of reference. Both the ag bag and sheet polyethlylene plastic sheets I've used have been fairly soft, so they did form a decent seal. Not perfect, however. I'm impressed with your former bosses bagging.
ZZZZZZZZZZZ!!!!!!