Almost done with my home-made vacuum press. Anyone else done one?

Okay... so I'm almost done with my home-made vacuum press. I was surprised at how easy it was to make, actually.
Anyway, I've got my home-made bag, the pump, and pressure switch all hooked up. As a test, I put my bathroom scale in the bag and turned on the pump. The pump was starting to top out at 250 lbs on the scale (and 100 sq. in. on the scale comes to a paltry 2.5 lbs/sq.in.), which isn't fantastic... but, over the 2'x4' surface that I'm planning to veneer, it's probably more clamping pressure than I could get with my clamps.
Anyway, I'm going to start checking the bag for leaks and sealing the vacuum connections better and then I hope to get up closer to 7-10 lbs/sq.in. We'll see.
My *problem* is this: I've got a pressure-actuated switch hooked up to the system, with the notion that I can get the thing to pump down to a certain vacuum and then shut off the pump... and then turn the pump back on when a certain amount of air leaks back in.
In theory, it's great. In practice, it doesn't work so well. When I start getting down to the target vacuum, each stroke of the vacuum pump creates a momentary vacuum surge which triggers the vacuum switch... which cuts the power to the pump for a moment, and the surge subsides, and the vacuum switch turns back on, etc. What this sounds like is like an engine running out of gas. The pump starts slowing down... starts "running rough" (even though it's electrical) and just keeps running slower and slower and never does get to the point where it shuts off completely.
My plan, at this point, is to build a little electronic circuit which will keep the pump going for a minute or so past the point where the vacuum sensor first triggers, so that I'll get well past the trigger point of the vacuum sensor. Then, the motor can shut off completely and it won't turn back on until the vacuum drops to the point that the sensor turns back on again.
Now, before I go and build this circuit, has anyone else made their own vacuum press? If so, how did they get around this problem? Did you just skip the vacuum switch and let the pump run constantly all night? What does the real VacuPress do?
- Joe
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On 6 Aug 2004 16:03:56 -0700, snipped-for-privacy@emenaker.com (Joe Emenaker) wrote:

sounds like the reverse of an air compressor switch....
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I don't know what you're using for a vacuum pump but most pumps used to evacuate a refrigeration unit can run for hours without any damage to the pump. Are you trying to avoid too much pressure on the bag or too low a pressure inside the bag? Why not just let it run? You might look into a vacuum pressure regulator that would bleed air into the system at a certain vacuum level. Most systems such as you describe have enough leakage that a pump will short cycle unless you have a good sized reservoir.
rhg
snipped-for-privacy@thanks.com wrote:

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Joe Emenaker wrote:

That is the wrong way to do it. Most things like that have two switches for an upper and lower limit. Let say your target is -7 psi. You would set the lower switch for -7 psi and the upper switch at -11 psi. The pump pumps until it is -11 and then quits. But it doesnt start back up until it goes down to -7. Does this make sense?
-Jonathan
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Somebody said it...reservoir. A tank (not all will take vacuum, might collapse, will smooth things out a lot. A quart or so would probably make a difference. Wilson

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A quart might smooth things out. A 20# propane cylinder (Bar B Que tank) will smooth them out more and the things are available used cheap. Also, a throw away cylinder used for refrigerant should be free from your local air conditioning technician.
bob g.
Wilson wrote:

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Cupla things Joe. First is that you need a switch with a greater ON-OFF differential. That can be done with two switches, but many switches have an adjustable differential and that would make things easy. Second, you can lessen the surge effect by adding a reservoir. That could be any kind of air tank, an old 20lb propane tank would work fine. You vacuum bag will have very little volume, and thus is sensitive to surges from your pump. The snubber tank will ease those surges and moving your switch away from the pump and closer to the tank will help as well.
As for leaving your pump running all the time, that depends on your pump design. Some will overheat if they are dead- headed. You need the specs on your pump to know that. You can allow a bleeder valve to allow a little air to flow all the time but that will also lower the vacuum level. A switch would be better all around instead of running for 24 hours. I am very interested in your project. How about some photos and more info when you get it working?
-- Bill Pounds http://www.billpounds.com/woodshop

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[...]

Vacuum pumps? I have never seen any vacuum pump which was switched of when the desired vacuum level was reached, they were kept running until they broke down after a few years or the regular service interval was reached. On the other hand, these were *real* vacuum pumps, not just "air thinners". And a plastic bag is a "huge leak" when it comes to vacuum applications, let alone the wood inside and the glue with it's water.
--
Dr. Juergen Hannappel http://lisa2.physik.uni-bonn.de/~hannappe
mailto: snipped-for-privacy@physik.uni-bonn.de Phone: +49 228 73 2447 FAX ... 7869
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On Sat, 07 Aug 2004 15:58:33 +0200, Juergen Hannappel

That's because you're posting from a physics lab, with real pumps. If you're using something like a Gast diaphram pump, or even worse an old fridge compressor, then it's useful to switch it.
--
Smert' spamionam

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What vacuum level we wanting here? I think somewhere around 29" will provide all the clamping pressure you'd want. If you want to get down into microns, two things to consider. A plastic bag probably isn't going to seal well enough to achieve this kind of vacuum. I've spent a lot of time chasing leaks and applying vacuum wax in a lyophylizer circuit. Doesn't take much of a leak to keep you from getting water evaporating levels of vacuum. If you want to dry the glue with the vacuum you'd want a cold trap in the vacuum circuit like a commercial lyophylizer, I'd think. My experience with vacuum is in refrigeration and lyophylization so talking about a bag veneer press, I'm trying to extend from experience to theory.
bob g.
Juergen Hannappel wrote:

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Right. The term I was going to use was "hysterisis", but I figured that would make most readers' eyes glaze over.

Upon reading the specs for the switch I got, the company makes one with two switches in it. Mine isn't one of them. Also, the switch is bulky enough (about the size of a grapefruit) such that I don't want to have two of them in this rig.

Thought of that, too.... but, like I said, I'm trying to keep the bulkiness down.

My first try was with a 110V pump taken out of a FoodSaver, and it would get pretty warm even after about a minute. My second pump, this spiffy German 24V one (rated to something like .03 atmospheres) runs much cooler. I might let it go for an hour and see how it does.

Well, I've already designed the circuit and simulated it with a circuit design program and it looks like it's going to work (and it only uses about $20 of parts from Radio Shack). I might build it anyway just for the nerd value. I'm concerned that I'll have enough leakage into the bag, however, that the pump will end up running most of the time, anyway.

Will do. When I have it working acceptably, I'll post the construction details on my webpage where I have my home-made T-square fence.
- Joe
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You need a ballast chamber - a sealed rigid vessel connected into the system, so that the total volume is much bigger. Assuming your leak is small, this will allow far longer between the need to re-pump. Use something like an old CO2 cylinder - a litre or two is usually adequate..
It will take some extra time to pump the ballast chamber down, so you might like to fit a valve onto the bag side of the circuit. Rather than just plugging the pump into the bag and switching on, this lets you pump the ballast chamber down first, then switch the vacuum circuit.
You shouldn't need two switches. The effect described is called hysteresis, and most air pressure switches should have some of this built in.
--
Smert' spamionam

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Remember, the most powerful vacuum pump imaginable with produce a pressure differential between inside and outside your reservoir of about 15 psi.
bob g.
Andy Dingley wrote:

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Robert Galloway wrote...

...assuming the setup is located at or below sea level. Best I can get in my shop is around 13.5 psi -- on a good day. Still, it's a helluva lot of pressure for larger surface areas.
Cheers!
Jim
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Lots of pressure on the veneer press. Not a lot to worry about when considering what to use for the reservoir.
bob g.
Jim Wilson wrote:

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On Sat, 07 Aug 2004 11:55:45 -0500, Robert Galloway

and that's at sea level. here at 2500 feet it'll be less...
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