The venturi vacuum generators work much like a simple lawnmower carburetor. Air passes through a restriction inside the carburetor at high speed, creating a low pressure vortex just beyond the restriction. In a carburetor this low pressure area draws in fuel through a small tube that mixes with the air flowing through the carburetor. With these Vacuum Venturi's , it's not fuel that's drawn in, but air coming in from your vacuum chuck, veneer vacuum bag, etc. Unfortunately, you either have to add a valve to close the vacuum line or keep the compressed air flowing constantly in order to have vacuum, and any tiny air leak in the vacuum line or vacuum chuck will let air into the cavity and you will lose the vacuum that you had created.
Vacuum is just the absence of air. Suck on an empty soda bottle and you create a vacuum in the soda bottle. Let air get past your tongue into the bottle and the bottle quickly fills up with air, and the vacuum goes away. Stick a suction cup on a window and if the seal is good, the suction cup will stay there, because the air around the suction cup is bushing the suction cup against the glass. If the glass is dirty, a tiny air leak will let air into the suction cup to a point where the air inside the suction cup is equal pressure to the air outside the suction cup. The vacuum (lack of air) that had been inside the suction cup is gone since air has gotten in. Once the air is equal pressure inside and outside the suction cup, the suction cup will fall off of the window.
Ok, now use the vacuum pump to remove all the air from the vacuum chuck. If there is no board on the chuck, air will continue to replace the air in the vacuum chuck as fast as the vacuum pump can remove it. Place a board, piece of glass, or metal on the vacuum chuck and the vacuum pump will work to remove the air from between the boarb, glass, or metal that is sitting on the vacuum chuck. If the seal between the vacuum chuck and the item placed on it is good, you can close the line to the vacuum pump and the item will remain stuck to the vacuum chuck, but if there is a tiny air leak in the vacuum chuck or vacuum line, air will leak in and take the place of the vacuum. As soon as the air pressure inside the vacuum chuck equals the air pressure outside the chuck, the board, piece of glass, or metal will fall off of the vacuum chuck.
Now, as long as your vacuum pump can remove all of the air from the vacuum chuck faster than air can leak in, your board, piece of glass or metal will remain stuck on the vacuum chuck. Turn the vacuum pump off, and shortly after the board, piece of glass, or metal will fall off.
Air is a fluid, just like water, and at sea level the air pressure around us weighs 14.7 pounds per square inch. It's pushing on us and everything around us with that much weight. Our bodies have air and fluids within us that counteracts this pressure, so we don'r get squished because the pressures (weight) is equal.
With a vacuum (absence of air) on one side and air at 14.7 psi on the other, the weight of the air will push toward the vacuum, trying to fill that space and balance the pressures (weight).
To demonstrate this, If we take a thin container, like a 1 gallon paint thinner can that is completely empty of it's original contents, and we put about 1" of water in the bottom of ithe container. Then we put it on the stove burner and heat the water up until the water boils heavily. The steam created will drive out all of the air inside the gallon can and only steam and a little water will remain. Once the boiling water has filled the can completely with steam, except for the little water remaining, remove the can from the stove and screw the lid on tight, sealing the steam and water inside, but also sealing the outside air from getting in.
As the can cools, the steam will also cool and return to a water state. Since no air can get into the can, a vacuum will form inside the can and the outside air surrounding the can will crush the can significantly. Why? Because the air around us and the can has weight and there is no longer an equal weight inside the can to keep it from collapsing under the weight of the outside air (a vacuum has been created inside the can).
Vacuum is just the absence of air or anything else. A suction cup remains stuck to a window because the air pressure is pushing on the outside of the suction cup and there is no air pushing back from the inside of the suction cup. The air outside is pushing the suction cup against the window.
The size of your vacuum pump will be determined by how large the inside volume of your vacuum chuck is, plus the inside volume of the line connected to the vacuum pump plus the amount of air leakage through the pores of your board and any leaks in the seals of your vacuum chuck and vacuum lines. Also, how fast do you want to remove all of the air from the inside of your vacuum chuck.
If you have a perfect world situation with no leaks at all, a very tiny vacuum pump could be used, but life and the things around us just aren't this way. No seal is perfect, nor is any wood so solid that air cannot leak through the fibers. So my recommendation is, the bigger the vacuum pump is, the more likely you will be able to suck the air out of the vacuum chuck faster than it's getting in. If you can do that with your vacuum pump it will hold the board to to the chuck very well. If you can't afford a big vacuum pump, you will be working hard to minimize the vacuum leaks so your smaller pump can attain an adequate vacuum in the chuck to hold the board in place.
Sorry, this is the best answer that I can provide. I hope that I have not confused anyone further. I've tried to explain it as simply as I can. Go try the gallon can trick with the steam to see for yourself, but make sure to boil the water long enough to drive out all of the air, leaving only steam inside, and get the cap on tight. The better you do this, the more crunching of the can you will get to see.
Charley
