Pressure vs Vacuum

Atmospheric pressure is roughly 14.7 PSI at ground level.

Removing as much air as possible from the bag will give you a maximum pressure on the bag of 14.7, if you could remove all the air from the bag, which you can't.

You have about 12.7 pounds of atmospheric pressure on the outside of your bag if your gauge is reading 2PSI.

Now after you have evacuated as much air from the bag as possible, you can add compressed air to your chamber to whatever degree is required. Depends on how much pressure you want to exert on the bag.

So to make the pressure the same as in the bag with about 2 lbs above ,I would have to put 15 psi in the box.

someone did this and tried to explain this on a website how he did it.He said that he "Vented the bag" meaning to make the both pressures the same, Then increased the outside air pressure because if their were any air bubbles that they would be Smaller.

John

To me "venting the bag" means that he is not using vacuum to extract the air from the bag, but rather just using compressed air pressure on the outside of the bag to drive the air inside the bag out. This will reduce the diameter of bubbles, as stated. This is how injection molded parts are produced with no VISABLE viois.

It will not remove the bubbles though, as using vacuum AND compressed air will. Using both gives you a better product, because the bubbles are gone, not just compressed.

Ahem! Sea level maybe?

I used the word "roughly" with guys like you in mind.

An autoclave is not usually a simple box, but instead a pressure vessel.

For many purposes, a vacuum bag is enough to get rid of all air bubbles and to compact laminations, etc. The big advantage of vacuum bagging alone is that the air pressure is applied everywhere, meaning that you can exert a lot of force without having to have (for instance) a large heavy press. If your differential is 13.7 psi (achievable) then a one square foot piece will have 144 x 13.7 lb of force on it: about a ton.

This points up the difficulty in using a box as a pressure vessel. If you pressurize the box, it will bulge, placing very large stresses on the edges and corners and surrounding areas. Your 48 x 36 panel would have 17280 lb of force on it if pressurized to 10 psi. You really need a pressure vessel if you are hoping to use any pressure substantially above atmospheric.

holy smokes, thats Alot of pressure.

Forgot one thing but dosen't make any difference, their is heat in the box to make a temp of about 150 deg F. this is to melt the EVA

tks Guys.

holy smokes, thats Alot of pressure.

This is the kind of statement that gets the physics police riled up. That's a lot of "force". The pressure is pretty low (10 PSI) but the force is high, because the pressure is acting on so much area.

The pressure difference between the underside of a 747 wing and the top side is less than 1 psi. The lift force, however, is enourmous.

"This points up the difficulty in using a box as a pressure vessel."

Agreed. Most autoclaves are round for that reason.

For those into ancient engineering history, the "Autocave" held Patent No. 3, issued to Fred Flintstone (who used it to smoke T-rex steaks).

(No. 1--Prometheus--fire; no. 2--Thag--wheel.)

(who used it to smoke T-rex steaks).

I tried that. Couldn't even get the slightest buzz.

John

The vacuum draws the atmospheric air out of the solar panel material and vacates the bag (assuming no leaks) to practical vacuum levels of 28 to 29 inches of Hg aka --13.75 to -14.25 PSI. With extra cost equipment and extra time you can get even closer to a perfect vacuum. The initial vacuum no doubt causes the air in the heated panel material to out gas and vacate.

Initially atmospheric pressure outside the bag is forcing the bag to collapse and hug the panel inside. When you close the box and add positive pressure it will compress or further collapse squeezing the volume from all nooks and crannies. The vacuum and the pressure are acting in the same direction to squeeze the maximum dead space out of bag so minimal air remains inside the bag.

When you close the box it will have one atmosphere inside, approx. 14.7 (some use 14.5 but it actually depends on the barometer that day and elevation). At this point the box has neutral force balance inside and out.

When you add pressure to the box, assuming it is sitting on something solid, the lid has 1728 square inches of area. For each 1 psig (pound per square inch) the force on the lid from within will be 1728 pounds. The sides and ends will have 960 and 720 lbsf respectively. Please be cautious or use a heck of a strong box.

Tom

Autoclave is a sophisticated machine used in hospitals to sterilize the infected instruments. It is steam operated. Process consist of pre treatment , treatment and post treatment. If you want to remove only air bubbles why cannot you use vacuum pumps instead of autoclave machines to avoid the complicated system.

Might be worth a look here for ideas.

Also look at this, from a previous thread: http://cr4.globalspec.com/comment/651810