Weight Versus Vacuum for Veneering Pressure

A little rough math about how high a stack of MDF you would need to match one atmosphere's worth of vacuum. November 14, 2009

Question
I have a 6'8" door in a bag that is pulling 21" HG. My friend came in and said he would have just stacked 3/4" MDF on it to do the same job. How many sheets of MDF would it take to attain the same force as 21" HG?

Forum Responses
(Veneer Forum)
From contributor G:
I feel mathy this morning. I'll take a stab at it.

If your door is 32" wide, 80" X 32" = 2560 sq in which at normal air pressure of 15 lb/sq. in. receives 38,400 lb of pressure on its area.

At 21" of vacuum, it would get 2560 X 21 = 53,760 lb total pressure.
53,760 – 38,400 = 15,360 lb

If a 3/4 sheet of MDF weighs 70 lb, you'd need 15,360 / 70 = 219.4 sheets

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From contributor T:
We're gonna need a few more sheets. 21" hg will net maybe 10.3 lbs/inch at sea level. There's more, but we'll let others play.

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From contributor J:
I can't follow contributor G's math, but I agree it would take an impractically large amount of MDF, not to mention one heck of a forklift to place it where you need it.

21" HG is about 10.3 PSI, so a 32" x 80" door in a vac press is receiving more than 26,000 pounds of pressure, or the weight of around 375 sheets of MDF (at 70# each). Does your friend stack a 40' tower of MDF on his projects?

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From contributor D:
I just love it when science meets myth. It always wins. And this from a guy that told his high school math teacher he would never need math since he would never be so stupid as to have a career that requires it. Duh'oh!