does vacuum equate to pressure?

[tapkoote]

I'm a retired hobbyist, with a basic under standing of math, science, physics

I want to glue a thin wood veneer on to deck of a radio controlled hydroplane. I want to clamp it with a clear plastic vacuum bag system. the hull is giber glass but basically hollow. The pump will produce 5 "hg, just under 2 1/2 pounds per square inch. this is the part I can't get my mind around. The hull is 50" X 24", lets say 1200 square inches. At a half pound per square inch, am I applying 600 pounds of force to all sides of the hull?

Thats like having two great big guys stand on my little boat.

I've tried other forums, hope you can help.

Tap

 

Otto von Guericke, (1602-1686), a German physicist, born in Magdeburg, performed a famous experiment: the "Magdeburg Hemispheres".

These were two halves of a large, hollow metal ball. When all the air was sucked out of the ball, two teams of eight horses couldn't pull them apart, because the pressure of the Earth's atmosphere created a very large force on the ball.

You may have seen a smaller version of this experiment in school.

[swansont]

You're applying 600 lb of force, but not to all parts of the hull. You are applying .5 lb to each square inch. The concept is the same as why snowshoes work. Snow can't support 200 lbs in the cross section of a boot, but it can support 200 lb if that area is increased by a factor of two or three; the force on any section has gone down.

[John Cuthber]

It depends on how the gauge is calibrated but the force might be a good deal more than that.

What does the gauge read when there's no vacuum? is it about 0 or about 30?

 

Normal air pressure is about 30 inches of mercury.

If you reduce the pressure to 5 inches then there is a 25 inch difference- that's 25/30 times atmospheric pressure.

Normal air pressure is about 15 PSI so 15*25/30 =12.5 pounds per square inch.

1200 square inches of area means 1200 * 12.5 =15000 pounds of force. That's not a couple of big guys standing on it, that's the weight of a couple of small trucks parked on it.

[tapkoote]

It depends on how the gauge is calibrated but the force might be a good deal more than that.

What does the gauge read when there's no vacuum? is it about 0 or about 30?

 

Normal air pressure is about 30 inches of mercury.

If you reduce the pressure to 5 inches then there is a 25 inch difference- that's 25/30 times atmospheric pressure.

Normal air pressure is about 15 PSI so 15*25/30 =12.5 pounds per square inch.

1200 square inches of area means 1200 * 12.5 =15000 pounds of force. That's not a couple of big guys standing on it, that's the weight of a couple of small trucks parked on it.

 

thank you John

I'm sure these guys use gauge pressure

because they speak of holding 25 to 28 inches for 4 hours on model air plane wings and the pump I want to use isn't capable of more than 5 inches hg

I'm sure if they used absolute pressure it would crush the styrofoam

Thanks for the help

Pat

PS think I'll test it on a coffee can first

[John Cuthber]

Gauge and absolute pressure are a bit unwieldy when you are measuring vacuum.

Do you actually have a vacuum gauge?

[tapkoote]

John

here is one article I subscribed to.

http://www.rcgroups.com/forums/showthread.php?t=1321258

Since this just a one time use I want to keep cost down, I know of a guy bagging hulls for carbon fiber

his vacuum system alone is over $1200

there are a few who get about the same gauge reading from these pumps

I'm in the process of locating the material I need

Thanks again

Tap