# A little math help

Discussion in 'Home Power and Microgeneration' started by No One, Jan 1, 2004.

1. ### No OneGuest

Ok I think my numbers are correct but I'd like others to look over them. As

I want to make some vacuum devices using pvc pipe. I'm trying to get some
rough numbers before I start. Here's what I've come up with. Correct or am
I missing something?

I'm looking at 1/2", 3/4" and 1" pipe.

My known/assumptions are:

pi*r^2=area area*14.7 psi = max vacuum

For the:

1/2" pipe. . . . .14.7 * [3.141 * (0.25 * 0.25)] = 2.88
3/4" pipe. . . . .14.7 * [3.141 * (0.375 * 0.375)] = 6.49
1" pipe. . . . .14.7 * [3.141 * (0.5 * 0.5)] = 11.54

My biggest question is how long of a pull do I need to get the max vacuum.
I'm thinking I'll just have to find this by trial and error.

2. ### Bert MenkveldGuest

Hello No One,

Your calculations look fine to me. How much of a vacuum you actually get
depends on the ratio of volumes. Presumably you intend to insert some kind
of plunger inside the PVC pipe, push it down near the bottom of the pipe
with the end open, then seal up the end and pull up the plunger to create a
vacuum inside the pipe.

To calculate how much vacuum is developed, you need to know the volume of
air left under the plunger at its lowest point. For simple numbers, let's
say your plunger has a flat face, and you leave exactly 1" of space below
the plunger at its lowest point. If you now seal up the end of the pipe and
pull the plunger up to 2" (ie. 1" higher than its lowest point) you have
doubled the volume below the plunger. Since the same amount of air remains
in this doubled volume (assuming there are no leaks), the pressure will be
cut in half, for an effective vacuum of 14.7/2 = 7.35 PSI.

Similarly, if you pull the plunger back another 1" up to the 3" mark, the
vacuum will become 14.7/3 = 4.9 PSI.

Make sense yet? I would think the hard part is getting things sealed. Good
luck!

Happy new year.

3. ### Bob PetersonGuest

doesn't matter what the pipe size is, the maximum vacuum you can get is
still 14.7 psi because that is atmospheric pressure.

4. ### No OneGuest

Ok if I got this I take the ratio of the orig to final air column and
multiply that by the area of the plunger THEN by 14.7 to get close to the
vacuum.

Using your example of a 1" to 3" air column with a 1" pipe:

1 / 3 * 14.7 * (3.141 * .5^2) = 3.85

5. ### Bert MenkveldGuest

Yep, that's right (in pounds of force).