I don’t know how to calculate vacuum. I will give a little work problem with two slight variants.
I have a sealed pipe that is 10 ft long with a 4" diameter ( About 12.5 sq inches) At the top of this pipe I have a foot long sealed shaft that weighs 42#. If I let this slide down the pipe I should have about 3.36# pressure on one end of the pipe minus the vacuum holding it up and ??? vacuum on the other end.
The other scenario would be that the pipe had a vent at the bottom and the shaft was allowed to slide down until vacuum held it up. How much Vacuum would that be?
Atmospheric pressure is about 15 pounds per square inch. Which is way more than enough to hold up your weight, being pulled down at 3.3 PSI. So I don’t think the weight will fall at all, whether the bottom is sealed or open. (Assuming the top actually is sealed, and the weight forms a seal with the tube, both of those seals capable of holding in the range of 15 psi.)
I may be misunderstanding the problem, but I know vacuum does not exert a force.
What you apparently have is a sealed pipe with a 42# weight in it that is free to move. You tilt the pipe and the weight slides down until the air pressure equals the weight. As I understand the issue (42/12.5 = 3.34 lbs/in2 which << 14.7 lb/in2) the weight won’t move at all. The ambient air pressure in the pipe will more than support the weight (ideally). That weight would have to be >184.3 lbs (W>=AP*Area)) for it to move assuming no leakage and no friction.
I look forward to the smart folks around here explaining the problem better than this very old physics major can.
A more direct question would be how much downward force on the sealed shaft to create a 2# atmosphere on the low side? Suppose I was using two cyl’s of the same size connected by a tube at the top. One was just for the weight and the other was a low pressure chamber.
1 bar is roughly 1 atm and is roughly 15 psi (absolute)
So 2 bar is 2 atm or 30 psi (absolute)
psi is also expressed in gauge and absolute. So the standard atmospheric pressure is 15 psi absolute or 0 psi gauge. And 30 psi absolute is 15 psi gauge.
So do you want 2 atm pressure which is also 30 psi gauge or 15 psi absolute ?
Or do you want 2 psi gauge which is 17 psi absolute ?
I want about 1/7th of atmospheric pressure or. So I guess I want 2 psi absolute. So I need to know how much downward force on the piston I would need to lower the pressure that much.
Now I am thinking it might just be a little less than 30# per sq inch?
Let us presume a perfect seal above the weight: 0 bar. (This does not change when the chamber above the weight changes in size).
So now we just have to calculate the pressure required to hold up the weight.
Pressure is expressed in N/m2. To go from N to kg you divide by g (9.81:: let’s round that to 10)
So now we have a generalized formula:
Your mass M (in kg)
The area of your Pipe A(in m2)
The pressure in the chamber below the mass p (in Pa: 10000Pa = 1 bar)
p = 10*M/A
Let’s plug in your numbers:
M~19 kg (42# is pounds?)
A~0,00806 (12.5 sq.inch)
p = 23573Pa or 2.4 bar absolute
In the other scenario your plunger would never stop. (Take a syringe and seal it with your finger; after the initial pull you do not have to pull harder to increase the size on the vacuüm. The force required to pull out the plunger stays the same)
The weight relative to the diameter is enough to create the vacuum.
Any fault in your seals will create a 1:1 increase in pressure under your weight. If you only get a 90% vacuum (0.1 bar) the pressure below the weight will be 0.1 bar higher.
Are you trying to vacuum dry your bows ?
I am asking because this kind of arrangement won’t work. You are much better off buying a cheap vacuum device like a car brake bleeder or more expensive vacuum pump.
I will do the calcs anyways when I get home but hope you understand that the piston in the pipe thing won’t work.
I wasn’t thinking I would get a vacuum as much as just lowering the atmospheric pressure? If it won’t work I won’t even bother. We are constantly having discussions on different things we can try and this just came to mind. Wouldn’t require any power.
HoneyBadgerDC - the volume increase of water from liquid to vapor is about 1600. So one drop of water when evaporated will give you 1600 drops equivalent of volume. So the piston above will have to constantly remove this vapor to maintain the vacuum (which it cannot if it stays in one place)
Also vacuum is very very prone to leaks. If power consumption is a concern, you can hookup a solar powered vacuum pump.
@HoneyBadgerDC: what are we trying to accomplish here? It feels like you are working on something different than the highschool physics problem in the OP. What is the vacuum for?
It’ll definitely require power, no matter how you do it. You have to be able to open it up to put pieces of wood in it, which means you have to be able to cycle it, and cycling it will require power in at least some stage of the cycle (lifting up the weight, say).
And I was wondering how this was going to get back to bowmaking.
A lot of times after building a bow we like to fast dry it for a competition if they happen to be a little high on moisture. As a rule just baking some rice and putting a bow and dry rice in a sealed tube will work pretty well in just a few days. Most of the time you are only looking to drop the moisture another 2 points or so. The dry wood performs better.
HoneyBadgerDC : vacuum drying is a great idea. The wood dries faster, needs less temperature and warps less too. However, you do need to use a vacuum pump like this one : https://www.harborfreight.com/25-cfm-vacuum-pump-61245.html
Alternatively, In place of rice, you can also use silica gel beads which is easy to regenerate in home oven or microwave. Like this one :
Just for fun, we had anhydrous liquid ammonia in some chemical (refrigeration ) plants. We would put wood sticks in the ammonia and we could dry and bend them in odd shapes very quickly.
What I found interesting with the ammonia bending is that a wood like bloodwood that will not usually allow us to bend it with heat or steam readily bends easily with the ammonia treatment. That is something we need to really look more closely at as we have several varieties of good bow wood that we can’t bend.
