Off Topic Hobbies > Mechanical Engineering

Putting vacuums in series (pneumatics)

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Capernicus:
So if you put two air pressure vessels in series,  its still the same psi.  (pressure/resistance= air flow.)

But if you put two vacuums in series (I mean just vessels,  not pumps/motors.) - is it the same psi - or do they add?

If you were pulling on a vacuum, the more you pull it the more force it has against you pulling?  this is what I'm trying to figure out,  but I dont have anything to test it with,  but I do have a 3d printer,  but I was wondering does someone have the answer for me already?

TimFox:
When you pull on a piston, where there is a partial vacuum on the other side, the force you are pulling against is the difference in air pressure from your side to the lower pressure on the vacuum side of the cylinder (i.e., the "gauge pressure" of the vacuum chamber).  However, if the vacuum seal is good, and no air leaks around it, all you are doing is pulling against the atmospheric pressure.  If there is a gas sealed inside, it acts as a spring with an approximately linear Hookes' law force where the extension is proportional to the net force.
If you connect that cylinder to the piston of a second identical cylinder, I assume the force you exert splits evenly on the two pistons.
Can you draw exactly what you mean?

Capernicus:


Thanks so much for the help,  I just need something simple cleared up.

My idea is,  having 2 evacuated vessels in series just counts as a longer cylinder,  so it would be more force?

But 2 vessels with the same high pressure, only count as 1!  (no force increase.)

TimFox:
In your drawing, with only one “vacuum”, assume for discussion that the absolute pressure at the right side of the cylinder is 0.5 atmospheres when the volume is 10 cm3.  When you pull the piston to the left until that volume increases to 100 cm3, then the absolute pressure in the vacuum side decreases to 0.05 atmospheres.  The force resisting you increases from (0.5 atm) x (piston area) to (0.95 atm) x (piston area).  This assumes a sealed piston and cylinder.  If the chamber is connected to a pump that maintains a constant (negative gauge) pressure, then the force doesn’t change.

Capernicus:
So if i double my position towards left of the piston in the cylinder,  given all other parametra the same -  its not double the force?

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