Author Topic: Putting vacuums in series (pneumatics)  (Read 4668 times)

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Capernicus

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Putting vacuums in series (pneumatics)
« on: July 06, 2021, 09:20:23 pm »
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?
« Last Edit: July 06, 2021, 09:21:56 pm by Capernicus »

TimFox

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Re: Putting vacuums in series (pneumatics)
« Reply #1 on: July 06, 2021, 09:27:36 pm »
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?

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Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #2 on: July 06, 2021, 09:49:24 pm »

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.)
« Last Edit: July 06, 2021, 10:07:40 pm by Capernicus »

TimFox

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Re: Putting vacuums in series (pneumatics)
« Reply #3 on: July 06, 2021, 10:08:03 pm »
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.

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Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #4 on: July 06, 2021, 10:17:25 pm »
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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Re: Putting vacuums in series (pneumatics)
« Reply #5 on: July 06, 2021, 10:21:52 pm »
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?

force = pressure * area

no magic involved

Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #6 on: July 06, 2021, 10:23:47 pm »
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?

force = pressure * area

no magic involved

Oh Ok thanx for telling me,   I was thinking it was going to be harder to pull the piston the more evacuated space was behind it...  im getting confused...

TimFox

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Re: Putting vacuums in series (pneumatics)
« Reply #7 on: July 06, 2021, 10:28:43 pm »
The relevant pressure in the force equation is the pressure difference between the two sides of the piston, where the outside is constant at 1 atm (approximately 100 kPa) and the inside varies inversely with the volume.  If the initial vacuum is extremely low absolute pressure, then the change in force will be small.

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Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #8 on: July 06, 2021, 10:32:33 pm »
So you can pretty much pull a blocked syringe for a mile and it never gets harder to pull it.

TimFox

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Re: Putting vacuums in series (pneumatics)
« Reply #9 on: July 06, 2021, 10:34:23 pm »
That depends on the initial vacuum pressure, as I stated.  This is a quantitative problem, and you will have trouble maintaining the sliding seal over absurd dimensions.  There is an old saying about giving an inch and taking a mile.
« Last Edit: July 06, 2021, 10:37:04 pm by TimFox »

Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #10 on: July 06, 2021, 10:36:18 pm »
I mean pressure 0.    So then it makes no distance whatsoever.  (outside 1atm)

Long vacuums have no force difference over short ones, thanks for explaining.
« Last Edit: July 06, 2021, 10:39:17 pm by Capernicus »

TimFox

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Re: Putting vacuums in series (pneumatics)
« Reply #11 on: July 06, 2021, 10:40:27 pm »
I don’t understand your interpretation of my answer.  I think you mean that the force on the piston depends only on the pressure difference, not on the volume of the vacuum chamber.  However, the internal pressure follows the gas law when the volume changes.
“Vacuum” does not necessarily mean 0 pressure, which is good since I never achieved lower than about 10-13 atm in laboratory chambers.  It means pressure lower than local atmospheric pressure in most applications for vacuum activated devices.
« Last Edit: July 06, 2021, 10:43:13 pm by TimFox »

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Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #12 on: July 06, 2021, 10:45:03 pm »
Yes I understand what you said.

But how can u even change the pressure of ~0 pressure?  it just stays ~same...  ~0...  no matter what the pull length was,  or how much evacuated space there was,  its just the same back force regardless.
« Last Edit: July 06, 2021, 10:47:27 pm by Capernicus »

TimFox

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Re: Putting vacuums in series (pneumatics)
« Reply #13 on: July 06, 2021, 10:47:47 pm »
How do you obtain zero pressure?  Even in my extreme example in a pumped laboratory chamber with no sliding seals, there is a small pressure.  You can’t say it is negligible if you insist on mile-long extension.  Some estimates of the pressure in interplanetary space are better than my lab, roughly 10-16 atm, but the distances are also very large.
« Last Edit: July 06, 2021, 10:51:27 pm by TimFox »

Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #14 on: July 06, 2021, 10:50:31 pm »
Just push the piston to the end, and put the plug in.  (mechanical)  Or close a bellow,   or squish an eyedropper.

TimFox

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Re: Putting vacuums in series (pneumatics)
« Reply #15 on: July 06, 2021, 10:52:00 pm »
No.  Have you ever pumped a vacuum?

Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #16 on: July 06, 2021, 10:55:09 pm »
I never have, hence my simple and confused questions.
I havent even pumped a car tyre before in my whole life.
« Last Edit: July 06, 2021, 10:56:47 pm by Capernicus »

TimFox

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Re: Putting vacuums in series (pneumatics)
« Reply #17 on: July 06, 2021, 11:02:09 pm »
Your mechanical methods work to produce a “partial vacuum” which can be adequate for a vacuum actuator, but still have measurable absolute pressure.
Practical vacuum actuators in automobiles use a negative pressure that is only a small fraction of atmospheric pressure to get a usable force on a sealed diaphragm, avoiding sliding seals.

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Re: Putting vacuums in series (pneumatics)
« Reply #18 on: July 06, 2021, 11:22:03 pm »
How do you obtain zero pressure?  Even in my extreme example in a pumped laboratory chamber with no sliding seals, there is a small pressure.  You can’t say it is negligible if you insist on mile-long extension.  Some estimates of the pressure in interplanetary space are better than my lab, roughly 10-16 atm, but the distances are also very large.

still the change is very small, 1atm - 0 vs. 1atm - ~0

Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #19 on: July 06, 2021, 11:22:03 pm »
What the hell I'm going to do an experiment.

I'm going to resin print it and see if when I pull the left piston out to just inside,  the right one is harder to pull out or not.

Then I can finally put it to rest - and move on.

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Re: Putting vacuums in series (pneumatics)
« Reply #20 on: July 06, 2021, 11:25:26 pm »
With the vacuum you can achieve, there should be a measurable absolute pressure inside, and your sliding seals will leak.  It will be hard to interpret the experimental results.

Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #21 on: July 06, 2021, 11:32:40 pm »
I bet it'll be air tight,  I trust my anycubic photon,  but I spose, I haven't actually done it before, I'm just guessing itll be air tight.

The vacuum will be near 0% if its air tight, solid against solid,  except that little bit at the end, where the noddies meet, otherwise it wouldnt be air-tight, I think.

I could add a softbody membrane to it to be sure... let me think, maybe...
« Last Edit: July 06, 2021, 11:35:10 pm by Capernicus »

Capernicus

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Re: Putting vacuums in series (pneumatics)
« Reply #22 on: July 06, 2021, 11:40:27 pm »
Ive got an easier one, made out of odds and ends around the house.

Itll only evacuate the kebab sticks diametre tho,  so maybe u stuff 3 or so in, depending on the radius of the straw?

Then once uve stuck it in,  you put some tacky on the end, and then u should have the vacuum prepared.
« Last Edit: July 06, 2021, 11:42:41 pm by Capernicus »

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Re: Putting vacuums in series (pneumatics)
« Reply #23 on: July 06, 2021, 11:41:10 pm »
I bet it'll be air tight,  I trust my anycubic photon,  but I spose, I haven't actually done it before, I'm just guessing itll be air tight.

The vacuum will be near 0% if its air tight, solid against solid,  except that little bit at the end, where the noddies meet, otherwise it wouldnt be air-tight, I think.

I could add a softbody membrane to it to be sure... let me think, maybe...

just do the math, force = pressure * area,  pressure * volume = constant (for constant temperature)

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T3sl4co1l

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Re: Putting vacuums in series (pneumatics)
« Reply #24 on: July 07, 2021, 06:45:55 am »
Pulling on a vacuum costs zero work.  Pushing against the atmosphere however requires work.

The force, of which, is defined by the rate of volume change.  For a piston in linear motion, the cross section is constant, and volume proportional to displacement.

So the force is constant.

This can be a good way to make constant force springs; though it depends on atmospheric pressure (so, varies with altitude).  It's more common to do it with compressed gas ("gas spring"), the pressure of which will increase some with compression, which itself can be a valuable feature (if the minimum volume is zero, then force increases hyperbolically with displacement, making a softer stop than a hard piece of metal, or a rubber bumper).

I don't know what you mean by vacuum vessels "in series".  You can connect them in parallel, joined with a pipe; this simply increases the total volume and equalizes the pressure between them.  Surrounding a rigid vessel in any other pressure (greater or lesser than what's inside) has very little effect, because of its rigidness (small change in volume for large change in pressure).  The situation is different for, say, a balloon (volume varies with pressure differential, in a nonlinear manner), or a bellows or other very flexible structure.  (High altitude balloons are usually little more than loose polyethylene bags; the gas expands at high altitude (lower pressure), eventually inflating the bag to full.)

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