Hey guys,
Hope all is well.
I'm still working on my volume test rig and was thinking about using a vacuum to increase accuracy...so I have been studying the effects of vacuum and on liquids and such.
With that, I have a question that might be best presented as the following. I guess this is more of a physics questions than an engineering one, but I thought it was interesting enough to share. Hope this sparks some interest and hope that it something we can share. I wrote this and this and I really don't know the answer.
Experiment/Question:
If you have a piston in a cylinder in which the piston is perfectly sealed, and there is no void between the piston and the top of the cylinder, (a perfect fit) what happens to the space (volume) that is created when you pull the piston down?
As this new volume contains no gases or matter, I would assume that it is immediately a perfect vacuum....right?
And as you pull the piston further down, creating a larger volume, I would assume that it requires a greater and greater force to do so, but would that not mean that the measurement on the empty volume (let's assume it has a gauge on it) be something greater than an absolute vacuum measured at 29.9hg?
At what point would you be pulling the piston further and further down yet not see in increase in required force?
THANKS EVERYONE!!!
Dave
I think what you are describing is that that there is already a perfect vacuum between the piston top and cylinder. Assuming that there is one atmosphere of pressure on the opposite side of the piston (~14.7 psi) it will always require one atmosphere of force to move the piston.
Of course, I just got out of bed and am not quite awake yet......
Anybody else?
Last edited by Kelly_Bramble; 04-25-2014 at 04:00 PM.
I agree, ignoring little things like absolutely perfect sealing, zero friction, and other bothersome facets of reality.
Originally Posted by Kelly Bramble
yes...vacuum as a state of nothingness.
But, I assume there is a substantial amount of force required to move the piston down the cylinder other than the force related to absolute pressure to that of the piston size (F=MA).
Sort of like removing a suction cup.
But what would that force curve look like? Is is linear the further the piston is moved? And if it is, how is that possible is a vacuum has a limit?
Or am I completely out of whack on something?
Yes...let's just assume all the little details aside and look at what the forces are, and what is happening within the chamber.
Thoughts?
"it will always require one atmosphere of force to move the piston."
Linear to a max, then flat.
Thanks!!
So if you put the piston/cylinder test set-up into a vacuum chamber, it would not require any force to move the piston in and out.
I think this is right because I know a suction cup will not hold in a vacuum chamber.
I don't think I like the word vacuum. Seems like it would be better to refer to this as "void". A vacuum should be an adjective, and not a noun. Okay...I'm getting punchy.
When you pull the piston, you will vaporize some of the liquid and the "vacuum" volume will be full of vapor at the vapor pressure of the liquid. The force required will be one local atmosphere minus the vapor pressure times the area. Note that the temp of the liquid will drop a bit from the heat of vaporization and the vapor pressure is temp dependent. Some liquids have very low vapor pressures (think vacuum oil) and some have relatively high vapor pressures (think volatile solvents). If you use water, the vacuum volume will be filled with steam. Of course all of this is also somewhat time dependent, but for your thought experiment you can probably ignore that complication.
Timelord
Posting Permissions
Reply With Quote