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Anonymous
 Posted on Monday, April 5, 2004 - 8:21 pm:

All the vacuum pump page says about pressure is "up to 26 inches of mercury". What is that in psi?

Simon Quellen Field (Sfield)
 Posted on Tuesday, April 6, 2004 - 2:49 pm:

Inches of mercury is an unfortunate choice of units.
But it is intuitive, and is what my pressure gauge happens to use.

The unit to use is the Pascal.

Because mercury changes density with temperature, and the gravitational
constant is not known to a precise value, the unit "inches of mercury"
can only be an approximate number of Pascals. The number we will use
here is 3,386.39 Pascals per inch of mercury. Thus 26 inches of mercury
is 88,000 Pascals, or 88 kiloPascals. Note that since we only have two
digits of precision in our "inches of mercury" measurement, we do not say
88,046.14 Pascals, because we only have two significant digits.

One psi (pound per square inch) is about 6,894.76 Pascals. It, like inches
of mercury, is not a unit that can be precisely measured, since the pound
is also defined by the graviational constant, which is not known to high
precision.

So 88 kiloPascals is about 13 psi after rounding to two significant digits.

Alessandro Carcione (alessandro)
New member

Post Number: 6
Registered: 2-2005
 Posted on Tuesday, February 22, 2005 - 3:49 pm:

Thats only 1.5psi short of a complete vacuum

Simon Quellen Field (sfield)
New member

Post Number: 232
Registered: 12-2004
 Posted on Tuesday, February 22, 2005 - 3:59 pm:

Yes, but don't let that fool you into thinking that the last 1.5 psi
is as easy as the first. It keeps getting more difficult as you get
to lower pressures. You can get to 24 inches of mercury by sucking
on a tube with your mouth, maybe higher.

lysdexia
Unregistered guest
 Posted on Sunday, March 6, 2005 - 7:16 pm:

Why would it get more difficult removing air? If one stores air, the pressure continues to rise because there's more substance. But if air is brought outside, the pressure increases only infinitesimally. I'm thinking about lossless methods to pump or exchange air that don't involve scaling up an integral-order lossy mechanism to reach greater changes.

Simon Quellen Field (sfield)
Senior Member

Post Number: 288
Registered: 12-2004
 Posted on Friday, April 15, 2005 - 10:17 pm:

It's hard to get close to a real vacuum.
You have leaks, you have outgassing, and you have
to chase around those few remaining molecules to
get rid of them. It's somewhat like asking why it is so
hard to get to absolute zero when it is so easy
to get to 273*2 degrees Kelvin.

Dave Jones (Unregistered Guest)
Unregistered guest
Posted From: 67.23.73.86
 Posted on Wednesday, February 15, 2006 - 6:12 pm:

What does this mean in relationship to a vacuum bag sealing chamber ? Vacuum pump capacity:4m3/h is this any reference to inches of mercury ie: 29" Thank You Dave Jones

Simon Quellen Field (Sfield)
Senior Member

Post Number: 1060
Registered: 12-2004
 Posted on Wednesday, February 15, 2006 - 7:44 pm:

Four cubic meters of air can be pumped in one hour by that pump.
It is not a pressure measurement. It is volume per hour.

Paul Simon (Bl4h)
Member

Post Number: 14
Registered: 2-2006
 Posted on Thursday, February 16, 2006 - 11:42 am:

What size does the rubber stopper have to be? I went to a few Lowes and Home Depots but they said they didn't have anything like that (I even showed them a picture of what it should look like). Today I found a site that has them but I don't know what size to order. I followed your instructions for the second vacuum pump and bought 3/4" PVC tube, etc. Here's the link to the site: http://www.sciencestuff.com/ctgy/L-tStoppers.

Simon Quellen Field (Sfield)
Senior Member

Post Number: 1063
Registered: 12-2004
 Posted on Thursday, February 16, 2006 - 2:01 pm:

At 20 cents each, you could buy one of each and still have lunch
money left over. Or you could measure the inside diameter of the
tubing and select the stopper that makes a tight fit but can still
be pushed in (with a little vaseline for lubrication).

I'm sorry I didn't specify the size in the text -- I just bought a
bunch of different sizes at the Ace Hardware store and used one that fit.

The screw makes the stopper expand a little, so you have some control.

Paul Simon (Bl4h)
Member

Post Number: 15
Registered: 2-2006
 Posted on Thursday, February 16, 2006 - 7:54 pm:

I built it. I had to improvise and use sand paper to sand one of the rubber door stopper things I got yesterday. It didn't go in without lube though, which is why I thought it wouldn't work. After lubing it up, it worked. It's a 4', 3/4" wide pump with a 4 food wooden dowel. I pumped water with it today and even though I didn't do an entire gallon yet (didn't try) it pumps like 2 quarts in like 4 strokes. Amazing.

If the water doesn't leak, gas shouldn't leak either, right? It took me like a week to collect a bit of hydrogen gas and I want to compress it into a smaller volume but I don't want to lose it why I pump it out of one container and into another.

Michael (Michaelt)
Member

Post Number: 11
Registered: 12-2005
 Posted on Thursday, February 23, 2006 - 3:02 pm:

Water has larger molecules than does any gas. This is why you can fill one balloon with water, and another balloon with hydrogen, and 2 weeks later the hydrogen balloon will have deflated by quite a bit, but the water balloon will be largely unchanged.

(Message edited by michaelt on February 23, 2006)

Simon Quellen Field (Sfield)
Senior Member

Post Number: 1094
Registered: 12-2004
 Posted on Friday, February 24, 2006 - 11:55 am:

Well, no.

Water is H2O. It has a molecular weight of about 18, and a
width of a little over an angstrom.

A gas like hot uranium hexafluoride is quite a bit larger, with a
molecular weight of about 244, and a width over 2 angstroms.

Xenon gas has a width of over 2 angstroms as well.

Liquid water bonds with itself, so comparing it to a gas is not really
very useful.

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