| Author | Message | ||
     Anonymous |
All the vacuum pump page says about pressure is "up to 26 inches of mercury". What is that in psi? | ||
| Simon Quellen Field (Sfield) |
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 Username: alessandro Post Number: 6 Registered: 2-2005 |
Thats only 1.5psi short of a complete vacuum | ||
| Simon Quellen Field (sfield) New member Username: sfield Post Number: 232 Registered: 12-2004 |
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 |
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 Username: sfield Post Number: 288 Registered: 12-2004 |
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 |
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 Username: Sfield Post Number: 1060 Registered: 12-2004 |
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 Username: Bl4h Post Number: 14 Registered: 2-2006 |
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 Username: Sfield Post Number: 1063 Registered: 12-2004 |
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 Username: Bl4h Post Number: 15 Registered: 2-2006 |
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 Username: Michaelt Post Number: 11 Registered: 12-2005 |
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 Username: Sfield Post Number: 1094 Registered: 12-2004 |
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. |
