Vacuum pump interpretation

[Mark Blumer]

Hi All,

I'm interested in building a vac bag system. I have been to Joe Woodworker's site and have learned a lot. My problem is understanding the relationship between different methods of describing the specs on pumps. JoeWW says that a pump should hold at least 25" hg and VacUPress systems also describe their pumps in terms of inches of mercury. However, when I look at offerings on eBay or on the internet, they often talk about microns of vacuum. How do I compare microns of vacuum to inches of mercury? Also, I assume that the CFM rating just indicates how fast the pump will get down to its maximum vacuum capacity. Is the size of the electric motor that drives it (I'm not interested in a compressor driven system) or the mechanism of the pump (piston, rotary vane, etc.) of major significance to a woodworker?

Thanks for your help,
Mark Blumer (East Lansing, MI)

[Steve Roxberg]

If you have 220 available I would just buy the Gast pump that Surplus Center has. You get it to your door for about $100. It's 1/4hp, continuouse duty. Exactly like the ones that sell for over $300.

I bought one and love it.

[Mark Blumer]

Steve,

Do you have a web address for the Surplus Center?

Mark

[Chuck Wintle]

Well vacuum pressure can be measureed in many units. The units I use most often are Torr and inches of mercury. Atmospheric pressure is 760 Torr or 29.92 inches of mercury. For our usage a good mechanical vacuum pump should give 30 milliTorr or better which is about 29 or better inches of mercury. Other types of vacuum pumps will go down into the microTorr region.

[Jamie Buxton]

Mark --

There's lots of different ways to measure vacuum -- microns, inches of mercury, torr, etc. Some googling around will get you conversion formulas if you want. However, typically the micron style is used in systems where it is important to get every last air molecule out of an enclosure. That's a higher standard than needed for veneer pressing. We just need to get most of the air out, because then we'll get most of the atmosphere's pressure on our veneer. For us, it doesn't matter if we get 90% of the air out, or 99.9999%.

The CFM rating tells you how fast you can pump down a bag. It also tells you how big a leak you can tolerate while maintaining clamping pressure. Real-world bags and check valves do leak, particularly as they age.

In the big picture, the type of pump mechanism doesn't matter. You just want vacuum.

[Ken Garlock]

Hi All,

I'm interested in building a vac bag system. .... I How do I compare microns of vacuum to inches of mercury? ....
Thanks for your help,
Mark Blumer (East Lansing, MI)

Hi Mark. You posed an interesting question that I think I understood, but didn't know exactly how to explain. So, off I went to good old google, and after a couple refinements, I found an excellent explanation at Los Alamos National Laboratory (http://www.lanl.gov/orgs/f/f6/pubf6stds/om/pdfs/Approved/418_VacuumPumps.R.0.pdf)

The short explanation is as follows:

In the English system, vacuums are measured in inches of mercury(hg.)

Pressures are measured in either pounds per square inch(psi) or in inches of mercury. The term STP means Standard Temperature and Pressure which is defined to be a column of mercury 29.92 inches high, 68F, AT sea level. (First day ground school class for pilots.)

29.92 inches of mercury is also equal to 760 millimeters in the metric system.
'
A micron is one millionth of a meter. Hence 29.92" hg is 760,000 microns.

We know that as you increase your altitude, the air pressure decreases, and hence the the column of mercury is less. What you are actually reading is the air pressure on the pool of mercury in the bottom of the barometer.

A perfect vacuum zero inches of mercury OR zero microns of mercury.

THEREFORE: If we put a column of mercury in a vacuum chamber, and start the vacuum pump we can see the column of mercury go lower and lower. With a respectable vacuum pump we can expect that column to go so low that it must be measured in microns, say 3 to 5 microns. When your HVAC man has to do major work on your A/C, he will/should pull a vacuum in the area of 3 microns in order to get "all" the air and moisture out of the cooling system prior to putting in the refrigerant.

Hope this helps

[Chuck Wintle]

Ken,
Are you sure the HVAC man pulls a vacuum to 3 microns? What I have seen from my experience is in the order of 30 milliTorr which is about 30,000 microns

[Ken Garlock]

Ken,
Are you sure the HVAC man pulls a vacuum to 3 microns? What I have seen from my experience is in the order of 30 milliTorr which is about 30,000 microns

Hi Charles. I was relating what my HVAC man said last week when he replaced my Trane evaporator. I have not watched the meter myself, and have never done the job, so I might have misunderstood him or he was wrong and didn't know it. I do know that he ran the vacuum pump for over 45 minutes.

OH, I see the problem.

1 Torr = .001 meters(1 millimeter) = 1000 microns
1 milli Torr = 1000/1000 = 1 micron.
Hence 30 milli Torr = 30 microns.

Now 30 microns gets us both in the same ballpark. ;)

[Wes Bischel]

Mark,

www.surpluscenter.com

Look in the upper left hand corner for "air" and follow the "vacuum pump/motor unit"

Good luck, Wes

[Mark Blumer]

Thanks for the responses everyone.


Mark

[Chuck Wintle]

Hi Charles. I was relating what my HVAC man said last week when he replaced my Trane evaporator. I have not watched the meter myself, and have never done the job, so I might have misunderstood him or he was wrong and didn't know it. I do know that he ran the vacuum pump for over 45 minutes.

OH, I see the problem.

1 Torr = .001 meters(1 millimeter) = 1000 microns
1 milli Torr = 1000/1000 = 1 micron.
Hence 30 milli Torr = 30 microns.

Now 30 microns gets us both in the same ballpark. ;)
Ken,
It all makes perfect sense now!
I was thinking in the units that made sense to me.

[Jamie Buxton]

Mark,

www.surpluscenter.com

Look in the upper left hand corner for "air" and follow the "vacuum pump/motor unit"

Good luck, Wes

My first vacuum press (a homebrew I built 25 years ago) used a Gast pump exactly like that, except that it ran on 110. It worked well enough, but it was LOUD. I couldn't stay in the shop with it. I built a muffler for the exhaust side, and that helped a great deal. My first muffler was a tin can stuffed with an old sock. It did the job, but occasionally the pump would blow the sock out of the can. Some time later I found a more official device, a sintered bronze water filter. Acoustically, it worked about the same as the sock, but it did tend to stay together better.

[Bill Turpin]

No matter what pump you have or how low a vacuum you want, what you suck thru is just as important. The speed of the pull is inversely proportional to the square of the orifice or length of hose. If you double the length of the hose between the pump and the vacuum chamber, it takes FOUR times as long to do the same amount of work. A 3/8" hose will pump five times faster than a 1/4" hose. The smallest orifice in the system will determine the rate even if most of the system is larger. Some people use quick disconnect air fittings for vacuum service. The actual orifice on a 3/8" hose fitting is usually less than a 1/4".

A refrigeration mechanic uses vacuum to remove moisture from the system. He has to reduce the pressure to a point that the water in the freon tubing will BOIL at room temperature. When the vapor pressure of the water is greater than the atmospheric pressure inside the tubing, the water will boil, allowing the pump to then suck out the water vapor( low temp steam from the water boiling at 70 °F). This happens below 50 microns of pressure. If the water is not removed, it will freeze, stopping up the expansion valve which shuts down the cooling system.

On Mount Everest, with much lower atmospheric pressure, water boils considerably below 212 °F. Pressure cookers have the oposite effect. The boiling temp of water inside the cooker is raised because the vapor pressure has to overcome the higher local atmosheric pressure. Therefore the food cooks faster.

Vacuum can be expressed in quantities from either end of the scale. Thirty inches(29.92) is the amount below atmospheric you have gone.Zero pressure guage, the pressure at sea level is really 14.7 psi above true zero pressure(absolute). When we refer to torr or microns we are quantifing how much above absolute zero pressure we are at. 29.92" Hg of vacuum = 0 torr= 0 microns=0 psi absolute. To further add to this confusion, The US Weather Bureau corrects local barometric pressures to sea level. Here in the NC mountains when the televised BP is ± 29", it is really 26.5" due to our altitude of 2 to 4000 feet.

Bill in WNC mountains