Vacuum Pump Question?

[George Skinner]

I need to get a vacuum pump for some Air conditioning work and was wondering if there is any reason the same pump could not be used for vacuum veneering and clamping.
These types of pumps are oil pumps and not dry vane pumps but I don't see why it would not work.
If there is no problems anyone can think of I will get a good pump and not a real cheap economy one.

Thanks

[Alan Tolchinsky]

Hi, Check out joewoodworker.com for info on pumps. I just bought a used vacumn pump for around $80 and it works fine. Any pump that will pull down to 25 lbs. Hg will work fine and you shouldn't have to pay more than $100 on Ebay or elsewhere for a used one.

[Steve King]

George,
I am using a RobinAir automotive vacuum pump at home for veneering. I had to buy a new R12 and 134A Freon recovery machine at work and didn’t need the stand alone vacuum pump at work. This pump is so fast. After installing a check valve between the pump and bag; I can pull a vacuum for 20 minutes, shut the pump off and it will hold a vacuum overnight.
Steve

[Ken Garlock]

George, I don't see any concern, after all you are pulling air out of the vacuum bag, not pump anything into the bag. As I recall the A/C pull-down pumps can get down to the microns of mercury . So, 25 INCHES of mercury is just a walk in the park. Also, remember that an A/C system does not tolerate contamination thus the pump must be particularly clean on the input side. The only concern I would see is that you need to watch that you don't pull too much vacuum on the bag and rupture it where it wraps around a corner or the like.

function (rant)
Often times I see posts on various boards referring to vacuum as measured in psi. That is patently wrong. Vacuum is always measured in INCHES OF MERCURY! In the aviation world, there is a term called "standard temperature and pressure." That STP is 68 deg F, and 29.92 inches of mercury, or 760 millimeters, at sea level. 29.92 inches of mercury translates into approx 14.7 psi. Zero inches of mercury is a perfect vacuum which is defined as a space which contains nothing, no atoms, ions, etc, nothing. Most likely such a thing does not exist in the universe, even in deep space.
end (rant)

[George Skinner]

Thanks Everyone

I did not think it would be a problem but just in case I was missing something I wanted to here that someone else was using one with good results.

Thanks Again

[Jim Becker]

Zero inches of mercury is a perfect vacuum which is defined as a space which contains nothing, no atoms, ions, etc, nothing.

Help me out here, Ken. When we are using a vacuum chucking system at the lathe, the HIGHER the number on the vacuum guage is what we shoot for...as close to 25hg as possible. When the guage reads, "0"...it's gonna fly! How does the marking on the guage relate to what you state? It's just confusing to me!

[Kevin Arceneaux]

Jim, I taking a shot in the dark here, but when the gauge is at 0, that is showing it is at atmospheric pressure. When showing 25 inHG, that is the vacuum it is pulling.

[Jim Becker]

Jim, I taking a shot in the dark here, but when the gauge is at 0, that is showing it is at atmospheric pressure. When showing 25 inHG, that is the vacuum it is pulling.

Yes, that's what I'm trying to rationalize with Ken's explaination. I'm likely just being daft...the latté hasn't begun to "work" yet this morning!

[larry merlau]

Jim, I taking a shot in the dark here, but when the gauge is at 0, that is showing it is at atmospheric pressure. When showing 25 inHG, that is the vacuum it is pulling.

//in reguards to the 25 no. on vacume it is the pulling amount and can go closer to 30 with a good pump. they use vacume in graphic arts to make offset plates to contack film to plate for exposing.

[Kevin Arceneaux]

Yes, that's what I'm trying to rationalize with Ken's explaination. I'm likely just being daft...the latté hasn't begun to "work" yet this morning!

I just talked to one of our power plants and I asked and was told that is right.

Latte? Down here, that is the equivalent of drinking colored water. I drink Community Dark roast - it will really open your eyes. My Grandmother used to make it on the stove, she didn't like Mr. Coffee. It was stout, you could almost stand a spoon in it.

[Jim Becker]

ILatte? Down here, that is the equivalent of drinking colored water. I drink Community Dark roast - it will really open your eyes. My Grandmother used to make it on the stove, she didn't like Mr. Coffee. It was stout, you could almost stand a spoon in it.

I don't drink "regular" coffee...can't stand most of it and certainly not black. But latté and other espresso drinks I enjoy. Don't be fooled...it's not weak in flavor! If I can't have that, I drink tea. The machine that Dr. SWMBO and I have has paid for itself many times over since we stopped going out for breakfast on the weekends. When it dies, that awesome new KitchenAid machine will likely take its place (and match the mixer )...oh what we invest in our habits!

[Ken Garlock]

Jim, I see your confusion. I would say that it is matter of labeling of the chuck, the zero is not a pressure, but rather a reference point of zero vacuum.

Consider that 29.92 in. Hg is the reference point. You often hear the weather man say that the barometric pressure is 29.92 or the like. That means that the pressure of all the air in a column extending from the ground to outer space will support a column of mercury 29.92 inches high. The density of mercury is 13.6 times that of water, a column of water one inch square and one foot high weighs .434 lbs, approximately. Take 29.92 times 13.6 and you find that it would take a column of water 406.9 inches high, or 33.9 feet high. Since water weighs .434 lbs/sq in, we multiply 33.9 by .434 and get 14.7 psi for the air pressure at sea level. These are approximate values due to rounding. Also remember that these values will vary with the air pressure, but the .434 psi is a constant.

So now we have established that the reference for a vacuum at sea level is 14.7 psi (and zero inches of vacuum.) What does this mean when using a vacuum pump in a shop? What you are trying to do is create enough pressure on a surface to to accomplish your needs. Perhaps it is a vacuum bag to "clamp" veneer to the substrate, or it is pressure to hold a lathe piece in place. How do we compute that force on the veneer or chuck? We can go through the same computations above, substituting 25 in Hg. That exercise yields 12.3 psi. But is that the actual pressure on the veneer?

If we start from beginning with no vacuum in the bag or on the chuck, we already have an atmospheric pressure of 14.7 psi, or zero vacuum. We calculated the 25 in. Hg to be 12.3 psi or as some people like to say 1771 pounds per sq ft. What does this mean?

Let us start with a bottle of mercury into which we insert vertically one end of a 3 ft. length of straight glass tubing that is open on each end. At the top we attach a vacuum pump and start it running. When the column of mercury reaches 25 inches, we have created a 25 inch vacuum. As we saw above, that is 12.3 psi (ANSWER TO YOUR QUESTION.) If we continued to run the vacuum pump for a very long time, given it does not leak, we could approach the 29.92 inches of vacuum which translates to 14.7 psi. I think your confusion is thinking that zero inches of mercury translates to a perfect vacuum, rather than no vacuum. May be I made it read that way in my post above

Do you remember going out to a farm in the country, and using a hand operated water pump with a long handle. Most of the time they wer painted red for some reason. These pumps can only lift water from the well if the well is less than 34 ft. deep. Why, because the only thing that is causing the water to come up the pipe is the difference in the air pressure pushing down on the water, and the vacuum created in the pump mechanism. The operator of the pump is supplying the effort to move the column of water from the pipe going down the well, and the pump interior, to the outside world while maintaining the vacuum in the pump. Why 34 ft? We know that a column of water one sq in. by one foot high weighs .434 lbs. IF we divide the 14.7 psi by .434 we find that 14.7 psi can support a column of water 33.8 ft high.

Physics is phun

[Robert Cox]

I had to learn all that stuff for a project once.

Long term dry storage of spent nuclear fuel.

We pull the canisters down to 3 torr. (A torr is a mm of mercury)

The canisters are 5' dia by 14' long, roughly.

It's a large vacuum pump!

[Bill Turpin]

Barometric pressure at sea level is 14.7 psi. We call it zero psi guage. Standard guages reading zero psig are realy 14.7 psi. Vacuum of 14.7 or 30" Hg (mercury) is actual pressure of zero psi. The closer you get to 30 " Hg vacuum, the closer you are to zero psi and the better or lower vacuum. The lower the vacuum... the better the vacuum or MORE suck. Refrigeration pumps have oil to seal the rotor, so they will pull better vac than dry pumps. Change the oil often because moisture absorption by oil competes against the vacuum pump. Vacuum is sort of negative pressure being subtracted from the actual pressure of 14.7psi which is zero psig.

Remember: the speed of pull down is inversely proportional to the square of the orifice or length of hose. An orifice one half the size will take four times as long to do the work. Four foot hose will take four times as long to pull as two foot.

Bill in WNC mountains
NC refrigeration license #2821

[Anthony Yakonick]

When the big hand points to 30 (no bifocals so it looks like 30) it's good!