HF Vacuum Pumps

[Pappy]

I saw in a recent flyer that HF now has vacuum pumps. As usual, the price is much better than the big brands.

http://search.harborfreight.com/cpis...rd=vacuum+pump

Anyone familiar with them yet? Would the 2.5 CFM unit be enough for a vacuum press for veneering or will I need the bigger 9 CFM model?

[Daryl]

Huh, they are measuring vacuum in cfm? My experience is it is in inches of mercury. Maybe they are trying to describe how fast it will pull a vacuum from a given volume, where I don't think it matters about the size once you don't have anymore air to suck out.

[LCHIEN]

I saw in a recent flyer that HF now has vacuum pumps. As usual, the price is much better than the big brands.

http://search.harborfreight.com/cpis...rd=vacuum+pump

Anyone familiar with them yet? Would the 2.5 CFM unit be enough for a vacuum press for veneering or will I need the bigger 9 CFM model?

If you have no leaks then any size vacuum pump will work once the air is pumped out. So it depends on the amount of enclosed air and how fast you want it to pull down. I would think a vennering press would have only a fraction of a cu foot of air so it would pull down in less than a minute or so. Think of how big a hole it would take to leak 2.5 cu feet per minute!

[Tom Slick]

Considering you can buy vacuum bagging kits that use a tiny hand pump I think 2.5 CFM, even 1.2 CFM would evacuate the bag fast enough. Any of their vacuum pumps pull a deep enough vacuum for vacuum press work which would be the main consideration to look at.

[just started]

They are run from an air line which is why the cfm rating and the low price, no electric motor. The package blurbs say they are for drawing down auto air conditioners before refilling them with coolant.

[Daryl]

They are run from an air line which is why the cfm rating and the low price, no electric motor. The package blurbs say they are for drawing down auto air conditioners before refilling them with coolant.

Two of the bottom ones are air powered, but the three top ones are stated as being 115/60.

[jonmulzer]

Huh, they are measuring vacuum in cfm? My experience is it is in inches of mercury. Maybe they are trying to describe how fast it will pull a vacuum from a given volume, where I don't think it matters about the size once you don't have anymore air to suck out.

They are run from an air line which is why the cfm rating and the low price, no electric motor. The package blurbs say they are for drawing down auto air conditioners before refilling them with coolant.

Think about it fellas, a vacuum pump is essentially an air compressor where you hook up to the intake instead of the exhaust of the pump unit. Vacuum pumps have cfm ratings just the same as air compressors do. Think of a ShopVac that converts into a leaf blower. Hook a hose up to one side and you can suck up sawdust off the floor. Hook up to the other and you can blow the leaves off your walk. And either way you go it is going to move a certain cfm.

Either of these vacuum pumps would work sufficiently well for veneering purposes. It really does not take a large pump at all to pull a vacuum on those bags.

[pecker]

The pdf manual for the cheaper two pumps shows the vacuum as being "10 Pa (2.95 x 10-3 in Hg)"

You need about 20 in Hg for vacuum veneering...so it's hard to tell what these pumps actually do. Their rating is confusing, if not outright wrong.

page 7 of http://www.harborfreight.com/manuals...8999/98074.pdf

[LCHIEN]

The pdf manual for the cheaper two pumps shows the vacuum as being "10 Pa (2.95 x 10-3 in Hg)"

You need about 20 in Hg for vacuum veneering...so it's hard to tell what these pumps actually do. Their rating is confusing, if not outright wrong.

page 7 of http://www.harborfreight.com/manuals...8999/98074.pdf

Not really confusing.
Pressure as you are used to seeing it is rated in units relative to 1 atmosphere, (e.g. the point at which most of our world exists. this is also called Gauge Pressure or PSIG. Vacuum is a negative pressure in this system. If you call it a vacuum, you can also use pressure units but drop the negative sign. So a pressure of -20 inches is a vacuum of 20 inches.

Pressure for really good Vacuums is measured relative to absolute vacuum, in PSIA, because very small vacuums are hard to define (mostly because atmosphereic reference is always changing.) For really good vacuums, you always measure relative to a true vacuum. In this case a good vacuum can be identified by the smallest number. There can be no negative pressures in this case. since basically zero is defined by no gaseous molecules present - you can't get lower than that.

Sort of like Centigrade and Kelvin temperatures, in a way*.

Given that:
1 ATM ~= 30 inches of Mercury ~= 15 PSI ~= 101,000 Pascals
Lets just use inches of Mercury for the following example.


If I were to define a poor vacuum, like whats you suggested was needed for a vacuum press, then I would say I needed -20 inches of mercury pressure or 20 inches of vacuum (vacuum being negative pressure).

a good vacuum pump that got close to zero would do (in the case of this HF pump) nearly -30 inches of mercury or 30 inches of vacuum. However for scientific and engineering people looking for a good vacuum, its not known at the moment if 1 ATM=30.1" or 29.9" so we really want to know in absolute terms how far down the pump pulls. So we go relative not to 1 ATM but to a true vacuum (e.g absolute pressure). In this pump it goes down to 2.95 x 10 (to the -3 power) inches of mercury which in non-scientific notation is approx .003 inches of mercury. A fairly good vacuum, but I have actually worked with vacuums of .00000003 inches of mercury and that was only a so-so lab. To achieve that you need to use quartz glass containers and tubing and fittings and very low vapor pressure stop cock grease - even these boil off molecules and therefore set the minimum pressure.

There's nothing confusing nor wrong about HF's specification at all, if you know how to measure pressures and vacuums.
In this case its way better than needed for veneering. 20 inches of vacuum is about 2/3 the way to a vacuum so you will get about 10 pounds per square inch on your veneer. Even 10 inches of vacuum would get 5 pounds per square inch, over a 1 square foot veneer job that would be equivalent to having 720 pounds holding the veneer down.


* Warning - if you get easily confused, stop here. If you are technical you can go on:
there is a difference in degrees C & K vs PSIG and PSIA.
Between C&K there is an offset which is exactly 273.15 degrees and does not change.
Between Gauge and Absolute pressures, there is always a variable difference of 1 ATM (1 standard atmosphere is fixed) but your local pressure always varies a bit.

[Garasaki]

sooooooo

would those pumps work well as veneer pumps!?

[LCHIEN]

sooooooo

would those pumps work well as veneer pumps!?

way better than needed (see my previous post)

Oh, if your press fixture does not leak significantly, then you may be able to fit the vacuum plumbing with a valve. Once you pull it down, you should be able to close the valve and turn off the pump, not having it run continuously for a day... as long as you fixture does not leak more than a few PSI over a day.

OTOH, if your fixture does not leak much, then one of those venturi vacuum pumps that works by blowing air through it to create a suction, this $13 one should work
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=96677 or http://www.harborfreight.com/cpi/cta...emnumber=92475
will provide more than enough vacuum and you just need to run it long enough to evacuate the fixture, then close off the valve. Should be a lot cheaper than a motorized pump. Warning - it will take 4.2 CFM from your compressor at 90 PSI to suck this baby down, though.

Or, use your shop vac.

[Daryl]

You can check out Surplus center and see if they have what you want.

http://www.surpluscenter.com/sort.as...&search=blower

Opps, wrong thread, but I will leave the link