1. ## Vacuum Pumps........again.......

Recently some half decent pumps are going on auction here for reasonable money, so I'm in the market for one, thing is they use Pascal (Pa) here for vacuum pump ratings, not the "Inches of Mercury" (inch Hg) like I'm used to.

I think, IIRC that I should be looking for at least 15" Hg, better to have closer to 20" Hg, right?

From what I can see, 1" Hg equals 3386 Pa

So say 18" Hg would be about 61000 Pa.

Now on the various websites I find here, they list this a 6.1 x 10-4

Sorry but my math sucks on stuff like this, does 5 x 10-4 = 50000

Or some such thing, must be an "Engineer" thing I guess

2. Geez I'm stupid

10-4 = 0.0001 right?

man these pumps are REALLY low pressure, what gives?

that means 5x10-4 is the same as 5 x 0.0001

Right?

So 0.0001 x 5 = 0.0005 Torr or 0.00001969" of Hg........??????

Man I can pull more vacuum with a straw in my gob..

OK, I'm REALLY confused here, what would you need a pump that pulls so little vacuum for?

Something is out of whack here..............

3. Calculator
Not that I understand it
Last edited by Jeff Horton; 08-09-2007 at 02:17 PM.

4. Member
Join Date
Nov 2006
Posts
538
Interesting page here - http://www.npl.co.uk/pressure/faqs/vacuum.html. Look at the table marked "Pressure Ranges". Smaller Pascal measure means "higher" vacuum (further away from atmospheric pressure). 5 x 10-4 is, on this scale, in the "very high vacuum" range. At 120l/min I would also think that this pump will not only pull a strong vacuum but do it reasonably quickly.

looks like a bargain to me.

5. Good find Ian, that seems to make sense.....

I too saw the 120L per minute and thought it must pull some serious vacuum.

Cheers!

6. A couple numbers that may help...

Standard atmospheric pressure is 29.97 inches of mercury, then you adjust for the altitude (about an inch lower for each thousand feet of altitude), then you adjust for weather... this is what the weathermen report ("barometric pressure is 29.45 and falling, prepare for a storm").

If you get rid of 28 inches of pressure equal to 28 inches of mercury, you have gotten rid of most of the atmospheric pressure... darn good. Not a perfect vacuum, but great for a vacuum bag (or most other things you want a vacuum for).

At sea level, the air weighs (presses down) at 14.8 pounds per square inch (most people round to 15 psi). So anything that doesn't squish must also push back at 15psi. If you get rid of 28/29.97 or 93% of the atmospheric pressure that was pushing back from inside the bag, then you end up with 93% of 14.8, x 144 square inches in a foot, or almost a ton of air pressure pressing down on each square foot of the vacuum bag.

In practice, you won't get that much vacuum inside the bag - but it is reasonable to expect 1500-1700 pounds per square foot.

Watch out for the psi readings. If you put a gauge in the air at sea level, it should read 14.8 psi, so you would spend a lot of time adding or subtracting 14.8 (depending on whether you were making pressure or vacuum). So most gauges are calibrated to start at 0 rather than 14.8 ... or actually report the difference between the measured pressure/vacuum and the atmospheric pressure at that point. This is referred to as psig or psi on the gauge. And you are right, that sometimes vacuum is just a negative number... how much below the 14.8 psi or "0 psig".

Now, it is up to you to convert to Pascals, stones, newtons, or whatever other unit is the local favorite.

Hope this helps

7. ## THE best source of vacuum pressing info.....

is: http://www.joewoodworker.com/veneering/welcome.htm

Great source of information and supplies!

8. Thanks fellas, now I got to find a decent vacuum pump for a decent price, they want over \$600 for a new one

Cheers!

9. Found one, but I think it would be OVERKILL.........

(that cart is it sitting on is nearly 3' long)
D-650D They want \$300 for it, says......

200V 50/60Hz
CYCLES 50/60Hz
DISPLACEMENT 507/618ℓ/min <- That is 134/163 US Gallons a minute
REVOLUTION 950/1150rpm
MOTOR POWER 1.1kW <-1.5 Hp
OIL CAPACITY 1.8ℓ

Think it would work well for a veneer bag press

200V would be a bit of a pain, and it is LARGE, I think I'll stick to a smaller unit......

10. What are you going to use it for? The guys who stabilize wood blanks for pens pull about 23 inches and it seems to work. The pen forums talk a lot about vacuum pumps. Same dilemma there, finding one that works well without spending a fortune. Old refrigerator compressors seem to work OK except they must run a long-long time to get results. But the price is right.

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