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  #1  
Old 07-25-2008, 12:10 AM
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$75+ shipping Air Conditioning Vacuum Pump

From Harbor Freight-Air cinditioning Vacuum Pump $74.99.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=98074

The say it will pull -75 micron/10 Pascal maximum vacuum. What is this in American terms???
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  #2  
Old 07-25-2008, 12:19 AM
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It's not -75 microns, it's +75 microns. That's above an absolute vacuum, although I'm not sure what the rating means in US terms. You won't notice the difference between that and an absolute (and unattainable) vacuum on any manifold gauge set, but a micron gauge will show it.

My pump claims 25 microns, 1/3 HP dual-stage 5 cfm, but it was more than twice the price. It's a Mastercool and I got it from Arizona Mobile Air (ackits.com). They also sell smaller pumps, and are likely competitive with Harbor Freight. Their customer service is also very good. I can't say if it's better than Harbor Freight since I've not experienced theirs, but I'd lay odds on AMA being better. Check their site before you buy from HF.

Whatever pump you get, buy extra oil. I changed mine during the operation, to get the best possible vacuum. That is, I pulled an initial vacuum which polluted the oil, changed it, and pulled vacuum again. You should change the oil in an electric pump with every use. Fortunately, the oil isn't expensive.
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  #3  
Old 07-25-2008, 01:21 AM
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+1 what Matt says. Oil needs to be changed every use.

Why not rent from your local Autozone?
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  #4  
Old 07-25-2008, 01:32 AM
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Nice to see you two guys at least agreeing in this thread.
$15 bucks more gives you a 2.5 vs that 1.2

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=98076
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'85 300TD 235K "The Wagon" Texas Friendly White
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  #5  
Old 07-25-2008, 01:59 AM
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Quote:
The say it will pull -75 micron/10 Pascal maximum vacuum. What is this in American terms???
The "micron" is a fairly traditional unit of (low) pressure measurement in some fields. It's "one micrometer of mercury".

760 millimeters of mercury is one atmosphere.
There are 1000 micrometers in one millimeter.
So, 75 microns would be about 0.0001 atmosphere.

What would that be in inches of mercury?
Well, 75 micrometers is about 0.0030 inches.
So, 75 microns (micrometers of mercury) is about 0.0030 inches of mercury.

But wait! Don't those mechanical gauges show something like 30 inches of mercury for a "really good" vacuum?
Yes, they do, but only because that scale has been inverted so that zero is atmospheric pressure and the numbers get bigger as the pressure goes down from there. If you want to know what 75 microns is in their "inches of mercury", you'd have subtract the number above from 29.9247 inches of mercury:

29.9247 - 0.0030 = 29.9217 inches of mercury.

Think about this a bit, and it should be clear that the mechanical Bourdon tube gauge in an AC service manifold just isn't capable of confirming that such a vacuum pump has really reached an ultimate pressure of 75 microns.

(Some of this was lifted from this article about pressure units from the Bell Jar.)
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  #6  
Old 07-25-2008, 02:06 AM
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Quote:
Originally Posted by Eskimo View Post
The "micron" is a fairly traditional unit of (low) pressure measurement in some fields. It's "one micrometer of mercury".

760 millimeters of mercury is one atmosphere.
There are 1000 micrometers in one millimeter.
So, 75 microns would be about 0.0001 atmosphere.

What would that be in inches of mercury?
Well, 75 micrometers is about 0.0030 inches.
So, 75 microns (micrometers of mercury) is about 0.0030 inches of mercury.

But wait! Don't those mechanical gauges show something like 30 inches of mercury for a "really good" vacuum?
Yes, they do, but only because that scale has been inverted so that zero is atmospheric pressure and the numbers get bigger as the pressure goes down from there. If you want to know what 75 microns is in their "inches of mercury", you'd have subtract the number above from 29.9247 inches of mercury:

29.9247 - 0.0030 = 29.9217 inches of mercury.

Think about this a bit, and it should be clear that the mechanical Bourdon tube gauge in an AC service manifold just isn't capable of confirming that such a vacuum pump has really reached an ultimate pressure of 75 microns.

Some of this was lifted from this article about pressure units from the Bell Jar.
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'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John
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  #7  
Old 07-25-2008, 08:49 AM
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http://www.lds-vacuum.com/conversion.html
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"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
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  #8  
Old 07-25-2008, 12:28 PM
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I use a Welch 1397. It will pull 17.7 cfm down to 1 x 10 -4 torr (which is a high vacuum). It is a monster, weighing in at over 200 pounds and it will probably suck the engine out through the exhaust pipe if I tried it (well, okay, maybe not).

You can get one here for $4,617.60: http://www.welchvacuum.com/products/welch/welch_vacuum_products/laboratory/belt-drive_high/large.jsp

I got my through DOD surplus for just over a $100.00 here: http://www.govliquidation.com. Run a search on the word "vacuum", but also misspell it as "vacume", vacumm", etc., and a bunch of other ways because the people running the place can't spell for beans.
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  #9  
Old 07-25-2008, 02:55 PM
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just don't anybody bid against me on my welch pump on ebay... please?
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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
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  #10  
Old 07-25-2008, 03:02 PM
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Quote:
Originally Posted by Whiskeydan View Post
+1 what Matt says. Oil needs to be changed every use.

Why not rent from your local Autozone?
For about 3 1/2 years I was a mechanic at a Refrigerated Container company. While I mostly kept their vehicles, air compressores, diesel powered generators and other engine powered equipment running one of my jobs was the keep the AC vacuum pumps in order. I changed oil and also replaced the vane pumps inside when needed.
The do get full of alot of nasty crap!
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  #11  
Old 07-25-2008, 03:10 PM
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Quote:
Originally Posted by Eskimo View Post
The "micron" is a fairly traditional unit of (low) pressure measurement in some fields. It's "one micrometer of mercury".

760 millimeters of mercury is one atmosphere.
There are 1000 micrometers in one millimeter.
So, 75 microns would be about 0.0001 atmosphere.

What would that be in inches of mercury?
Well, 75 micrometers is about 0.0030 inches.
So, 75 microns (micrometers of mercury) is about 0.0030 inches of mercury.

But wait! Don't those mechanical gauges show something like 30 inches of mercury for a "really good" vacuum?
Yes, they do, but only because that scale has been inverted so that zero is atmospheric pressure and the numbers get bigger as the pressure goes down from there. If you want to know what 75 microns is in their "inches of mercury", you'd have subtract the number above from 29.9247 inches of mercury:

29.9247 - 0.0030 = 29.9217 inches of mercury.

Think about this a bit, and it should be clear that the mechanical Bourdon tube gauge in an AC service manifold just isn't capable of confirming that such a vacuum pump has really reached an ultimate pressure of 75 microns.

(Some of this was lifted from this article about pressure units from the Bell Jar.)

You are right; when we see let us say a pressure guage and look at Zero on the guage we do not account for the Atmospheric Pressure. Sometimes I have seen pressure listed ad "psig" instead of "psi" meaning the pressure as read on the guage.
Anyway you answered my question the pump pulls enough vacuum to do the job.
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  #12  
Old 07-25-2008, 03:16 PM
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Quote:
Originally Posted by JimmyL View Post
Nice to see you two guys at least agreeing in this thread.
$15 bucks more gives you a 2.5 vs that 1.2

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=98076
2 good points!
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  #13  
Old 07-25-2008, 03:58 PM
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If you take a close look, I think that you will conclude that most of the lower priced vacuum pumps being peddled these days came off the same assembly line, regardless of which brand name is on the label. I would not pay more for additional CFM if the pump is only going to be used for an occasional evacuation. A higher flow rate is only going to pull a vacuum a few seconds faster.

I have a pump that looks very much like the one depicted; it works great.
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  #14  
Old 07-25-2008, 04:04 PM
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Quote:
Originally Posted by Eskimo View Post
If you want to know what 75 microns is in their "inches of mercury", you'd have subtract the number above from 29.9247 inches of mercury:
To be meaningful, the actual absolute (not seal level adjusted) atmospheric pressure at the vacuum pump needs to be used for the calculation. (Not the scientific standard sea level pressure.)
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  #15  
Old 07-25-2008, 04:09 PM
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Originally Posted by Whiskeydan View Post
Why not rent from your local Autozone?
They are not available at all locations.
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