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confused about vacuum pumps

bigkev on Tue June 17, 2008 1:46 AM User is offline

Im getting ready to buy a new vacuum pump but Im a little confused about some things. I noticed that almost every
vacuum pump has a rating around 25,50,75,100 etc...
Ive tried to find out on my own which one would be good enough for me but im not sure about something.
look at this chart I found:
I see that the cheaper the pump the less vacuum it can pull. My problem is however that if I go by the chart
100 or even 200 microns would be plenty to boil off moisture. Im concerned that these ratings are really 25k,50k,75k,100,k etc...
If this is the case then I can only imagine a 25 as being enough to pull a good vacuum.

also, Ive seen single stage, two stage, three stage pumps but im not sure about the benifits of having another stage. I see single stage
pumps that can pull more vacuum than some two and three stage. Maybe the two and three stage are just faster at achieving the pumps ultimate vacuum?

Fat Freddie on Tue June 17, 2008 4:23 AM User is offline

Thanks for the link to the scale. As for a dual vis a single stage pump both will do the job it might make a difference if you where making a living doing this kind of work but for out use as long as the unit will pull the necessary vacuum to pull the moister out of the system that is good enough.

Here is a discussion that should help:

navy article [email protected] writes:

Getting your system recharged at places that do not properly
evacuate the system is almost unavoidable for most people.

SO TRUE.... I estimate over 90% do not do good vacuums..

The vacuum pump used must be in good working order with regular
oil changes of the proper oil to even come close to moisture
free evacuation. A micron gauge (an expanded scale vacuum gauge)
must be used to verify that the required 29.90 in hg is pulled.
A perfect vacuum is 29.921. A guage set will be not accurate
enough. A bad or improperly maintained (probably most) pump
will not pull that deep at all.

This is very true... 29.0 inches of vacuum is 25,400 microns,
29.921 is 0 microns.. water boils out at about 1500 microns..
Good practice requires 500-700 microns.. with the vac pump
"blanked off" (valved off), with the micron gauge reading
the system vac.. for 5-10 mins.. If any water is left, vac will
float to 1500ish microns.. if leaking, it will go higher (worse vac).

A pump that is oversized may freeze the moisture before it
can be removed by dropping the vacuum too fast.

This is not completely true.. no such thing as an oversized pump..
a much larger pump will not evacuate any faster in many cases..
The water may freeze for awhile.. but it will "sublime" (vaporize
directly)... or melt as heat returns to the area.. It takes lots
of water to make chunks of ice.. and your system is probably
ruined if it had that much water..

The point is that some moisture being left in the system has
a high probability.

Very true

TRB on Tue June 17, 2008 1:02 PM User is offlineView users profile

Dual stage pumps will pull deeper vacuum quicker. For the average DIY person. The Mastercool 3 cfm or Mastercool Econ 5CFM dual Stage will be fine. 1000 microns to boil off moisture and the vacuum pump listings are all under laboratory conditions. Be hard to find a pump that is going to pull down to 25 microns in the real world.


When considering your next auto A/C purchase, please consider the site that supports you:

Edited: Sun July 06, 2008 at 8:24 PM by TRB

wrenwright on Tue June 17, 2008 4:05 PM User is offlineView users profile

A gauge set will be not accurate....

Is this correct? If so, why do manifold gauge sets have a vacuum reading capability?

bigkev on Wed June 18, 2008 1:59 AM User is offline

Thanks for all the input guys, I really do appreciate it. I went for one of the mastercool units. It looks much nicer than my fridge compressor in the milk crate with a light switch.

Fat Freddie on Sun July 06, 2008 8:20 PM User is offline

Thank you I too have been looking at Vacuum pumps, what I would like too know with so many brands out there which one's are good.

Edited: Mon July 07, 2008 at 12:24 AM by Fat Freddie

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