On Fri, 21 Dec 2012 11:09:45 -0500, " Sir Gregory Hall, Esq·"
åke wrote:

"Bruce" wrote in message
.. .


But your question brings up something... The American refrigeration
people I've worked with all insisted on long periods of vacuuming
systems. the Thai's on the other hand hook the pump up, pause to light
a cigarette, and then charge it with refrigerant.

But I've never had a Thai serviced system freeze up :-)


Americans tend to believe the old adage that if a little is
good, a lot is better and too much is just right.

Respecting drawing a vacuum in a refrigeration system,
it can't possibly matter how long the vacuum is in place.
One minute is as good as one hour. In a vacuum one
draws out molecules. As many water molecules will be
drawn out in a vacuum held for one minute as for one hour.

American's also believe in ripping off the customer. If a
service technician can charge you and extra hour labor
while he sits and twiddles his thumbs or smokes cigarettes
he will do so.

Here's basically how it works. A "decent" electric vacuum pump is
capable of pulling down to about 200 microns.
The pump comes equipped with the standard vacuum gauge, which in the
U.S. is measuring inches of mercury vacuum. inHG.
Not accurate enough to measure microns. It will be bottomed when
there's still more vacuum to be had.
But a decent micron measuring gauge cost twice as much as the evac
pump. A good HVAC guy buys one.
Typically refrigeration specs call for evac no higher than 500
microns.
Since the evac pump has to work harder and takes longer to draw down
those last microns, let's say from 10000 down to 500, but the inHG
gauge needle has stopped moving, you let the pump run on for a while.
It could take the pump 10 minutes to bottom the inHG gauge, and
another half hour to actually reach 500 microns vacuum, which is what
indicates an acceptable amount of moisture has been boiled off.
Some people - like me - can "guess" by the sound of the pump when it
has stopped pulling vacuum. But without an accurate micron gauge,
it's really just guessing in the end. I usually end up letting the
pump run about 5-10 minutes after both the gauge is bottomed and
there's no tonal sound difference from the pump. But without the
micron gauge, it's voodoo anyway. I don't sweat it.
I've read that pros with micron gauges sometimes have to run the pump
for hours to get down to 500 microns. I can buy that.
In the past I've repaired my R-12 auto A/C's with no evac, and run
them for years after.
I started maintaining family cars with R134 some years ago, and wanted
to treat them better than I did my old cars. So since one of my kids
is a mech and they had an air-operated vacuum pump in his garage, I
had him do evacs at work a couple times. That was a hassle for him
and me because of timing. So I looked into buying a vacuum pump.
First thing I found is air operated vacuum pumps are plain incapable
of achieving 500 micron vacuum. Lucky to even reach 2000 microns.
And that's with a high capacity air compressor running a looong time.
So I bought a mid-level Mastercool electric.
About 120 bucks and about 30 for a couple quarts of oil, which is
important maintenance on a vacuum pump.

Now the other main part is about maintaining vacuum. That doesn't
require running the pump, but does require time. I usually let the
system set for 2-8 hours with the pump off and isolated when I'm done
pulling vacuum and before I charge. Depends on how lucky I feel.
That's to see if the system has a leak.
Again, that's mostly voodoo without a micron gauge hooked up.
The last time I maintained vacuum on the inHG for a couple hours,
after replacing the compressor and dryer, Gauge never moved.
I still lost R134 from a very small condenser leak, and a year later
the car stopped cooling.
Supposedly a micron gauge would have picked up that leak because of
its sensitivity.
But I might be wrong about that, and the condenser wasn't leaking when
I tested leakdown. No way for me to prove the timing of events.
Anyway, having your own electric evac pump and a gauge set is a good
idea for the DIY car or boat guy. Inexpensive enough to pay off the
investment. A micron gauge is kinda stretching it, and I don't plan
on buying one. I don't mind a little voodoo.