Jim,

You've gotten good at it so now you don't need to do it as much anymore. I'm glad to hear that you're staying cool without having to go into the shop and sweat to make it happen.

I agree that those last 100 Microns are not quite as necessary, but the more moisture you get out, the less corrosion that can take place from the combination of moisture and refrigerant. How much less, I wouldn't have a clue, but a deep vacuum is a good thing if you have the necessary equipment to make it happen. If not, and you can pull 28 inches, you're doing better than a few shops I've seen.

It is not just the depth of the vacuum that is important...
the moisture is in the oil in the system...and that oil is spread around in high and low spots... it does not boil off evenly... one section may need to get down to a certain level before the next starts being effectively addressed by the vacuum situation...

So deep vacuum applied as long as is practical does the best job of getting out moisture.

__________________
1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Oil is exactly the problem. The weight of the oil supplies additional pressure to the surface of any water droplets. Oil is a lot heavier than air, and the water doesn't care what supplies the pressure.

My Mastercool two-stage pump is rated at 25 microns, but it won't do it. It will get to about 100 when dead-headed against the gauge.

I have used brand new Yellow Jacket 2 stage 5cfm pumps that would not get to 25 microns. An older auto ac system evacuated to 100 microns would be very good. The ambient temperature is a consideration, the higher the better. On critical systems I have run a torch along all the available exposed piping just to flash more moisture into the vaccum. Not possible on a auto ac system, but worth a mention in regards to temperature.

seriously, if you are getting the mobile a/c system below 1100 microns you are doing very good.

I agree. But you can't tell 1100 microns from 25000 on a typical low-side mechanical gauge. Well, I can't.

true...
a lot of pawn shops have the thermister led micron gauges.
too bad we can't evacuate, flush and re-evacuate anymore...

If you have a Micron gauge to monitor with, once a pump starts losing performance, you will probably be surprised at the improvement by simply changing the pump oil.

Many years ago I worked in the instrument shop at North Texas State University Biology/Chemistry department. Along with keeping the Spectrophotometers and Liquid Scintillation Counters going, Vacuum Pumps were part of our responsibility. It was very common for students to turn the wrong valve on their apparatus and suck some serious contamination into the pumps. We had a Micron gauge that we tested with, and simply changing the oil would provide drastic improvements.

These were big, two stage SERIOUS vacuum pumps that cost a ton. Sure wish I had one.

I have two welch two stage pumps. SERIOUS pumps. one is a 15CFM model. HUGE! but it pulls down a large chiller in no time!

OK, guys , it is time for you to tell us that do not have a micron gauge which type/s you recommend..... what are the most important features to look for in figuring out what to buy?

__________________
1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Yeah, WELCH. I've forgotten alot in 35 years. I wish I even had a small Welch pump.

I'll have to look, but I think mine is a TIF. It has a LCD display reading in microns, once you get a partial vacuum. It doesn't work at atmospheric pressure or just below, just showing a "full scale" reading.

The biggest downside is that the sensor can't be replaced without calibration (presumably by the factory), but the upside is that the sensor is actually replaceable.