How much vacuum

[Mogman]

I am sorry as I am sure this has been beat to death but all the searches I try say the words are too common.
I am looking at getting a 2 stage pump,as I understand it they will most likely give the deepest vacuum but that is just what I have read.
It will not be used very often so I am looking at the cheap Chicom pumps.
But it is difficult trying to compare pumps as many use different terms and I am not having any luck making out the conversions.
Add to that that some are rated compared to atmosphere and some to absolute vacuum and I get a headache.

It seems most pumps are more concerned with CFM than ultimate vacuum which makes no sense to me but for commercial users where time is important and they probably don't use the cheap Chicom pumps anyway.

So how much is enough? I see 22.5 and 25 Micron, .3pa, 19.9" HG etc

I see pumps from $109 shipped on up, I do not mind spending 2-2 1/2 times that much if it makes a significant difference.

Again I apologize but even when I do this search on the internet I get many conflicting results and I trust the opinions of this forum members.
MM

[Tim]

Suggest a trusted brand that will more than fulfill your needs.

43-MT1721

[bohica2xo]

That is a solid pump Tim sells. A good value.

Pump CFM only matters if you are running flat rate labor on 3 bays worth of cars all at once.

I have been telling people for decades on the generations of this forum to buy and use a Micron gauge. It is worth having, both to check the system for leaks, and to check the overall health of your vacuum pump and service manifold / hoses.

Target vacuum for servicing a system with a mechanical shaft seal is anything below 400 microns.

Direct drive pumps like the one Tim sells will go well below that. This is a good thing. You can test a new pump at say 75 microns. Then you can pull a vacuum on your whole manifold / hoses / service connectors and see if it leaks.

[Mogman]

Thanks!

[Cusser]

Suggest a trusted brand that will more than fulfill your needs.

43-MT1721

THIS.

[Tim]

I find it odd that all my direct suppliers. Do not carry Micron Gauges, but know them as well. The response would be to get it from the tool store.

[bohica2xo]

Tim:

MVAC was always the red headed step child of HVAC. I learned HVAC from the RSES courses for the refrigeration techs who worked at the big plants. I took those classes in self defense after paying a shop a lot of money to do a poor job on a car I owned.

The first micron gauge I used in class was a 110v unit with a vacuum tube to amplify the voltage swing on the thermistor. But as soon as I understood the difference between 29.5" on the service gauge and 400 microns, I never was satisfied with the 20 degree arc on the low side compound gauge.
 
Back then all MVAC was R12. Removing the non-condensing gasses from the system was performed by vacuum pump if you had one. People used engine vacuum at times and even just R12 bled through the system to replace the air with refrigerant.

Times have changed. A lot. Systems hold very little refrigerant. We tell people to change vacuum pump oil frequently. But I prefer to have an actual measurement of performance.

A micron gauge is a lot cheaper in todays dollars than it once was. I know Robinair sells the RAVG-1 but it is about twice as much as an import gauge from the usual sources. Supco still makes the VG-60 and it usually sells for under $150. I am surprised your suppliers don't list some kind of micron gauge.

[Mogman]

I bought a digital "micron" gauge, albeit a cheap one, it has 13 different scales but I see nothing about microns..
It does not have a mTorr scale

[bohica2xo]

1 Micron = .001 Torr

or

1 micrometer of Hg = 1 milli Torr

We started with a column of Mercury 30 inches tall. Except Atmospheric Pressure limited this to 29.92 inches. 30 inches being an absolute vacuum.

While small fractions of the last inch of mercury were good enough for Crookes to make vacuum tubes with, we needed smaller numbers to describe vacuum with. So we converted that 29.92 inch high column of mercury to millimeters. 760 millimeters. But this was still not a fine enough measurement, so we divided again, this time by 1000 and got Microns of mercury.

The metric system mixed with inches, divided by 1000. Messy.

https://www.engineersedge.com/calculato ... _15037.htm

So your target vacuum of 400 Microns would be 400 mTorr.

Confused yet?

[Mogman]

So 1 Torr = 1 mmHg yes? so 1 micron would be .001 mmHg yes?
I guess it depends on the resolution of the Chicom meter which has diddly squat to say about the resolution in the "manual"
So a target of .4 to .5 mmHg would be correct?