Charging A/C system.. - Page 2 - PeachParts Mercedes-Benz Forum

baudenfj · #16 08-22-2009, 10:22 AM

I am pretty familiar with high vacuum equipment and techniques but I don't understand the units on my HVAC pressure gauge.

I have a relatively expensive pressure gauge used in labs. I am sure it works and is properly calibrated. I have a pressure in the system of 10^-2 mbar. I know you guys are not very familiar with the units so I do a little conversion.

- 1 bar = 7.5 * 10^5 micron Hg or mtorr

Therefore I have a vacuum of 7.5 micron Hg or mtorr. I trust that number.

Now the confussing part my HVAC pressure gauge shows in green that my pressure is sitting at 25-26 in of Hg. I can get a better pump and get it down more but I do not understand that meter/units!

If I do the conversion into in of Hg...

1 mtorr = 3.9 * 10^-5 in Hg therefore I should have about 30 mtorr or microns...

WHY does my meter show 25-25 in of Hg they must be different units... ! Can anybody explain?

Should I now take one of my vacuum pump stands which gets my equipement down orders of magnitude lower (10^-6 mbar) and try it but I don`t really want to risk contaminating my vacuum pump. What is the vacuum pressure of the compressor oil?

Matt L · #17 08-22-2009, 12:17 PM

Something isn't right, Baudenfj. One micron is 10^-6 meter Hg. Atmospheric pressure is 760mm Hz or 760,000 microns. So the first part looks right. But you aren't getting there or your mechanical gauge is way off.

As for near 30" vacuum, yes, that is true that this is what you want to see. But on my mechanical gauge, the difference between -29 (possible) and -30 (impossible) is about .010 inches. Maybe .020 at most as I have not measured. But that is 25,000 microns of difference.

When my real gauge goes from 10,000 microns to 500, there is no discernible movement of the mechanical gauge. The mechanical gauge is not trustworthy at all, but sometimes it's all you have.

Matt L · #18 08-22-2009, 12:21 PM

Quote:

Originally Posted by LarryBible

In my experience, I have found very few a/c pro's who are even aware of the measurement of vacuum in terms of microns. If you have a vacuum meter that will give you microns, it would be very good to see 100 microns.

If the truth could be known, I fully expect that most a/c systems rarely get evacuated much below 1,000 microns. If for no other reason than the fact that many people don't change the oil in their vacuum pumps often enough.

I have given lots of thought to coughing up the money for such a vacuum meter, but other needs keep cropping up diverting the funds.

My pump is rated at 25 microns, but in reality it will get to a solid 100 microns if it is dead-headed into the gauge. Add hoses and things get tricky. Add a car and things get impossible. The pressure goes slowly down, pops up a few hundred microns, slowly goes down, etc. Each time the pressure popped up, it popped up to a lower maximum than before, so it was working. Four hours later and the spikes were all below 400.

I don't recall the chart, but 1000 will get the job done in warm weather. It won't be enough if you're doing the job in the winter.

baudenfj · #19 08-23-2009, 12:47 AM

Quote:

Originally Posted by Matt L

Something isn't right, Baudenfj. One micron is 10^-6 meter Hg. Atmospheric pressure is 760mm Hz or 760,000 microns. So the first part looks right. But you aren't getting there or your mechanical gauge is way off.

Yes, my mechanical gauge most be off I do not even understand the units.. Pressure is force per unit area. If pressure is getting smaller the number must get smaller ! However on my mechanical gauge the numbers [inchers of Hg] are increasing as the pressure is getting samller!

Here is the missing link which I found on the internet

http://www.belljar.net/units.htm

The inch thing really got out of hand when manufacturers of rough pumps came on the scene. Rough pumps are arbitrarily defined as those used for: in-house vacuum systems; meat packing; impregnating lumber and transformer coils; making freeze dried coffee, tea or foods (got ya!). That is, any pump that hauls great loads of gas and vapor day-after-day to a modest vacuum level. These manufacturers noted that if atmospheric pressure was 29.92 inches Hg, they would be shooting for 0 inches. That would look bad in their brochures. So, they calmly inverted the scale. Atmospheric pressure is 0 inches Hg and the best possible vacuum is 29.92 inches Hg, they said. Which left the rest of us struggling with converting inches Hg to torr. (First, subtract the given inch pressure from 29.92 inches, then multiply the answer by 25.4.)

As for near 30" vacuum, yes, that is true that this is what you want to see. But on my mechanical gauge, the difference between -29 (possible) and -30 (impossible) is about .010 inches. Maybe .020 at most as I have not measured. But that is 25,000 microns of difference.

I agree it does not really tell us much in terms of microns... !

When my real gauge goes from 10,000 microns to 500, there is no discernible movement of the mechanical gauge. The mechanical gauge is not trustworthy at all, but sometimes it's all you have.

Yes, my mechanical pressure gauge is at least 5 inches off! Therefore a good vacuum is 25 inches on my gauge..

Nice day in TN. 77F in August no need for the A/C but I can not wait very much longer before my wife rushes out an buy a minicooper...

Keep those Mercedes running... thanks forum... I hope I don`t have to post another message related to A/C ... vacuum .. Cheers

samiam44 · #20 08-24-2009, 01:56 PM

I own a micron gauge.

My comercial friend says freezers or chillers which are way below freezing are much more critical of moisture, hence 200 microns is what he shoots for. I imagine with all the rubber hoses, o-rings etc you'd be hard pressed to get a leak-free automotive system below 400 microns after it sits for a half hour or so. I've flushed systems and then pulled them down, and you can watch the vacuum decrease as volitles boil off. Very cool stuff.
I find the gauge good to test the vacuum pump. Still PO at my robinair expensive pump which is rated at 4 microns and one day does 400 and the next 150. My comercial friend retires his pumps when they won't pull 200.
Harbor-freight had a 3 CFM, 2 stage 25 micron pump on sale this past week for $120.
Oh- and the digital micron gauge is helpful in leak checking your manifold/hoses etc.

Michael

#21 08-24-2009, 03:24 PM

I read that some of the oils used in a/c systems can suffer a break down at vacuums below about 450 Microns for extended periods of time.

That said, I have absolutely no idea as to the validity of that information.

baudenfj · #22 08-25-2009, 01:41 AM

Dear Forum:

I learned how the scale on our mechanical gauge works. The units are based on the arbitray zeroing 1 atmosphere to 0 inch of mercury and the real pressure is 29.92 in Hg - whatever your mechanical gauge tells you. Mechanical vacuum gauges could be way off at least at very low pressures!

Here is what I did.

- I got a professional grade vacuum pump which gets down to microns of Hg and evacualted the system for about 3 day and achieved a base pressure of about 10 micron with the pressure gauge right on the pump. Which means the pressure in the system was higher but not two orders of magnitude!

- Now I shut off the pump and discounected everything for charging the system.

- I got 3 cans of R134a (340 g each) and a simple charging hose with this piercing valve ...

- I connected the charging hose to the low pressure side and heated up the can with a bowl of warm water. It nicely streamed into the sytem. Beautiful trick.

- I am pretty sure everything went in.

- I tested the system as it says in the manual and measured the temperature at the center console straight after charging. I obtained a lowest temperture of 7.5 deg Celcius. Watching the pressure I could also observe that the compressor switches off as the temp of 7.5 deg Celcius is reached. Can anybody confirm...

Neverless thanks again forum and I have now a ice cold A/C in TN>... Now the car of my wife is fixed and I enjoy working on my 300SD.