Strange micron gauge behavior

[MushCreek]

I'm trying to vacuum down my new mini-split system. I have a CPS VG-200 micron gauge. I have a valve removal tool in place, which has a shut-off to the pump, and a T fitting to mount the micron gauge. I've run the pump quite a while, but I'm still not getting a reading from the micron gauge unless- I turn OFF the valve going to the pump! Then, it starts reading, going down to maybe 33,000. If I open the valve to the pump, with the pump running, the microns go back up until it stops reading. Close the valve, and it drops back down.

I connected the gauge directly to the pump, and it sucked right down to 500 or so in a minute, so I know that the gauge is good,, and that the pump is good. Why am I not getting a reading until I shut OFF the vacuum coming from the pump? What is the gauge reading, the residual vacuum in the system? If there's vacuum in the system, then why does opening the valve to the pump make it lose vacuum? I would think that if there's a leak somewhere, it wouldn't pull a vacuum at all. I'm baffled, and not finding anything on google searches.

 

[fitter30]

Could have a leak. Did u happen to pressurized the system with 500 lbs of nitrogen pressure. Let sit for for few hours after leak checking the connections and the manifold. Since minis only have one gauge port move micron gauge to the other side of the manifold.

 

[American Locomotive]

Can you take a picture of your setup? It sounds like your tool might have a front seat and a back seat?

 

[Sumboodie]

vacuum gauge that measures in microns

 

[MushCreek]

OK, I finally got some success. It was either a lot of water vapor messing it up, or a bad hose. I changed hoses, and it pulled it down just fine. I wanted to **** out as much moisture and whatever else before pressure testing it, plus a vacuum test is easier to set up without dragging out the gas bottle until I know that there are no huge leaks. The vacuum pump was blowing off a lot more 'steam' than the last time I used it, but that was on a new set of lines. Once I got it down to the point where the micron gauge was reading, it stopped blowing off so much steam/smoke. It held good vacuum for an hour, so now I'm pressure testing it at 500 psi for 24 hours. If that looks good, I'll open 'er up and see how it works.

I'm still puzzled as to how it didn't read microns until I turned off the vacuum source.

 

[rawen2]

Maybe the hose was leaking close to the gauge? Or the valve was leaking in the open position but not in the closed position?

 

[Ohmthis]

From you description it’s hard to help you what may have been the issue. I will say that vacuum is “Not” the correct way to leak test. I have seen where the vacuum actually pulled a non brazed fitting together and held a decent vacuum. Always pressure test with nitrogen. It does more than just test for a leak. The nitrogen will also displace moisture, which speeds up the evacuation process.

 

[MushCreek]

After 5 hours, it was still holding at 500 psi. If all is good this afternoon, I'll pump it down and open the valves.

 

[fitter30]

While pump is on its pulling in non condensables. With micron gauge at the pump and a hose leak gauge is just seeing hose. Turning pump off it seeing the system. If u waited a few minutes gauge would rise to 0. Pulling a vacuum always pressure test first by vacuum testing first could be pulling in non condensables. Always use fresh oil has lower vapor pressure = lower vacuum.

 

[LS6 Tommy]

What core tool and hoses do you use? A lot of the core tools leak like crazy and older hoses can be permeable or have low loss fittings that won't hold vacuum. I replaced the compressor on a lab fridge a while back and couldn't get much below about 1400µ with my Supco core tools and my older Imperial Polar Shield low loss hoses. I decided it was a good time to get into the 21st century and got a Testo 557s Smart Vacuum, NRP HD vacuum rated hoses without low loss fittings (I use Uniweld EZ Turn low loss adapters for charging/service) and two Appion vacuum rated core tools. Hooked it all up the same way and pulled down to under 400µ in under a half hour...

Tommy

 

[MushCreek]

This was a while ago. As I said, it turned out to be a bad hose. The system held good pressure for 24 hours. I vacuumed it down, and again, it held good. It's been running good ever since. Thanks!

 

[fitter30]

OK, I finally got some success. It was either a lot of water vapor messing it up, or a bad hose. I changed hoses, and it pulled it down just fine. I wanted to **** out as much moisture and whatever else before pressure testing it, plus a vacuum test is easier to set up without dragging out the gas bottle until I know that there are no huge leaks. The vacuum pump was blowing off a lot more 'steam' than the last time I used it, but that was on a new set of lines. Once I got it down to the point where the micron gauge was reading, it stopped blowing off so much steam/smoke. It held good vacuum for an hour, so now I'm pressure testing it at 500 psi for 24 hours. If that looks good, I'll open 'er up and see how it works.

I'm still puzzled as to how it didn't read microns until I turned off the vacuum source.

If your using nitrogen it will savage water vapor because it's so dry.