Hey everyone, as some of you know, I am pretty new to the AC installing world and am still getting some decent tools together. I grabbed a CPS digital micron gauge the other day, and have just used it on the first split system.

I am seeing some weird behaviour, and I'm just after some clarification, or an explanation.

So, basically, once my pipe work is complete, I pull a quick vacuum to around 1000 microns, then break it with OFN, taking the pressure up to a few hundred KPA, letting it sit for half hour or so while I wire the rest of the unit up, etc.

Then I dump that OFN to almost nothing, then vac it down to 500 micron and leave the vac going for as long as I can. The vacuum gauge today indicated 330 microns after an hour and a bit of pumping. I figured this was good, so valved off the hose to the pump, preparing for a 3000KPA OFN standing pressure test.
As soon as the vac pump is gone, the gauge indicates more than 20 000 micron.
Not too sure why, so I put in more OFN, this time to 3000KPA, and let it stand for an hour at minimum. The gauge has not moved...

I'm sure it's something simple, I'm not a fridge mechanic, I'm a sparkie, I've only done the split installers course. So please don't flame me too much :-P