Micron gauge, deep vacuum technique questions

[Sparkletron]

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

[Rob White]

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Where is the micron gauge in relation to the vac pump?

Was it fitted to the gauge manifold?
Do your gauges leak?
Do your gauge line have isolating valves on the end?

Did you valve it off correctly before disconnecting the pump?

The torr / micron gauge would rise for a few reasons.

One is it is faulty, so prove it is not by connecting directly
to a vac pump and vac it out, close the valve and let it stand,
if it remains constant your gauge should be ok.

Once you have proved the gauge, the only way it will rise in situ
is if something was getting into the system to cause the rise.

You do it the other way to us in the UK, we pressure test first to
prove integrity and gas tightness then we vac out. Possible gas leak?
Possibility of moisture in the system boiling off? Possibility of the refrigerant
leaking out of the AC shut off valve.

Try it on another system and pressure test first?

Regards

Rob

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