Slow leak testing

[BradC]

G'day all, I recently fixed a 15kw ducted system for a mate. It is 9 years old and it had a blocked orifice piston (looked like a piece of cellotape had been floating around in the system since manufacture). He had a pro come and look at it. The "pro" said it was leaking and charged them for 3 kilos of R22 (did not recover and weigh first). Told them the leak was so small it'd never be found and the 3 kilos would do them a couple of years. That did not fix the problem, so they called him back. They watched him recover 7kg of R22 (which is what the system holds). He did some pretty nasty things to the condenser pipework, brazed it all back up and put the gas back (supposedly). No dice. Fast forward 2 weeks and $1400 to the "pro" and I get a desperate call for help.

So the first thing I do is turn it on, find the blocked piston (hot one side, cold the other - simple) and pull the gas out. Hey at $220/kg, if it's leaking we don't want to lose any more. I removed the teflon tape he'd put on the service valves, and fixed the mating surfaces he'd buggered up with his wrench. I then put 150psi of Nitrogen in there and left it for 5 days.

In 5 days I still had the right pressure (when allowing for temperature - I ended up with 160, but it was hotter that day).

There is no way the system had lost 3 kg of gas. It went from working perfectly one day to dead the next, so I assume he charged for 3kg and put nothing in. The owner also saw him pull out 7kg, and I only found 5 in the system. The service valves were leaking, but not that much to lose 2 kilos in 14 days. The Big-Blu bubble fluid indicated a very slow leak, so we assume he nicked some gas as he went. I pulled nearly 2 rolls worth of teflon tape from the service valve threads. Why is it muppets think teflon tape is doing to help seal a flare ?

Anyway, my question is would 5 days with no pressure drop on nitrogen be enough to be sure the system was not actually leaking? My experience has been that even tiny leaks show relatively measurable pressure drops over 24 hours, and they also tend to prevent pulling and holding a good vacuum. I got no drop in 5 days, and it held below 500 microns for a couple of hours. Having said that, I've not worked on a system larger than about 9kw and 3kg of gas before.

Given infinite time and a client who wants it done *right* regardless of time/cost, how would you do a leak test for a possibly tiny leak?

[edit] Why are all my paragraph and line spaces being stripped out? I can't get this post to look anything like what I fed in. I have to add html break tags manually to get anything to work?