Vacuum

[question mark]

Hi guys
over the week i was working on evacuating the system. new installation.
1 .manufacturer want 250 microns of vacuum.
2 well first we hooked up the vacuum pump and the Vac stat digital gauge.
3 it pulled about 850 microns. we left the vacuum pump over night running but when we went again over there in the morning someone tripped the circuit breaker. so we lost all the vacc only for 15 mins.
4 we again put nitrogen in and after half an hour evacuated the system. left the vac running over night. in the morning down to 550 microns.closed the gauges and see what vac stat says. it is slowly rising. like in the first 15 minutes from 550 to 600 and then in other 15 minutes 615.
5 so thought might be a leak . put nitrogen in again to see if there is any leak. 5 hour 380 psi no movement. left nitrogen in over the weekend.
6 remote condenser , air cooled. server room air conditioner.
Question. how many micron rise over a period determine moisture or leak in the system. i know some people say slow rise means moisture and fast rise means leak. but what number can we call slow and what no of microns rise is call fast. i guess depend on the ambient temp as well.

Thanks

[leegally1983]

sounds like moisture to me, never known of a exact way of determining which one it is. is the unit old or new? why was you repairing it in first place or is it a install? you may have a real small leak and nitro isn't getting though maybe try helium. without sounding like a c**t have you checked you gauges are not leaking etc

[chilliwilly]

If its holding with the OFN test, its definitely sounds like moisture. Did you pull a deep vac straight away, or did you open the vac relief to pull a more steady vacuum?

When I vac a system I always open the relief valve on the pump to pull a steady vac down to about 500 microns, then close the valve to pull down to about 170 micron. If it rises over 500 micron I know I've still got moisture in the system and the vacuum has caused the energy inside the system to get too low and cause the moisture to form ice. (Something I'd forgotten until I joined this forum). If it keeps rising I use the triple vac method to help dry it out, or put some heat on the out door coil if it has one, to help the moisture boil off.


Digressing from the thread.

I find that these days the manufacturers standards of sealing a system and pulling a vacuum is lower than what it used to be, especially on cabinets and the like. The amount of times I've been to under gassed systems that are new or still under warranty, decanted and pressure tested and found no leaks. And pulling a vacuum has taken much longer than anticipated, would suggest that they haven't bothered heating the coils and line set to dry out the system before sealing it, and just connected the system to a production line apparatus that pulls an instant prescribed vacuum too quick, and immediately give it a shot of gas and not bothered to check for sweat back or frost back on the capillary. Only for the owner to find that its not cooling because its under gassed and has non condensibles causing partial blockages.

I think its something that's going to get worse with the profit before labour and quality attitude that manufacturers have these days. Maybe the F gas regs or other countries similar codes may implement a system whereby the manufacturing and tightness testing of the system is done by the cheap economy countries, then commissioned in a warehouse at the port of entry before they go any further.