On Sat, 30 Jun 2018 15:27:42 -0700 (PDT), Cynic
wrote:

I've been online reading up a bit about air conditioning after seeing a neighbour having it fitted. I notice
some installation manuals stress the need for the pipework to be nitrogen purged and pressure tested then
a full(ish) vacuum be drawn before introducing the refrigerant. Other sources don't seem to mention this.
Is a small proportion of nitrogen in the system a serious defect and what would it's effect be during operation?

Professionally installed AirCon usually comes at ambient air pressure
and after installation is nitrogen flushed to remove water vapour and
taken to vacuum. It is then filled with the correct weight of
refrigerant. Doing it thus allows for pipes to be cut to size and
neatly fitted.

Twin units meant for DIY install often come with both indoor and
outdoor units pre-filled with refrigerant and sealed with push fit
fittings. The pipes are positioned, any surplus coiled out of the
way and the push fit joints made. The amount of refrigerant allows
for a small loss when joining the fittings. Shortening the pipes is
not an option.