You don't know if the service guy did put a thermometor on the system.
He might have done so, or he might have put a quick-readout guage on
the system and seen it was low.

In theory they should be able to pull all the refrigerant out of the system
with a machine, then put the same refrigerant all back in plus what
additional
was needed to get the system to the proper refrigerant level, in probably
the same time as adding refrigerant.  I don't know if
the usual practice at these places is to just dump the refrigerant to the
bottle
that goes to the refinery, though.

However, to be perfectly frank, if they are reusing the customers's
refrigerant
in his own vehicle, then that is a much better system than using
thermometers
and such.  With the thermometers your only getting a close approximation of
the system capacity, not the exact amount.  Even at a good A/C place it is
unlikely your going to get the master A/C mechanic to be doing a simple
topping off of a low system, instead your going to be given the entry level
boob
who may or may not be any good.  I'd rather have him working off very
simple diagnostic instructions (look at that guage, turn it off when the
guage
says 24 ounces have gone in) than complex ones.

Also, to be frank, checking the A/C system state of charge should be SOP
at any A/C repair place.  Not because it has anything to do with the
problem, but
because it is very quick and easy to do, and if the tech discovers a leaking
system,
then he should sweep the system for leaks.  This is a
good-for-the-environment
sort of thing that helps all of us.

Ted