each time you disconnect the hose from a can, you connect to a new can, you do two things, you introduce air and water, and you loose the speed of charging a evacuated system. also, CHARGING hoses are not meant to hold vacuum... so they leak. there are barrier hoses made specifically for evacuating systems.
best way to charge an evacuated a/c system, is evac, purge with N2, evacuate again, connect full bottle of refrigerant on scale, purge charging hose, weigh in full load of refrigerant in one shot.
only way to do this is with a jug larger than capacity of the system...
with cans, you put in 12 oz, disconnect, hook another 12ozer in there, etc etc etc...
quickly the vacuum is gone, and pressure is such that operation of the compressor is needed to draw in more refrigerant. not good for the compressor to operate in low charge conditions. oil circulation is reduced, and no matter how good you are with purging the hoses, moisture, and air is introduced each time the connection is broken...
also, notice how quickly the can gets cold? when charging an evacuated system, the entire charge can be dumped in on the condenser side, as a liquid, simple!

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!