Acutally, 1/20 a drop is for a capilary(sp) system., like a house portable
dehumidifier. A system with a reciever/dryer  should not have much
circulating moisture. Maybe a  uncaptured drop of moisture will shut it
down. It will combine with r12 to make phosgene gas.

 When charging a system, or just topping off with a refrigerant, purge your
hoses first. The air inside has moisture in it and it will be injected into
the a/c ststem.