Why do I need to use and exact amount of Freon?

Today's A/C systems use smaller amounts of Freon. No longer is it 2 to 4 lbs, but rather 18 to 38 oz. With tight compressor tolerances and lubricant mixed with the Freon, the mixture must be just right to enter the compressor as a mist. Wrong mixtures will either under lubricate or slug the compressor causing premature failures. You don't know how much liquid Freon is in a system so you should recover the old Freon, evacuate and add the correct charge. Always check the OEM spec for the correct charge.