The oz. on the refrigerant cans are indicative of weight, as with all gases. It's not a liquid except when under extreme pressure like in the can or in half the a/c circuit. The big tanks of 134/22/12 are not x gallons, they are x pounds.

The sight glass is actually pretty good. May have to clean it with a qtip and cleaner. But the bubbles are very visible when there is boiling in there. It's a lot easier to read than the Delco dot.

PS dude WD40 on the battery terminals is a bad idea because petroleum oils are dielectric.