How to fully empty refrigerant tank during recharging AC

[timsch]

I'm looking to do some AC work on some of my vehicles and I've purchased a recovery system along with a 30# tank (empty), vacuum pump and a scale. Looking at videos on how this works, during the recharge, the car/truck is run with the AC on, which is supposed to pull in the last remaining refrigerant from the tank. Every time I've used manifold gauges and looked at low & high side pressures, they are always positive. Even with the tank contents going into the low side, there's positive pressure, so how will the tank ever become empty?

Setup like this:
https://youtu.be/O8s5SzUXtiQ?t=1165

[JohnHere]

I'll hazard an explanation with the following presumptions: Recovery of the entire contents (refrigerant only) of a system into the vacuum of a recovery cylinder, and then attempting to recharge the same system with the same refrigerant contained in the recovery cylinder.

The low side of the system, while charging, will be at a negative pressure at first (presuming charging into a vacuum) and rising in pressure as charging proceeds and the compressor engages, which creates suction again. The pressure in the recovery cylinder will be positive all the time and significantly higher than the low-side suction pressure (until the completion of charging), depending on the temperature of the recovery cylinder. Therefore, the recovery cylinder's contents will flow back into the system's low side (higher pressure flows into lower pressure). The recovery cylinder probably won't *completely* empty at that point, but close to it, just as a small 12-ounce can of refrigerant would mostly empty into the low side.