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 similar to this

 

[03TDICommuter]

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 similar to this

Your recovery tank will have liquid and gas in it. The majority of the refrigerant weight and volume is the liquid form. Your low side pressure will be below the pressure of the refrigerant at ambient temperatures, so your car's ac will consume all the liquid from your recovery tank. When done, yeah you'll have some pressure, but it won't be much volume nor weight.

What recovery machine did you buy? I've been looking to get one for a while and can't decide.

 

[timsch]

I bought a Vivohome model like in the link above. Gave it a test run on a truck that I know has a leak, so not much R134a left in it, but it worked fine. Time will tell how long it'll last.

 

[timsch]

Regarding gas vs liquid in the tank, I'm doubtful there was liquid in that 30# tank with the smaller amount I put into it. I was careful as directed in the video to slowly extract the refrigerant from the truck and never saw any liquid in the manifold gauge site glass. With a vacuum in the tank initially, I'd expect it to remain gas. Once I get to the vehicle I bought this setup for that has a full charge, even that will be only about 2 pounds of refrigerant. I'm wondering how much I will not be able to get out of the tank.

I'd assume that the small R134a cylinders for consumers are under pressure so that the contents are forced into the system. I've heard of heating it somehow to force even more out of it. That'd be more difficult to do with this larger tank, although it'd be possible. I could put it in a kettle and heat some water.