I've never had a high pressure switch cutout on me, but then I generally only charge AC in the summer. In the winter I don't need it.

As a very rough guide, I've found that the system off pressure when everything has equalized and the car is shut off and sitting in the garage at about 20°C is around 70-75psi.If for some reason I can't have the car running when charging, I bring the system pressure up to that and it's pretty close to a good charge.

 

Then there's another interesting refrigerant called R152a. It's interesting to me becasue I use it almost every day as an IT consultant. The "can of air" or "air duster" can with the pull trigger is an interesting case. These cans are filled with R152A which has a GWP of 132. Lots of R152A is dumped in to the atmosphere each day by me and others on purpose and It also has mild flammability. I've even seen YouTube videos where a can of "air duster" is side tapped and the tap is used to feed the R152a into an automotive a/c system where it seems to work fine as very low cost replacement for R134a.

These cans can be purchased for less than 4$ making them a very cost effective alternative to R134a with lower flammability than hydrocarbon blend replacements.

 

R152a isn't compatible with mineral oil in an R12 system though, so the oil would need to be flushed out. Apparently about 10% less cooling capacity than R134a.

 

It's a bit more complex than that. R134a efficiency rapidly decreases as condenser temperature goes up. R152a increases in effiency relative to R134a as condenser temps rise. Many of us have experienced this performance decay with R134a where it cools fine on an 80 degree day but cools terrible on a 90 degree day. This phenomenon also presents with R134a as cooling well when moving but not cooling so well when stopped in traffic, then vent temp will start to rise. R152A's performance doesn't decrease as fast so at 100F condenser temp R134a will cool 10% better but when the evaporator is at 120F (hot day, slow traffic) R152a will actually cool 5% better than R134a. This makes R152a better in automotive situations where condenser temps can rise, like slow traffic on a hot day. Most automotive a/c system are limited by condenser size since the engine turns the compressor at speed it will easily overcome the performance decrease at lower condenser temps.

https://journals.utm.my/index.php/ju...icle/view/9661