Hello, I will not pretend to be an HVAC or Auto A/C tech or technical authority. That said, I have sufficient capability to "top off" a system in good condition, using real, US-made high/low gauges and a 30# honeywell bottle of r134a. Problem is, all the good information that I had online I cannot find. I want to be definite with high/low pressures... Perhaps someone can provide their information/insight so that this can be a good reference thread. I was going to top up two cars, one a 91 BMW retrofitted to R134a about 5 years ago, and blowing cold but cycling a bit much, and one, my 2004 saab 9-3 which seemed to be louder than normal when the compressor was cycling on, but always blowing nice, cold air. So, the BMW... hasnt been used for over a day, cold soaked. Ambient temperature, 78F. I cannot find a good static pressure chart, so I pulled out my chemical engineering thermodynamics book, and looked at the chart. At 78F, it looks like my gauges should read roughly 83 psi to indicate that I have a dual-phase condition in there, and are not too terribly low... Well, I read about 75-78 psi. So that would indicate a slight undercharge, and explain why the compressor seems to be cycling a bit more than normal, right? On the saab, I didnt bother to read a static pressure because the vehicle was used quite a bit today, with the AC on, and then vehicle was heat-soaked. The real issue is high/low pressures. Ive seen general information online that indicates a low pressure of 25-45 is correct, at 1500 RPM. Ive seen high pressure info that ranges from 2.2xambient temperature through "don't worry about it, just watch the low side". On my BMW, it started at 25 psi when running the AC at idle on full fan, and would drop lower when the RPMs were increased. I filled it until I saw 30 psi on the low side at 1500 RPM, and the high side started creeping past 200 psi (which is roughly 2.5x ambient). On my saab, it was down at about 30 when running at idle, and dropped a bit when at 1500 RPM. I added some so that I was solidly at 30 psi when at 1500 RPM. THe AC in the car blew cold before (it did in both, in fact), but it got colder when I did this... Additionally, what sounded like a roughness in the sound from the engine area when the compressor was engaged, seemed to go away! Thing with this car was, the high side never really fluctuated or varied from 175psi. I was expecting to see it creep, at least when we increased the engine speed and added some refrigerant. So, long story short, an good information online? Any good sources of charts, data, etc? And, what is the right range to see the high and low pressures, both when at idle and when at 1200-1500 RPM? Thanks!!