Be aware that while the sight glass worked for assessing charge level on R12 systems it does not work for 134a. Yes, you will see bubbles, but if you charge up to where the bubbles disappear it will be overcharged. Pretty much all conversions wind up with a lot of judgment as to how much 134a to put in. The sight glass isn't reliable, nor are the conversion factors for weight (they give a range, not an exact figure for all systems) although they are probably the best starting point if you have a fully original system. I have always targeted the amount by weight but watched the pressures. My general guideline is watching the low side pressure (bypass the low pressure switch if there is one so the compressor won't cycle). When the system is less then full the lower side will draw down pretty low, like into the low 20's. Engine should be perhaps running 1500 - 2500 rpm. As you add more 134a watch for the low side to stop going down and start rising. Stop charging and let it stabilize. I feel the sweet spot is for it to be stable at 1500 - 2500 rpm at a low side pressure of about 32 to 40. When it engine goes back to idle low side will rise. Speed it up to 2500 and you want it to not go below about 30, maybe 35. Every system is a bit different. Too little 134a and low side pressures will be low, too much and they will run high, like 45 to 50. Also be watching the high side, it you are getting too much in it will start to go up. Sometimes in the driveway with limited air movement it will read higher than ideal. I have had a couple instances of doing this where I wound up with too much 134a in the system and when I took it on the freeway I could feel the compressor short cycling from excessive high side pressure. On newer systems which don't have much storage capacity for excess 134a, I would say the difference between the perfect charge and too much can be as little as 1/4 can, maybe half a can. That's only 3 to 6 oz.
