I came across another version of the charging-by-increments and monitoring pressure. It was embedded in this thread
and here is a quote:
bohica2xo wrote:You can add small amounts of refrigerant while looking at the pressures to sneak up on a full charge with a TXV system like that.
On a custom system where the charge level is only a guess I handle it like this:
Watching the high side with it idling & the cabin fan on low, add a half ounce of refrigerant to the low side & wait for a minute. Did the high side stabilize at the same pressure? Or 1 psi higher? That means the refrigerant is being stored in the Receiver as liquid, where it takes up very little space.
If you add 0.5 ounces + 0.5 ounces + 0.5 ounces, and see little or no high side rise, it is still stacking up in the receiver & the line from the condenser.
If you see a 5 psi rise with only a half ounce of refrigerant, it is filling the condenser itself. This is the "spike" One should recover the charge at that point & weigh it. Then deduct an ounce & use that for a charge spec.
The problem is you only know what "enough" is, by going too far...
Add a little refrigerant, monitor pressures & the vent temps. Remember to let the system stbilize for a minute after adding. Log the resuls & post them for us.
These is one part I don't understand about the quoted method described by bohica2xo. He states that post-pressure spike is "too far" and that you'd have to deduct an oz after finding this point. At this point the condenser has just started to fill with liquid. I thought the condenser should have some saturated liquid refrigerant in the bottom so it can begin to subcool before it goes back to the receiver. In this thread
where DetroitAC described the increment-pressure method the OEs used, it sounds like they would stop incrementing after they have ~20F subcool coming out of the condenser. But of course they are also looking at maximum load conditions in a controlled climatic wind tunnel with the car running 25-30mph; the method described by bohica2xo is at idle with low blower speed.
I am still looking for a good description of charging by sight glass and a comparison with the incremental pressure method. I'm also curious how ambient conditions affect the approach. i.e. if it's 80 deg outside what do you do differently from if it is 90deg outside.