Thanks for the feedback.
I totally agree. The weight of the complete refrigerant charge is the most accurate way to charge the system.
I've googled the subject and most of the technical articles have mentioned that in the days of R-12 refrigerant, the sight glass on top of the receiver-dryer would look clear when the system was right, indicating liquid refrigerant with no vapor bubbles. However, this is not applicable for R-134a . It uses a different type of oil, polyalkaline glycol (PAG). PAG oil does not fully emulsify the way the mineral oil used with R12 would in the past. The result is that the sight glass looks slightly cloudy even if the system is fully charged, except at very cool outside temperatures". The "sight glass charge inspection" can be used if the following conditions are met :
  • Ambient temperature below 95 deg F. >>
  • Humidity below 70%. >>
  • High condenser fan speed (>=1500 rpm).(It's normal for some bubbles to appear in the sight glass of R134a systems in low fan speed).
  • High-side system pressure below 240 psi (1640 kPa)>
Summarizing: R-134a is very hard to adjust the recharge rate to so it's usually weighed into the system rather than depending on the sight glass method.