Back when i went thru Trade School and beyond...youd see ALOT of sight glasses used on TXV split systems ; made charging the system not only a breeze, but, accurate also. Used to see nearly every TXV system with a field installed Sight Glass -- same on automobiles. I understand the approach today is via the subcooling chart (hopefully) with the A/C unit, but, i sure miss watching the bubbles disappear thru the Sight Glass. They're only $8.00 each.