Non-condensable's and Sight Glass Bubbles

[Saturatedpsi]

Howdy to everyone...

I'm trying to understand the effects of NCG's in high temp systems. Hours of research haven't as yet answered my question(s). I staged an "experiment" with an R22 split heat pump system (running in the cool cycle), beginning with decent performance numbers, then added a small amount of nitrogen to the system. TXV fed coil.

Beginning numbers:

Suction/SST...71 psig/41.6˚F
Superheat...13˚F
Liquid/SCT...213 psig/106˚F
Subcooling...11˚F

Sight glass full, located 12" downstream of liquid service valve

After adding nitrogen:

Suction/SST...73 psig/43˚F
Superheat...12˚F
Liquid/SCT...225 psig/110˚F
Subcooling...17˚F

Sight glass gave appearance of undercharge...less than full.

Anyone know what causes the bubbles/voids in the sight glass, and what they are? Nitrogen or *****?