sight glass is "empty", not clear

[jd1010cc]

hi, first of all, i want to thank for your guys. i learned a lot of from this website. i have a question regarding sight glass. i saw sight glass has bubble in it and sight glass with full of liquid and " sight glass with nothing in it". my question is "sight glass with nothing in it", is it normal? i came across this situations a couples times. the unit was running fine(cool down the box) for walk in cooler(freezer). can anyone explain to me why sg is empty instead of filling with liquid. what can cause that problem?

i also get the rule of thumb for this talk, 20 to 40 degree of overal system superheat is normal for refrigeration application. does it apply to small units like undercounter and bain marine unit use with cap tube?
i was told superheat should be around 10-15 degree.

[smurphy]

the sight glass was full of liquid not empty.

[gerryboy00]

do not mix compressor superheat (total superheat) with evaporator superheat reading

[referguy123]

I am curious, when the sight glass was "empty" did you happen to have gauges on the system? What were the readings? Did you happen to check the superheat of this particular system at the suction line of the compressor?

One of the things the metering device in a refrigeration system needs to have is a a solid column of liquid refrigerant supplied to it to operate properly.

In my experiences if the sight glass is "empty", and the unit is not at the tail end of a pump down, then there is usually something wrong (That is if you are using a sight glass, I caution you to never fully rely on a sight glass) Was there a solenoid valve near the sight glass?