Sight Glass

A sight glass does two jobs on a liquid line, and both are diagnostic. First, it shows whether the line is full of liquid: bubbles passing the glass mean flash gas, which points to a shortage of charge, a restriction upstream, or a condenser that is not condensing properly. Second, the element inside the glass changes colour as the moisture content of the refrigerant rises, so a glance tells you whether the drier is still doing its job or is close to saturation.

The SG series is available in two end styles. The flare versions, SG2F to SG6F, use 7/16-20UNF to 1-1/16-14UNF threads and cover 1/4 inch to 3/4 inch SAE connections; the solder versions, SG2S to SG7S, cover 6.35 mm to 22.22 mm copper tube, which is 1/4 inch through 7/8 inch. The body is brass, with the indicator element and the glass window sealed into it, so there are no serviceable gaskets to perish at the joints.

Mounting position matters. The glass goes in the liquid line, downstream of the filter drier and upstream of the expansion device, in a place where it can actually be seen during a service visit. Horizontal installation with the window facing the technician is normal; if the glass is fitted vertically, fit it with flow upwards so the window stays full of liquid and bubbles are visible as they pass.

Reading the indicator: the element is green when the refrigerant is dry and moves towards yellow as moisture rises. A yellow element does not mean the system is wet, it means the drier is approaching saturation and should be replaced before the moisture reaches the expansion valve and freezes there.

Sizing is by line diameter. Because the glass slightly reduces the bore, fit the size that matches the tube rather than going one size down; the pressure drop of a correctly sized glass is negligible.