Sight glass
Also known as: moisture-indicating sight glass, liquid indicator
Short definitionA small glass fitting in the liquid line. It shows the refrigerant passing through: bubbles mean the liquid supply is short, and a colour change on the indicator signals moisture.
It is fitted just ahead of the expansion valve and works like a window cut into the pipe. When the system runs properly, clear, unbroken liquid shows in the glass. The coloured ring in the centre is the moisture indicator: on most products green means dry and yellow means wet, while some makers use blue and pink. An indicator that has turned yellow says it is time to change the filter drier.
It is the first place to look at commissioning and during maintenance. Constant bubbles or foam show that vapour, not liquid, is reaching the valve; the charge may be short, the filter blocked or the subcooling insufficient. Brief bubbling as the compressor starts is normal. Charging by the glass alone is the best-known way to overcharge a system. The final call on the charge is made with superheat and subcooling measurements.