SGN sight glasses are used to indicate: the condition of the refrigerant in the plant liquid line, the moisture content in the refrigerant and the flow in the oil return line from the oil separator. The SGRN is used to indicate the liquid level in a receiver or the oil level in a compressor crankcase. The SGN and SGRN are fitted with an indicator which changes colour to show the moisture content in the refrigerant in HFC or HCFC plants.