Sight glasses are related to filter driers in a certain way. In a system with an expansion valve, a sight glass is usually fitted in the liquid line immediately after the filter drier. In theory, the sight glass can be fitted at any desired location on the liquid line. However, placing it close to the expansion valve is especially advisable. Generally speaking, a standard sight glass with an indicator also has a dual function. It monitors whether the moisture content of the refrigerant is within an acceptable range, and it indicates whether liquid refrigerant is always present at the expansion component. If the moisture content is OK, the indicator colour is green and no further action is necessary. If the indicator is yellow, there is a problem with the moisture content of the refrigerant. Excessive moisture content can cause icing of the expansion valve if the evaporating temperature is 0 C or lower. In addition, excessive moisture in the system can react with the oil normally used in modern refrigeration systems (polyester oil) to form acids. Frothing in the sight glass indicates either a shortage of refrigerant or that the refrigerant in the liquid line is not entirely in the liquid state. Neither of these situations is good for the system, so they should at least be recognized. Variants Standard sight glasses are available with solder fittings and flare fittings with male threads, as well as with male and female threaded fittings for direct connection to a filter drier. Sight glasses are also available without housings for screw-in installation, and as saddle-mount versions. Saddle-mount sight glasses are fitted to the sides of especially large-diameter pipes using suitable adapters.

• Variants Standard sight glasses are available with solder fittings and flare fittings with male threads, as well as with male and female threaded fittings for direct connection to a filter drier. Sight glasses are also available without housings for screw-in installation, and as saddle-mount versions.