13. June 2023

Sight glasses in refrigeration systems

Sight glasses provide important information on the condition of refrigeration and air conditioning systems.


Sight glasses are somewhat related to filter driers. The sight glass is normally installed in the liquid line directly after the filter drier in systems with an expansion valve. Theoretically, the sight glass can be installed anywhere along the pipe run in the liquid line. however, positioning close to the expansion valve is particularly advisable. A standard sight glass with indicator also usually has a dual function. it monitors whether the moisture content of the refrigerant is within acceptable limits and whether liquid refrigerant is always present at the expansion valve. if the moisture content is OK, the indicator colour is green and no further action is required. if the indicator is yellow, there is a problem with the moisture content of the refrigerant. If this is too high, it can even lead to icing of the expansion valve if the evaporating temperature is 0 °c or below. In addition, too much moisture in the system can react with the oil (polyol esters) usually used in modern refrigeration systems and lead to acid formation. a bubbling sight glass indicates either that there is not only liquid refrigerant at this point in the liquid line or that there is a lack of refrigerant. Both variants are not really good for the system and should therefore at least be recognised.



Standard sight glasses are available for soldering in and as flared versions with external thread, but also with external and internal thread for screwing directly onto the filter drier. In addition, there are sight glasses without a housing for screwing in and as saddle versions. Saddle sightglasses are mounted on the side of particularly large pipes with a corresponding adapter piece.



With soldered-in versions of sightglasses, special care must be taken during the mounting process. In general, brazing should always be carried out under nitrogen - this is well known - to avoid the formation of scale in the tube or sight glass. In the case of the sight glass, the use of nitrogen is particularly advisable, as otherwise it will quickly fog up black from the inside. This internal scale formation can be avoided with the general use of (dry) nitrogen.

Standard Schaugläser zum einlöten    Standard Schaugläser als Bördelversionen mit Außengewinde