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Sight Glass/ Moisture Indicator
Often in commercial HVAC and refrigeration, you will either find or install sight glass/moisture indicators. The sight glass portion is simple; it's just there to show if the liquid line has a full line of liquid or if it has bubbles, which shows it's a liquid/vapor mix.
A clear sight glass on a running system generally means a full line of liquid (or totally flat, but you would know that already if you have gauges attached). Reading subcooling essentially does the same thing as a sight glass; it simply proves that the system has a full line of liquid. In HVAC, subcooling actually gives you more data than a full sight glass in that it tells you the actual amount of heat that the refrigerant has lost past the condensing temperature.
In refrigeration systems with receivers, a sight glass is an excellent tool and can be relied upon as an indicator of liquid refrigerant to the metering device.
The moisture indicator shows you if the system is dry or if it has moisture content.
First, make sure you are aware that older sight glasses may not be sensitive enough to pick up wet conditions with HFC refrigerants that contain POE oil.
Second, when installing a sight glass, keep it sealed as long as possible before installing. If you open the indicator to air prematurely, it may change color due to moisture in the air. If that does happen, most indicators will change back after being installed, having a proper vacuum pulled, and the system is run for several hours. If it still reads wet after that time, the system likely is wet; new line driers should be installed and a deep vacuum pulled.
Your best defense against a wet system is fresh line driers, good installation practices that prevent moisture entry, and proper evacuation confirmed by an accurate micron gauge.