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.


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7 responses to “Sight Glass/ Moisture Indicator”

  1. There is also a conversation to have about not having a fully clear sight glass with HFC refrigerants even though you have enough subcool.

  2. What’s your take on putting eight glasses on residential systems. I live in south Florida and a previous employer insisted on them. I work for myself now and want more information to justify doing it on residential systems.

  3. What’s your take on putting sight glasses on residential systems. I live in south Florida and a previous employer insisted on them. I work for myself now and want more information to justify doing it on residential systems.

  4. Please, complete your article about SIGHT GLASS / MOISTURE INDICATOR.
    How to install a SIGHT GLASS / MOISTURE INDICATOR is important.
    I´m talking about positioning it, vertical or horizontal. Most manufacturers are writing in instructions that the installing position doesn´t matter.
    However, I always try to install the SIGHT GLASS / MOISTURE INDICATOR in HORIZONTAL position because when the liquid line is in HORIZONTAL position the liquid refrigerant will always fill up the liquid line which give a more accurate reading and will show up bubbles in the top of the SIGHT GLASS / MOISTURE INDICATOR. In vertical position the refrigerant gas bubbles will go anywhere in the SIGHT GLASS / MOISTURE INDICATOR. Most manufacturers are showing in pictures in their instructions sheets that the SIGHT GLASS / MOISTURE INDICATOR should be installed in HORIZONTAL position.

  5. For brazed in sight glasses make sure the tubing is properly reamed. Standard practice. If the tube is not reamed refrigerant flowing past the curl may cause bubbles in the sight glass.

    Some moisture indicators can be replaced on sight glass/moisture indicators without removing the sight glass from the system. This is helpful on brazed in sight glasses on a system that has a system burn out.

  6. Sight glass can only show refrigerant condition at that point: superheated, saturated, or subcooled and no indication of the amount or sh or sc.

    A false subcooling due to non-condensables or/and pressure drop between where the readings are taken and the glass location can show saturation in the glass.

    I never trust the moisture indicators without an acid test to confirm.

    Noted on some refrigeration units “warranty voided if sight glass installed”.

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