Sight Glass / Moisture Indicator

Often in commercial HVAC and refrigeration, you'll come across a sight glass/moisture indicator on the liquid line. (Oil sight glasses also exist, but those are different from what I'm talking about today.)

Sight Glass

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. If we see bubbles, we'll know there'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. Subcooling tells you the actual amount of heat that the refrigerant has lost past the condensing temperature. Sight glasses let you know if you have a full column of liquid, but that's the full extent of the data.

In refrigeration systems with receivers, a sight glass is an excellent tool; that's because we can rely on it as an indicator of liquid refrigerant to the metering device.

Moisture Indicator

The moisture indicator shows you if the system is dry or if it has moisture content.

First, be sure you're aware of the age of the sight glass. 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. Should that happen, most indicators will change back after installation, proper evacuation, and several hours of runtime. If it still reads wet after that time, the system likely is wet; at that point, you should install new line driers and pull a deep vacuum.

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.


P.S. – If you're interested in learning more about refrigeration components, we have the perfect video or podcast for you! Brett Wetzel and Kevin Compass from the Advanced Refrigeration Podcast walked through rack refrigeration components on the HVAC School podcast. You can watch the walkthrough video HERE or listen to the podcast episode HERE.


12/21/16 at 01:19 PM

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

12/24/16 at 05:22 AM

Mention best location! Great articles and podcasts. Thanks for your work.

7/5/17 at 09:07 PM

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.

7/6/17 at 02:06 AM

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.

Alvaro Osorio
Alvaro Osorio
7/27/17 at 04:01 AM

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.

Gary Reecher
Gary Reecher
7/31/18 at 05:09 AM

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.

9/23/20 at 08:53 AM

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”.

6/7/23 at 07:36 PM

How do you actually use the moisture indicator? Does something happen to it that shows there is moisture in the system? Is it a color change of some sort? I see the two different colors indicated on the cap, but I’m not sure where to see how to use the indicator portion of this component.

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