Sight Glass / Moisture Indicator

By August 6, 2022

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.

—Bryan

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.

Check out our most recent video

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HVAC School – For Techs, By Techs

Real training for HVAC ( Heating, Ventilation, Air Conditioning and Refrigeration) Technicians. Including recorded tech training, interviews, diagnostics and general conversations about the trade.

Dominick Guarino from NCI joins the podcast to talk about the High-Performance HVAC Summit, an upcoming event hosted by NCI from April 17–20. The National Comfort Institute (NCI) is a training organization that helps technicians learn crucial skills and succeed through high-performance contracting.

High-performance contracting is based on the mindset of “don’t just promise performance; prove it.” To maximize technicians’ potential, NCI teaches technicians how to test equipment properly and monetize their skills. Techs can then use their knowledge to teach the homeowner about the system and solve problems with sales; they can earn money while being solutions-oriented.

NCI’s High-Performance HVAC Summit is a training event geared toward HVAC business owners and contractors. It started as a membership conference for education and networking, and it has since expanded to include like-minded HVAC professionals from all walks of life. Workshops are led by a mix of contractors and NCI staff, and there are breakout sessions and other various session types.

This year’s High-Performance Summit theme will be “It All Starts With Service.” The four focal points of the workshops will be lead generation through service and maintenance, CO safety, the lead handoff from service to sales, and maintenance agreements. One more session will focus on hands-on testing, called “Low-Performance Town” this year, and there will be a panel about the future of high-performance HVAC.

Dominick and Bryan also discuss:

Dominick and NCI’s history in the industry
High-performance maintenance
Consumers who do their research
The HVACR Training Symposium
High-Performance Summit Awards Banquet
NCI’s collaboration with other industry leaders and organizations

Visit https://www.gotosummit.com/ to learn more about the High-Performance Summit. If you decide to register for the summit, type HVACSCHOOL in the coupon code section for a $100 discount.

Learn more about the 4th Annual HVACR Training Symposium or buy a virtual ticket today at https://hvacrschool.com/symposium.

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7 responses to “Sight Glass / Moisture Indicator”

Genry says:
2023.03.27 at 13:19
There is also a conversation to have about not having a fully clear sight glass with HFC refrigerants even though you have enough subcool.
Reply
Bill says:
2023.03.27 at 05:22
Mention best location! Great articles and podcasts. Thanks for your work.
Reply
Jose says:
2023.03.27 at 21:07
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.
Reply
Jose says:
2023.03.27 at 02:06
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.
Reply
Alvaro Osorio says:
2023.03.27 at 04:01
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.
Reply
Gary Reecher says:
2023.03.27 at 05:09
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.
Reply
EHSX says:
2023.03.27 at 08:53
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”.
Reply

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