What is Refrigerant Sight Glass?

In this blog post, you’ll read:The refrigerant sight glass component is straightforward. It only shows if the liquid line fills with or contains bubbles. If bubbles are present, a liquid/vapor mixture is present.
Table of Contents

The refrigerant sight glass component is straightforward. It only shows if the liquid line fills with or contains bubbles. If bubbles are present, a liquid/vapor mixture is present.

An operating system’s clear sight glass indicates a full line of liquid. Reading sub-cooling only establishes that the system has a full line of liquid. Although, which is what a sight glass accomplishes. Subcooling in HVAC provides more information than full-sight glass. The process of sub-cooling reveals how much actual heat refrigerant is lost. As far as after reaching the condensing temperature. Sight glasses can tell you if a liquid column is entire, but it is the only information they provide.

A sight glass is an excellent tool for refrigeration systems with receivers. Yet we can rely on it to show the presence of liquid refrigerant at the metering device.

sight glasses type
sight glasses type

Working of sight glass in refrigeration

This part offers a visual means to tell whether the refrigerant is liquid or saturated as it moves through the system while operating. When the refrigerant is liquefied, it is subcooled, and bubbles are not visible. The presence of bubbles indicates that the refrigerant is saturated. Yet, the liquid line sight can be observed until the bubbles are gone. While the sight glass turns clear after the automatic pump down begins, and the solenoid closes. This indicates that nothing else is present at the site of the sight glass.

Some eyeglasses change color to indicate the presence of acid in the line. Place the sight glass before the TXV but after the filter drier and solenoid. If a restriction in the liquid line, such as a blocked filter, is present, it can be seen through the sight glass.

Regardless of whether the liquid entering the filter drier is subcooled, this would appear as bubbles in the sight glass. By looking through the sight glass, it is also possible to tell if the solenoid is open or closed. Refrigerant bubbles will still be seen at the solenoid during operation if it is supposed to be closed but is only partially sealed.

sight glass with indicator
sight glass with an indicator

Sight glasses functions

The sight glass’s duties include monitoring the liquid refrigerant level. However, looking for moisture in the system’s dry filter’s back end. A sight fit in the fluid stream right after the filter drier in systems with expansion valves. The sight glass can work on the discharge line at any desired place. But it’s especially wise to put the sight glass close to the expansion valve. But it’s especially wise to put the sight glass close to the expansion valve.

The sight glass’s color indicator displays the moisture content.

Green: The refrigerant doesn’t include any hazardous water vapor.

Yellow: The liquid line at the front end of the expansion valve has too much water vapor.

If you notice a bubble in the mirror, the following circumstances are actual:

The dry filter’s pressure drop is excessive and could be due to obstruction.

Undercooling is not enough. In the system, the total amount of refrigerant is insufficient.


MaterialSteel or Stainless Steel
Nominal SizeDN 10 -100
Nominal Pressure LevelPS 25, PS 24
Temperature Range for Standard Valves_50 C to +120 C
Temperature Range for  HT Valve Series-10 C to +200 C
ConnectionsWelding ends as per DIN and ANSI, threaded ends

Applications of sight glass in refrigeration

In refrigeration systems, liquid line sight glasses can fool non-technical individuals. They are inaccurate and display the amount of refrigerant in the system. They can deceive by showing bubbles at various points. While during the cycle when the system still has plenty of gas.

While the sight glass can show the system full of gas, it may have other problems that mimic a gas shortage. They will display bubbles in the liquid gas if the system is low on gas. Throughout the system’s lifespan, the gas should never need to change.

refrigerant sight glass
refrigerant sight glass

The sight glass indicates the following.

When charging refrigerant, you must ensure all bubbles have burst. The liquid lock position is that. Bubbles are a sign that no condensable gas is present. When the system is not vacuumed, this occurs.

The presence of bubbles indicates the presence of a system leak.

The other condition shows whether the system is dry or wet on the sight glass. It must always be in a dry state. The moisture in the system displays a damp condition, which must dry out before the gas charge.


The refrigerant sight glass is often placed wherever there is liquid flow. The technician can be sure that a solid column of liquid is getting to the expansion device. As if it fixes right before it. It can aid in troubleshooting when mounted at the condensing unit.

inside structure of the sight glass
the inside structure of the sight glass

Styles of Refrigerant Sight Glass

There are two primary types of sight glasses: simple and with a moisture indicator. The refrigerant is seen as it travels down the line using the plain sight window. The system’s moisture content can determine using sight glass with a moisture indicator. It contains a tiny component that changes color in the presence of moisture.

You can identify system flaws using the information displayed in the sight glass. It advises that you do not rely on the sight glass for diagnosis. Also, avoid diagnosing R134a systems using the sight glass.

One of three possibilities could be true if the sight glass is clear. Either the system is empty, complete, or overloaded. If there is foam, the charge is low; yet, if there are only a few bubbles, everything will be fine. It’s usual to see a few bubbles as the compressor starts up when there are oil streaks in the sight glass. Since the charge is likely low and the compressor is pumping oil from its sump. The desiccant bag has burst if the sight glass is foggy. However, an R134a system has a sight glass. But, using it or attempting to diagnose the system is not recommended. Consequently, bubbles are common in this setup.

hvac parts moisture indicator brass sight glass
hvac parts moisture indicator brass sight glass

Routine inspection:

Depending on the end user’s operational practices. Yet the interval between sight glass examinations may vary. Yet, plants follow a standard pattern whereby each sight glass examines once a month. To do this, isolate the sight glass and empty the gauge. Then review the glass under a flashlight for clear reflections as it would show a chip or crack.

Depending on the sight glass’s size, this takes 30 minutes. If the factory needs two employees to perform this task, and if so, at what salary? The cost of performing this procedure is roughly $75/hr. This translates to about $450/year for one sight glass inspection.


Over five years, sight glass maintenance has been the most fixed-cost contributor, routine inspection, cleaning, and repair/replacement. These are the three vital elements of a maintenance plan.

hvac parts
hvac parts


Every person who has ever used or seen a sight glass in action is aware that with time, the glass gets dusty and hazy, making it impossible to read the level. Yet, most plants have regular cleaning schedules. Some end users choose to wait until the liquid level is no longer visible before doing a cleaning. While it takes so long to clear sight glass. For this illustration, let’s assume that sight glass requires cleaning every two years.

What do bubbles in a refrigerant sight glass mean?

Some refrigeration systems are starting up, and the system has a heavy load. Then the sight glass downstream of the receiver may bubble and flash. A pressure drop at the receiver’s outlet tube’s entry is what’s causing the bubbling. Additionally, when loads increase, bubbling could happen.

What are the colors on refrigerant sight glass?

Most glasses feature indicators for moisture. The indicators change color. Whereas depending on how much moisture is present in the refrigerant. With the removal of moisture, some turn from yellow to green. Some people switch from pink to purple.

How do you charge refrigerant with sight glass?

To reach equilibrium, allow the system to run at this high heat loading for a while. Charge refrigerant vapor into the system’s low side if the sight glass is bubbling. And keep doing so until it stops.

HVAC 084 sight glass

What do bubbles in the refrigerant sight glass mean?

A low flow rate sight glass and a bubbling sight glass are very different. A pressure decrease results in liquid flashing. Also, an undercharge of refrigerant causes vapor. Yet liquid to exit receiver due to deficient sub cooling shown by entrained bubbles in the liquid.

What does a clear sight glass mean?

An operating system’s clear sight glass often indicates a full line of liquid (or flat, but you would know that already if you have gauges attached). Reading sub-cooling only establishes that the system has a full line of liquid, essentially what a sight glass accomplishes.

Solutions For Waterworks Plumbing

Get quote Now

Free Sample with OEM