AC Sight Glass in Automotive Systems

The sight glass will add some potential (albeit unlikely) leakage points, but it will help in trouble-shooting the system, especially for loss of refrigerant charge. It will also typically incorporate a moisture detector to check for unwanted water in the system.

A sight glass is an easy way to tell if the system is undercharged. It won't tell you much else.

Disadvantage, cost.

They're great for guys like me. I have an old car and it has a sight glass. I have a new truck, it doesn't. So, I need gauges or a mechanic if the truck AC has a problem.

The short and sweet of it;

Advantage- you can see status of refrig system.

Disadvantage- It does cost a little more to have it in there!

h

A bubble or two passing through your sight glass isn't necessarily a bad indication. It does show that the freon is present, and likely is in enough supply to preform. Lots of bubbles mean low freon and likely some trouble shooting to find where leaks may be.

With lots of bubbles available, as you recharge the system, you should see the bubbles diminish indicating that the recharge is being done correctly. Typically the glass in on the top of the dryer bottle. These bubbles are considered contaminates and that can be air or moisture.

Cheers

TMF

Such bubbles could be air, but this is fairly unlikely in an above-atmospheric system. Much more commonly they are refrigerant vapor. This can happen from too low a charge, uncovering the dip tube in a receiver; or a plugged filter-drier, resulting in flashing in the line. This reduces the mass flow capacity of the expansion valve (or tube), thereby decreasing the system performance.

Earlier design automotive systems, prior to 1993, used R12 refrigerant. When charging the system the refrigerant properties were such that when system capacity was reached the high pressure liquid would cause the reftigerant flowing beneath the sight glass to appear clear, no bubbles.

With later anfdcurrent R134a systems the sight glass only shows flow. If the sight glass is clear with the system operating, it is either overcharged or empty.

Bob

Hello wrench, you are correct.

And regarding the earlier comment made by another about the positive atmosphere with in the AC system not permitting contaminants from without, I suggest that he take a look at a little piece of equipment that is used to suck liquids from a small container and mix them with water flowing through a garden hose. A loose fitting or a small leak in a suction hose is all that is needed to permit air from entering the system.

TMF

The device you describe is called an eductor, among various alternative names. I am familiar with them and use them quite frequently.

I did not say that inward leakage was impossible; I said unlikely. The vast majority of such systems do not have this problem.

Hi tornado,

And I sure hope that you never experience the impact of a direct hit from one of those things, "I wouldn't wish that on an enemy" and certainly not on a fellow CR 4 responder.

Regarding the apparatus that contains the sight glass, For many years, that thing was called a dryer bottle. It's useful purpose was to trap unwanted moisture that managed to find it's way into the AC system. Even an evacuated system can contain minuscule droplets of moisture. These will show as bubbles in the sight glass. Thus we have a verifiable justification for the dryer bottle.

Regarding the recharge of a troubled AC system, typically mechanics and lay persons alike, use the bubbles, and their removal as a way to determine when the system is adequately recharged, when they do not have the proper equipment for that determination. This lacks perfection but it works when there is no other way.

Regarding the intrusion of contaminants into the system. After market products are woefully notorious for springing leaks due to the poor quality of their fittings and often hose clamps getting loose. Even factory installed systems will sometimes have a hose rub on other parts until it leaks. Nothing is perfect it seems, but there will always be a way to fix things when an emergency arises, and Mr. Fixit is the only help available.

Cheers

TMF

In that case, what does the sight glass look like when the system is properly charged?

It just shows bubbles irregularly appearing in a usually mostly solid stream of high pressure liquid refrigerant, as opposed to the R12 system that when fully charged shows no bubbles.

Bob