Thread: Sight glasses on refrigerant lines ???

I saw a new one for me - hundreds or more like thousands of AC units around here but I have never seen these before.

On the high pressure side piping of the two AC compressors there is a sight glass. One was clear - so clear that you cannot really tell there is fluid in there except an occasional teeny-weeny bubble passing by. The other glass is frothy and bubbly constantly. What are these glasses for and what can I learn about the system I am seeing when I look at these glasses?

Jonathan

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Bubbles in the sight glass in a indicator that refrigerant is missing.

rick

A sight glass is a tool to observe the refrigerant flow in the liquid line. Beyond that, you really need a primer on the refrigeration cycle, metering devices and such.

But IN GENERAL , Rick is right, bubbles in the sight glass indicate that there is not pure liquid present at the sight glass, indicating a shortage of refrigerant.
Now, there are lots of reasons for the shortage, restricted liquid line filter/dryer, high head pressure, unusual ambient temperature.
This is kind of like taking temperature differentials, you can have false positive and false negatives and everything in between.
Most residential a/c units do not have sight glasses for two reasons: cost and lack of proper usage by customers and techs.
Best thing I can suggest if you don't know about what to look for is to ignore them and pay attention to the evaluation of the rest of the unit.
I would not put anything about a reading from a sight glass in a report.

BTW, the colored dot in the center is a indicator of moisture in the system. Usually after they have been installed for a while the indicator ceases to function and the reading is worthless. It is a good indicator if you are cleaning up a contaminated system and need to see if the dryer needs to be changed.

Originally Posted by Fritz Kelly

I wish more systems had them. They are a great way to check the refrigerant level.

Yep, but only if you know how to use them.

Years ago all automobiles had sight glasses. It is an excellent tool for owners and techs. You could charge a system with a sight glass and never hook gauges to it.
I too wish they still had them.

Hey fellas.
Unfortunately site glasses are more common on commercial systems than on home a/c systems.

In my studies though, I have understood that if bubbles are found in the site glass, regardless of poor performance of the system, service should still be recommended.
But that is the choice for you guys

Cheers

In my studies though, I have understood that if bubbles are found in the site glass, regardless of poor performance of the system, service should still be recommended.

Might be an excuse to call for service, but it does NOT mean there IS a problem with the system. With a cap tube system, bubbles are normal.

Again the sight glass is a tool, but if you don't know how to use it, leave it in the box.
Not trying to sound superior, just trying to explain the pitfalls of using a tool you don't understand to diagnose a problem that may not exist.

Originally Posted by Jim Luttrall

Might be an excuse to call for service, but it does NOT mean there IS a problem with the system. With a cap tube system, bubbles are normal.

Again the sight glass is a tool, but if you don't know how to use it, leave it in the box.
Not trying to sound superior, just trying to explain the pitfalls of using a tool you don't understand to diagnose a problem that may not exist.

You're exactly right Jim.

Under low load conditions on a fixed metering device system a sight glass will bubble and it's perfectly normal.
It can happen on TEV equipped system also.

Most A/C guys who put them on don't have a clue to their limitations either.

That being said if it's on a commercial system that employs a receiver it can be a handy little accessory.
On residential cooling it's overkill.
From what I've seen the guys who put sight glasses in usually hook up the equipment to the worst duct systems.
Take the cash you paid for the sight glass and put it where it really counts like a properly sized duct system and some mastic.

Don't let the sight glass change your inspection. We use to put sight glasses on our condensor installations, sometimes I would install one on a system I just repaired and the customer would let me know if the bubbles started to come back but they never seemed to call if it was repaired properly. A good service technician don't charge by the sight glass anyway. I use to use head pressure base on the out door air temp or charge by super heat method or even sub-cooling or all three at the same time. The glass is in the liquid line after the refrigerant has been condensed from a hot gas to a liquid by going through the condensor.
Cap tubes are usually in window air-conditioners and orfices and TXV valves are in residential units. Orfices are usually in 10-seer and below units while expansion valves are mostly in 12-seer and up. There are so many variables in the new A/C units today. The new ones have VFD Variable frequency drives on the condenser fan motors to control head pressure on low ambient days. Some have head pressure controls by a pressure switch so if you see a fan cycling on and off, thats OK. Wait till you come across a two stage compressor and you hear it go to stage two. Also a scroll compressor can run backwards right after a power hit, so if you are running a scroll and the power goes off and on right away and you hear the compressor start screaming, turn off the disconnect and wait a few minutes and then you can turn it back on and it will be OK. Sorry for the long winded info.

Dan Hagman

Originally Posted by fritzkelly

So Jim, why don't you enlighten us?

Thought I did... at least I tried.

You have any specific questions?
I might not know the answer, but I bet DavidR could help us out.
Remember, I went to school back in the days of horse drawn air conditioners.

Originally Posted by Jim Luttrall

Remember, I went to school back in the days of horse drawn air conditioners.

Man, horses drew air conditioners? I have a hard time drawing those fins, wonder how the horses drew them?