One of the truly unique HVACR service tools in the past 50 years has been the electronic sight glass, because it is an exclusive HVACR tool that was excellently thought out for very unique purposes; detecting bubbles in refrigerant liquid lines, and detecting liquid droplets in refrigerant suction lines. However, the advertised purpose is at odds with HVACR technology, as its best use often goes overlooked; for it is billed as a charging tool, not as one of the best tools for preventive maintenance and solving difficult servicing problems.

I doubt that any HVACR manufacturer would recommend the electronic sight glass as a charging tool (but I could be wrong), because charging until there is no presence of bubbles in the liquid line of a thermostatic-expansion-valve system brings it to the point of total saturation (all liquid) but not to the 10-15 degrees of sub-cooling needed to reach maximum rated efficiency. And charging a restrictor-type metering device (such as capillary tube) system until there are no liquid droplets in the suction line carries you to the point of super heat, but not necessarily to the 10-15 degrees needed to provide compressor protection. In other words: close, but no cigar! Yet I have an electronic sight glass and I still consider it one of my best servicing tools. Why?

The electronic sight glass gives service technicians a non-invasive look into refrigerant systems and can show things that can be seen in no other way - not even with actual sight glasses! I believe that it is a servicing tool that should be used on all preventive-maintenance calls and to show up loss-of-capacity problems. In other words; it is a must for all top service technicians, because this is what it can accomplish:
    1. To check for the presence of bubbles in the liquid line just as it leaves the condensing unit, in order to show whether there is air (and other contaminants) in the system.

    2. To check for the presence of bubbles in the liquid line just as it leaves the condensing unit, in order to show whether the system is close to the proper charge.

    3. To check for the presence of bubbles in the liquid line just as it leaves the liquid-line filter-drier, in order to show whether the filter is clogged and needs to be replaced.

    4. To check for the presence of bubbles in the liquid line just as it enters the evaporator, in order to show whether there is too much lift to the line or if it is undersized.

    5. To check for the presence of bubbles in the liquid line just as it enters the evaporator, in order to show whether too much heat is being added to the line (as in a hot attic).

    6. To check for the presence of liquid droplets in the suction line at the condensing unit, in order to show whether the system is overcharged or if the thermostatic expansion valve is improperly adjusted, or has failed.

    7. To check for the presence of liquid droplets both before and after a suction line filter-drier, in order to show whether it is clogged and needs to be removed or replaced.

    8. To check for the presence of liquid droplets or bubbles in the suction line at the condensing unit, to indicate a condition of too much oil in the compressor or the use of mixed refrigerants.
As I mentioned in a previous article, I hate the thought of opening a refrigerant system with hoses, manifolds and gages for preventive-maintenance purposes. This usually contaminates the system, because of oil, moisture, and dirt in the gages and hoses, and because connecting the gages often causes refrigerant leaks, which creates service problems rather than eliminating them. So, I would much rather trust the indications of an electronic sight glass and a good thermometer on an operating system, than to allow a technician to hook up his/her gages and risk doing some damage. All of this is becoming far more critical as we transfer into the use of the new HFC refrigerants and their moisture-critical lubricants.

Actually, one of the first magazine articles that I ever wrote (in the News back in 1986) was about this subject, and it elicited the response from the then-president of TIF, “Try it Jim; it works” (speaking of the device’s charging abilities). I’ll agree that a technician who uses it to quickly charge a system may overshoot or undershoot the charge enough to end up with the right results. With many technicians, even close is better than what they are doing. However, I believe that the best uses of this product are being overlooked, and I wish that more understood its true value.