A sight glass in a convenient means of determining the refrigerant charge, showing bubbles when there is insufficient charge, and a solid clear glass when there is sufficient charge.This may be due to a restriction or excessive presure drop in the receiver outlet valve, a partially plugged drier or strainer, or other restriction in the liquid line ahead of the sight glass.
If the expansion valve feed is erratic or surging, the increased flow when the expansion valve is wide open can create sufficient pressure drop to cause flashing at the receiver outlet. Another source of flashing in the sight glass may be rapid fluctuations in compressor discharge pressure. For example, in a temperature controlled room, the sudden opening of shutters or the cyclinng of a fan can easuly cause a reduction in the condensing temperature of 10 F. F. Any liquid in the receiver may then be at a temperature higher than the saturated temperature equivalent to the lower condensing pressure, and flashing will continue until the system has stabilized at the new condensing temperature. Sight Glass In Refrigeration System Should Be Clear Full Reliance OnWhile the sight glass can be a valuable aid in servicing a refrigeration or air conditioning system, a more positive liquid indicator is desirable, and the system performance must be carefully analyzed before placing full reliance on the sight glass as a positive indicator of the system charge. 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. 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. What are these glasses for and what can I learn about the system I am seeing when I look at these glasses Jonathan Similar Threads: Traps on Primary Condensate lines Indirect waste lines. Electrical Lines Inspector PVC drain lines PVC water supply lines in crawlspace. 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. This is kind of like taking temperature differentials, you can have false positive and false negatives and everything in between. Most residential ac 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 dont 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. 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. Sight Glass In Refrigeration System Should Be Clear How To Use ItAgain the sight glass is a tool, but if you dont 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 dont understand to diagnose a problem that may not exist. Under low load conditions on a fixed metering device system a sight glass will bubble and its perfectly normal. Most AC guys who put them on dont have a clue to their limitations either. That being said if its on a commercial system that employs a receiver it can be a handy little accessory. From what Ive 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. 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 dont charge by the sight glass anyway.
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