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Can you Measure Negative Superheat on an air conditioner when checking the charge? The Answer is NO! Time to check your gauges and temp sensors for accuracy!
To briefly explain what is happening during the refrigeration cycle, the refrigerant exits the metering device and then enters the evaporator coil as a saturated refrigerant in roughly an 80% liquid and 20% flash gas mix. The refrigerant in the evaporator coil is lower in temp than the air crossing the evaporator coil. All that can happen is that the refrigerant will absorb heat from the air, not reject heat to the air. Because the refrigerant absorbs heat at the indoor evaporator coil, the refrigerant will either stay saturated or it will superheat.
(Saturated is a mix of liquid and vapor refrigerant.) (Superheat is the increase in temp of a vapor refrigerant.)
To learn more, check out our article on the Total Superheat Method: https://www.acservicetech.com/articles
Check out our Book and apply what you learn with our Workbook: https://www.acservicetech.com/ac-book
To learn more about the refrigeration cycle and superheat, check out these videos: https://youtu.be/yjmrCEA1zIA & https://youtu.be/s9I1aVugdkY
I think that if you have a lot of oil in the evaporator, the SH can give you a negative