I have a Carrier 19XR centrif with high cooler approach which at near full load is around 8-10 degrees F (5-6 degrees C) the oil level looks low in the compressor as well. The skimmer line sight glass was clear. At part load 60% amps and 30% guide vanes the appraoch is much lower (1.3 degrees C or 2.5 degrees F) there is liquid flashing in the skimmer line sight glass and the alert is gone. I checked the resistance of the thermistors which seem OK. The flows are near enough, only marginally low. My deduction is that there is a refrigerant shortage, as at fuller load the liquid in the cooler is below the skimmer line, which leads to poor oil return (the level is lower). Another identical chiller runs mormal oil levels, and a 0.2 degree C appraoch, with flashing in the skimmer line.

Any other thoughts on this ?