How do you calibrate, Dm? I ask because those bulbs are Uber sensitive, and both those methods are pretty rough treatment.
Also, both the calibration fluid and the storage fluid do have a shelf life.
I had a new grower a couple of years ago who's plants were beyond hammered, (I think he wound up culling them), every indication of severe lock out. But his pH was apparently spot on.
A fellow member finally caught the real problem: the calibration was so far off on his meter that he was basically adjusting to the low 5's
All I'm suggesting is make sure you trust it before you depend on it.