I calibrate mine after I replace any tubing that starts to harden.. usually from high Ca additives like Kalk or Calcium Carbonate. But they dont normally wander unless something is starting to go (like the tubing).
I've never had mine back syphon, but the potential does exist. I just mount my tubing above the water line so it can't pull anything back. generally good practice for any equipment that pushes fluids.
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