I have a Hayward Aquarite and I just replaced the T-15 cell.  The instant salt reading is -4200, but the average is 2500. I recalibrate, but over the course of 2 - 3 days the average declines back to 2500, triggering a low salt warning.  More confusing, the local pool company tests my water at 3100. 

Anyone have an idea what is going on?