EskimoPie, you missed another reason for the salt to drop (and it's because you, like I, have a cartridge filter), backwashing Sand or DE filters will also lower salt levels.
Another thing to note, SWG systems without a dedicated salt sensor is basically deciphering the amperage and voltage draw on the cell, then assuming what the approximate salt level is. This is all based on the conductivity of the system.
What can affect the conductivity, giving a false reading of low salt:
- Cold Water Temperatures: most systems shut down when temps fall below 65 degrees. However, to compensate (or trick the system), additional salt can be added when cooler water temperatures are maintained.
- Scaled Cell: blocks the electrical current from passing between the electrodes. Cells must be maintained clean and free from calcium scale. Follow the manufacturers directions to acid wash the cell, as needed. To ensure a scale free cell, test and adjust your water chemistry to ensure balanced water conditions according to the Saturation Index.
- Failing Cell: if your cell is clean and your salt level has been tested (not from the display on your system), a failing cell can falsely indicate low salt.
Periodic (monthly) inspections of your salt level should keep you on track as there are very few conditions that will allow the salinity to drop drastically from month to month. Heavy Rainfall and Leaks are the main culprits.
Hope this helps,