Water hardness

It's the amount of dissolved calcium. Target 200–400 ppm (150–250 in a spa).

  • Too low: "hungry" water that corrodes and dissolves cement and seals.
  • Too high: tends to form scale and deposits.

The saturation index (LSI)

It combines pH, alkalinity, hardness and temperature to tell whether the water is balanced:

  • LSI near 0 (between −0.3 and +0.3): balanced.
  • Negative: corrosive water (raise pH, alkalinity or hardness).
  • Positive: scaling water (lower pH or alkalinity).

The app calculates the LSI for you in the "Advanced chemistry" card when it has pH, alkalinity, hardness and temperature.

Why it matters

Perfectly chlorinated but corrosive water slowly eats away your pool and equipment. Watching the LSI extends the life of the installation.