Hardness (TH) measures the calcium and magnesium content of water, i.e. the presence of lime in the water. A low hardness produces aggressive or soft water (corrosive to equipment). A high TH produces lime scale and is therefore hard water (which accumulates lime scale). The recommended TH parameter is between 150 and 300 ppm, the closer to 150 ppm the better to avoid corrosion or scale problems. It is recommended to measure this parameter once per season, at the spring tune-up.