Total dissolved solids (TDS)

Total dissolved solids, or TDS, describes the combined amount of dissolved material in water, including salts and minerals. It is usually reported in parts per million. Fill water already contains dissolved substances, and water-care products can add more. TDS is different from visible particles trapped by a filter, and it is not a measurement of how much sanitizer is available.

A TDS result needs context, including the starting water, the treatment system and the spa manufacturer’s guidance. A meter may estimate TDS from electrical conductivity, so use the correct instrument settings and instructions. Legacy Spas includes TDS in its water-testing service. Keep the result with your other chemistry readings and refill date, rather than using a single number to judge the condition of the water.