Total Dissolved Solids (TDS)
TDS, total dissolved solids, is defined by a leading maker of test equipment as follows:
Total Dissolved Solids (TDS) are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm). TDS is directly related to the purity of water and the quality of water purification systems and affects everything that consumes, lives in, or uses water, whether organic or inorganic, whether for better or for worse.
In reality, many city water departments are in violation of the suggested TDS level because there is very little city water departments can do to reduce TDS.
For practical purposes, small handhell TDS meters to an excellent job of reading TDS. The most common use for TDS meters is in the verification of the performance of Reverse Osmosis units. Typically , Reverse Osmosis rejects 90% plus of the total dissolved solids, so with a tap water TDS reading of 200 ppm, a reverse osmosis reading of 20 or less would be expected.
Most fresh water that is consumed by humans falls in the <1000 ppm category. Here's how the Water Quality Association ranks water according to TDS:
Fresh: <1,000 ppm TDS