What is Total Dissolved Solids or TDS in water for parts cleaning?

What is TDS in water? Total dissolved solids (TDS) is the term used to describe the inorganic salts and small amounts of organic matter present in solution in water. These salts dissociate in water to form ions which means it has two parts: one with a positive charge and one with a negative charge, which separate and mix with the water (H20) molecules. This ionization allows for reading what the TDS is in the water via conductivity / resistivity which are discussed below.

The principal constituents are usually calcium, magnesium, sodium, and potassium cations and carbonate, chloride, sulfate, and nitrate anions. For example, sodium chloride (aka Table Salt) or NaCl is made up of sodium or Na positive charged ion and Chloride or Cl negative charged ion.  Other common salts that make up TDS in water are sodium Na+, sulfate SO42-, chloride (Cl-), calcium (Ca2+), magnesium (Mg2+), and bicarbonate (HCO3-).  All of these salts and minerals are found in varying levels based on the water source and the minerals in the surrounding soil.

(TDS) Total Dissolved Solids Measurement

Total dissolved solids in water are primarily measured by means of gravimetric and conductivity. Gravimetric is the most accurate and is usually done by evaporating the liquid water with TDS and measuring the mass of residues left. This method is generally the most accurate, although it is time-consuming and can’t not be done in process on part cleaning systems easily.

The concentration of dissolved ionized solids in the water is directly related to electrical conductivity / resistivity.   Ions from the dissolved solids in water create the ability for that water to conduct an electrical current which allows for in process measurement via a conductivity or TDS meter. Although not as accurate as gravimetric, the conductivity of water typically provides precise enough in process measuring for part cleaning and washing applications.

Measurement units for TDS: 1 grain of hardness = 17.12 mg/l or milligrams per liter = 17.12 ppm or parts per million.

Water can vary in TDS levels based purification method:

Municipal / City Source Water – maximum 1000 mg/l, but typically 140-400 mg/l

RO Water – 80-99% reduction of TDS from source water

DI Water – 99%+ reduction of TDS and often used in combination with RO

Article: Difference Between RO and DI Water for Parts Cleaning, Washing, and Rinsing