Significance of Total Dissolved Solids in Coolants

A total dissolved solid (TDS) is a measure of the combined total of organic and inorganic substances contained in a liquid. These solids are primarily minerals, salts and organic matters that are present in molecular, ionized, or micro-granular (colloidal sol) suspended form. Generally, the operational definition is that the solids must be small enough to survive filtration through a filter with 2-micrometer pores (nominal size). TDS includes both volatile and non-volatile solids. Volatile solids are the ones that can easily go from a solid to a liquid state. Non-volatile solids must be heated to a high temperature, in order to achieve this state change.

One very well-known example of TDS is related to water quality: high TDS can indicate the existence of hard water. Water quality plays an important role in many spheres of our lives, and therefor is strictly controlled by a number of industrial norms. TDS is also crucial for automotive industry as can result in unwanted effects in a diversity of systems.

In case of coolants however we do not deal with pure water, but with a mixture of a base fluid (typically, glycol) and water, that also contains dissolved additives (such as corrosion inhibitors, stabilizers, colorants, etc.). In this way, TDS of a coolant is represented by the amount of added chemicals and of water compounds. High TDS can promote deposit formation in cooling system, causing premature water pumps failures and leakages, hence coolant’s TDS requires monitoring.
In the field of coolants, TDS is particularly a concern in heavy-duty applications due the practice of adding supplemental coolant additives (SCAs) to the coolant. This practice can lead to overloading and to subsequent corrosion but also to water pump seal leakage and failure following too high amounts of TDS in the system.

The meters and procedures for coolants TDS determination are borrowed from wastewater analytical techniques: gravimetric next to electrical conductivity is used as test to estimate TDS. Conductivity has the advantage of being easily and quickly measured, not only in laboratory environment, but also in the field. Obviously, the higher the dissolved solids content the higher the conductivity of a coolant since pure water and pure glycol are essentially not conductive. It is strongly recommended to measure conductivity of a dilution of a coolant concentrate. General recommendations for glycol-based coolants is to have TDS < 5 weight %. Another way to measure the amount of TDS is to look at the ash content. The ash content (measured by the ASTM D 1119) is determined by measuring the amount of residue that remains after an antifreeze has been ignited. The nonorganic metals found in the antifreeze together with some non-volatile additives are the major constituents of the ash. Therefore, on several OEM specifications and international standards (such as ASTM D 3306), an upper limit is set to the limit of the amount of ash allowed in the coolant.