Conductivity and TDS

Conductivity and TDS

The CHEMTROL® PC3000, PC5000, PC6000 and PC7000 Programmable Controllers constantly monitor the level of dissolved solids in the water (TDS) with a conductivity sensor.

The readings can be displayed either in conductivity units microsiemens/cm (µS) using the CONDUCTIVITY MENU or in parts per million (ppm) of Total Dissolved Solids (TDS) using the TDS MENU, with an operator-selectable TDS conversion factor.

The menus can be used to set automatic replacement of water (dumping or bleeding) when the TDS value becomes too high, selecting either a Conductivity or a TDS control set-point to activate a dump valve. Backfilling of water is done simultaneously with a level control activated valve.

Total Dissolved Solids (TDS)

TDS represents the total concentration of all dissolved chemicals in water, mostly chlorides, calcium, magnesium sulfates, stabilizers, carbonates and bicarbonates, etc. that results mostly from the addition of chemicals and body wastes.

The normal range is 1000 to 2000 ppm.

TDS values above 2500 ppm can lead to damage to pipes, filters and pumps, loss of water clarity, “salty taste” and reduced chemical efficiency. High TDS values can also affect test kit and ORP readings. Above 2500 ppm, the water should be dumped and replaced with fresh water until the proper level is reached.

Conductivity Measurements

Since TDS is composed mostly of ionized species, it can be measured with a conductivity meter. The relationship is almost linear with a conversion factor of about 0.5, depending on the type and electrical charges of the ionized species. For instance, a conductivity reading of 2,000 micromhos/cm corresponds to about 1,000 ppm of TDS.

Replacement of Spa Water

High TDS problems are particularly frequent in commercial spas due to the constant evaporation of water, the large additions of chemicals and the large number of bathers. To prevent sanitation and corrosion problems, it is therefore recommended to replace the water on a frequent basis.

Many health departments recommend using the following formula: Baths = Gallons / 3

This means that the water must be dumped and replaced with fresh water after the above number of bathers have been in the spa.

For instance, a 3,000 liter spa should allow only 3,000/ 3 = 200 bathers before the water is dumped. With 100 bathers per day that means that the water should be dumped every 3 days.

On the other hand, a 4,000 liter spa with 10 bathers per day should dump the water only every 30 days.