Fresh water from municipal water systems or private wells contains a certain amount of dissolved solids. Some of these solids break down into electrically charged ions in their aqueous state. The classic physics lab example is common table salt (NaCl), which reduces to a positive sodium ion (Na+) and a negative chlorine ion (Cl-) in solution with water. The presence of these ions allows the solution to conduct electricity - the higher the concentration of ions, the greater the conductivity. Thus, a measurement of the solution’s conductivity gives a reliable clue to the concentration of dissolved solids in cooling tower water, boiler feedwater, process rinse water, or even our cool, refreshing summer drink. Conductivity is measured in Siemens (S) per centimeter (the inverse of Ohms per centimeter), and is the ratio of induced current to applied voltage across a one centimeter gap. The “per centimeter” portion is often implied rather than stated.

So how does all this science help? Consider a 1000 ton cooling tower system, freshly filled with 10,000 gallons of water from the municipal supply (pretty clean, 400 µS conductivity). As soon as it starts up, water begins evaporating because, hey, that’s what cooling towers do. However, the dissolved solids within the water do not evaporate. If the tower is operating at capacity, it is probably evaporating about 30 gallons per minute. At that rate, it will have evaporated an amount equal to its initial fill volume in a little over 5 hours. Thus, it will contain all of the initial dissolved solids, plus an equal dose from the makeup water, in the same volume. The concentration of dissolved solids will be twice as high, and the conductivity will be around 800 µS. This point, at which the system conductivity is double that of the makeup water, is referred to as the second cycle of concentration. In another 5 hours three cycles of concentration will be reached, and if no liquid is bled from the system the concentration of dissolved solids will continue to increase. At some point, though, the solution will become supersaturated and solids will begin to precipitate out. Minerals like silica, calcium carbonate, and calcium sulfate will solidify and form hard scale in the system. Also, reactive ions will be building up toward corrosive levels.

The ultimate goal is to minimize wasted makeup water by maintaining the highest level of dissolved solids that will not cause corrosion or scale. With a conductivity transmitter or stand-alone controller, an appropriate level of bleed or blowdown can be established to maximize the cycles of concentration in the system without allowing scale and corrosion. While the appropriate cycles of concentration for an application depends on the water treatment chemicals being used, typical values run around 4 cycles for cooling towers and 6 cycles for steam boilers. The 8225 Series of field-rangeable conductivity transmitters and controllers from Kele is a reliable choice for virtually any application in which dissolved solids must be controlled.