Cycles of Concentration from Chloride

Chloride is the referee. Calcium precipitates as scale, alkalinity boils off as CO₂, silica drops out at high pH and conductivity gets muddied by whatever you are feeding — but chloride stays dissolved through all of it and appears in no cooling-water inhibitor. So when the conductivity ratio says nine cycles and the chloride ratio says five, the conductivity number is lying: 250 ppm Cl in the basin over 50 ppm Cl in the makeup is five cycles, full stop.

Field technique matters more than the arithmetic here. A silver nitrate (Mohr) titration on a 50 mL sample resolves chloride to a few ppm, which is plenty when the makeup carries 30–80 ppm, but if the makeup is soft well water with 5 ppm Cl the ratio becomes noise and you should switch tracers. Watch also for chloride sneaking in from elsewhere — a leaking heat exchanger on a brine loop, or hypochlorite biocide slugs, both add chloride to the tower only, and both will make a well-controlled system look badly over-cycled.