What Does Conductivity Tell You About a Cooling Tower?

What the measurement is

Conductivity measures how readily water conducts an electric current, which depends on the concentration of dissolved ions in it. More dissolved salts, more conductivity.

That makes it a practical stand-in for total dissolved solids, and therefore the basis on which cycles of concentration and blowdown are controlled. It is fast, continuous, cheap to measure and robust enough for a plant room, which is why it is the parameter most cooling systems are actually run on.

What it cannot tell you

Conductivity is a sum, not a breakdown. It responds to total ionic content and says nothing about which ions are present.

• It cannot distinguish scaling species from corrosive ones. The same reading could be mostly calcium and alkalinity, or mostly chloride — two very different problems.
• It does not measure inhibitor residual. Treatment chemical contributes to conductivity, but the reading is not a check on the programme.
• It does not see suspended solids or biological load, neither of which is ionic.
• It does not indicate scaling tendency on its own — that requires the specific ion balance, temperature and pH.

Getting a reading you can trust

Conductivity varies significantly with temperature, so a compensated reading is essential — and the compensation has to be set for the water being measured, not left at a default that assumes something else.

Sensor type follows the duty. Contacting cells are accurate in clean water at lower ranges but are affected by fouling and coating. Electrodeless sensors tolerate fouling and higher conductivity, which is why they suit dirtier recirculating systems.

Location decides whether the number means anything. The sensor needs to sit in flowing, well-mixed water — not in a dead leg, and not immediately downstream of the chemical injection point, where it will read a slug of concentrate rather than the system.

Using it for control

Blowdown driven by conductivity holds cycles of concentration at a measured value. When evaporation raises the concentration above setpoint, the valve opens; when concentration falls back into the deadband, it closes.

The deadband is not a detail. A setpoint without one produces a valve that cycles constantly, wearing the actuator and destabilising the reading. Feed limits and lockout timers serve the same purpose — they are what make an automatic system stable rather than merely automatic.

The most useful thing conductivity gives you is not the instantaneous number but the trend. A slow upward drift points to blowdown not keeping up or a valve partially blocked. A sudden fall suggests dilution — a leak, an overflow, or make-up entering somewhere it should not.