Conductivity control is the foundation of every cooling tower water treatment programme. It keeps dissolved solids from concentrating to the point of scale formation, and it triggers the bleed-and-dose cycle that maintains chemical treatment levels. It is necessary. It is also not sufficient.
A cooling tower controlled only by conductivity is a system that manages one risk — scale — while leaving corrosion and microbiological fouling largely uncontrolled. In industrial cooling water applications, all three failure modes — scale, corrosion, and biofouling — operate simultaneously and interact with each other. Addressing only one of them does not protect the system; it just changes which failure mode dominates.
What conductivity actually measures
Conductivity in a cooling tower measures the total dissolved solids content of the circulating water. As water evaporates, the dissolved minerals, salts, and ions that remain behind become progressively more concentrated. Conductivity rises in proportion to this concentration. When it reaches the setpoint, the bleed valve opens to remove concentrated water and replace it with fresh makeup water, bringing conductivity back down.
This control mechanism is effective at managing scale risk. Calcium carbonate, calcium sulphate, and similar salts precipitate when concentration exceeds their solubility limits. By keeping conductivity — and therefore overall mineral concentration — below a defined level, conductivity control prevents the conditions that cause scale to form.
What conductivity does not measure is the chemical composition of the water. It does not distinguish between ions that cause scale and ions that cause corrosion. It does not detect the presence or absence of biocide. It provides no information about pH, oxidising potential, or microbial activity. It is a proxy for total dissolved solids concentration, and nothing more.
The corrosion gap
Corrosion in cooling towers is driven primarily by pH, dissolved oxygen, and the balance of aggressive versus protective ions in the circulating water. Chlorides and sulphates promote pitting corrosion of carbon steel and copper alloys. Low pH generates acid attack on metal surfaces. High pH can cause copper corrosion and deposit formation. Dissolved oxygen creates differential aeration cells that drive localised corrosion.
None of these corrosion drivers are measured by conductivity. A system with perfect conductivity control — maintaining its setpoint precisely, bleeding and dosing on schedule — can simultaneously be running at the wrong pH, with a depleted corrosion inhibitor residual, at a conductivity setpoint that allows chloride concentrations high enough to cause pitting. The conductivity reading tells the operator nothing about any of these conditions.
pH monitoring is the minimum addition required to make corrosion control meaningful. The cooling water pH should be maintained in the range of 7.0 to 9.0, with the specific target depending on the metallurgy of the system and the inhibitor programme. pH outside this range does not affect conductivity. A system where the pH has drifted to 6.2 due to acid overdosing or CO₂ ingress will show a normal conductivity reading while actively corroding its heat exchanger tubes.
ORP (oxidation-reduction potential) monitoring provides additional corrosion-relevant information. ORP reflects the overall oxidising or reducing nature of the water — a low ORP environment tends to accelerate corrosion of passive metal surfaces like stainless steel, while an appropriate ORP range supports the formation of protective oxide films on carbon steel. ORP also provides indirect information about biocide presence, as oxidising biocides increase ORP when they are active in the water.
The biocide control gap
This is the most significant limitation of conductivity-only control, and the one with the most serious safety consequences.
A timer-based biocide dosing programme — the most common approach in conductivity-only systems — doses biocide at fixed intervals regardless of whether the biocide residual is actually present in the water between doses. Biocide demand from organic load, biofilm, sunlight degradation (for chlorine-based products), and dilution from makeup water all consume biocide between dosing events. The residual that exists immediately after dosing may not be present eight or twelve hours later when the next dose is scheduled.
ORP monitoring provides real-time feedback on oxidising biocide residual. When free chlorine or bromine is present in the water above a threshold concentration, ORP rises measurably. When the biocide is consumed, ORP drops. A system with ORP-based biocide control maintains a defined ORP setpoint — which corresponds to a meaningful biocide residual — rather than dosing on a fixed timer that cannot account for variable biocide demand.
Free chlorine measurement takes this a step further by directly measuring the concentration of available disinfectant rather than its electrochemical proxy. In systems where regulatory compliance or Legionella risk management requires demonstrable biocide residuals at defined concentrations, direct chlorine measurement is the appropriate monitoring approach. ORP alone does not provide a quantitative concentration value that can be reported against a compliance target.
The consequences of inadequate biocide control in a cooling tower are not limited to heat transfer efficiency losses from biofouling. Legionella bacteria, which colonise the biofilm that grows under inadequate biocide conditions, can generate an aerosol risk to personnel in and around the facility. In Malaysia, a number of occupational health incidents have been linked to inadequately treated cooling towers. Conductivity control does nothing to prevent this.
What a complete monitoring programme looks like
A cooling tower water treatment programme capable of controlling scale, corrosion, and microbiological fouling simultaneously requires monitoring across all three risk categories.
For scale control: conductivity monitoring with automated bleed control remains the correct approach. The conductivity setpoint should be calculated based on the makeup water mineral analysis and the maximum allowable Langelier Saturation Index (LSI) or Ryznar Stability Index (RSI) for the system.
For corrosion control: pH monitoring with acid or alkali dosing to maintain the pH within the target range for the inhibitor programme. ORP monitoring provides supplementary information on water chemistry and oxidising potential.
For microbiological control: ORP or direct oxidiser monitoring to maintain a defined biocide residual in the circulating water. For systems under regulatory scrutiny or with documented Legionella risk, direct free chlorine or chlorine dioxide measurement should be used in place of ORP alone.
The Pyxis Guardian IK-203X series cooling water analyser integrates free chlorine, pH, ORP, temperature, and conductivity measurement in a single panel, with optional turbidity and corrosion rate sensors. This configuration addresses all three control categories simultaneously with real-time data and automated dosing control outputs — replacing the piecemeal approach of a conductivity controller plus separate timer-based biocide dosing with an integrated programme that responds to actual water conditions.
A system controlled only by conductivity is better than no control at all. It is not a complete programme for a system where the consequences of corrosion or Legionella growth are significant.
Autoflo Technology supplies cooling tower water treatment controllers and multi-parameter analysers, including the Pyxis Guardian IK-203X series. Contact us at info@autoflotechnology.com to discuss your cooling tower monitoring requirements.