Cooling tower water treatment controllers are most commonly evaluated on what they can control — conductivity setpoints, biocide dosing schedules, pH management. What is less often discussed is what they prevent: the accidental delivery of chemicals under conditions where those chemicals will not work, or worse, will cause damage.
Two features in the Aquarius Ultima cooling tower controller — pre-bleed lockout and lockout period — address exactly this. They are not widely understood, but they solve a real operational problem that affects dosing accuracy, chemical efficiency, and system protection.
How Conductivity-Based Bleed Control Works
Before explaining the lockout features, it helps to understand the control logic they work within.
Cooling towers lose water through evaporation continuously. As water evaporates, the dissolved minerals and salts in the remaining water become more concentrated. Left unchecked, this concentration increases the risk of scale deposition on heat exchanger surfaces and pipework.
Conductivity is a direct proxy for the total dissolved solids concentration in the cooling water. As concentration increases, conductivity rises. The controller monitors conductivity continuously and triggers a bleed valve — a motorised valve that opens to discharge some of the concentrated cooling water to drain — when conductivity exceeds the setpoint. This bleed is simultaneously replaced by fresh makeup water, which dilutes the system back to an acceptable concentration level.
The bleed event is also the timing trigger for chemical dosing. Inhibitor, biocide, and other treatment chemicals are typically dosed in proportion to bleed volume or timed relative to bleed events — because it is during and after bleed that the chemical concentration in the system is replenished.
This control logic works well under steady-state operating conditions. The problems arise at startup, after system shutdown, and after maintenance interventions — and this is where pre-bleed lockout and lockout period become important.
Pre-Bleed Lockout: Preventing Dosing Before the System Is Ready
When a cooling tower starts up after a shutdown — overnight, over a weekend, or after maintenance — the water chemistry can be in a very different state than it was when the system went offline.
During a shutdown period, the system water is stagnant. No evaporation is occurring, so conductivity is not rising. But if there was any residual make-up water addition, any rainwater ingress, or any chemical dilution event, the conductivity may actually be lower than the setpoint when the system restarts. The controller, seeing low conductivity, may interpret this as a need to start dosing to bring inhibitor concentration back up — even though the system has not yet reached stable operating conditions.
Pre-bleed lockout prevents this. It inhibits chemical dosing until the system has completed at least one bleed cycle. The logic is straightforward: a genuine bleed event only occurs when evaporation has concentrated the water to the setpoint level. If the system has not yet bled, it has not yet reached operating concentration — and dosing before that point risks over-treating water that has not been properly conditioned.
In practical terms, pre-bleed lockout prevents the scenario where a system restarts, the controller immediately begins dosing based on a residual low-conductivity reading, and the first genuine bleed then flushes out chemical that has just been added. The chemical is wasted, the treatment window is missed, and the operator sees inconsistent results that are difficult to diagnose.
Lockout Period: Protecting the System After a Bleed Event
The lockout period addresses a different but related problem — what happens immediately after a bleed event.
When the bleed valve opens and discharges concentrated system water, makeup water begins flowing in to replace it. This makeup water is at a much lower conductivity than the system water it is replacing. During the bleed event and immediately after it closes, the system conductivity is actively falling — the incoming makeup water is diluting the bulk water, and the conductivity reading at the sensor is dropping.
Without a lockout period, the controller may interpret the falling conductivity reading as a signal that the system needs more chemical. It triggers a dosing event — adding inhibitor into water that is currently in a transient dilution state and not yet at chemical equilibrium. The result is inconsistent chemical loading: some dosing events add treatment to concentrated system water, others add to freshly diluted water. Over time, this produces a treatment programme that swings between under-treatment and over-treatment rather than maintaining the stable residual that good corrosion and scale control requires.
The lockout period solves this by suppressing dosing for a defined time window after each bleed event — long enough for the system to reach chemical equilibrium after the dilution transient. The controller waits for conductivity to stabilise at the new steady-state value before making any dosing decisions based on it.
The appropriate lockout period depends on the system volume, the bleed flow rate, and the time constant of the chemical mixing in the system. In the Aquarius Ultima, this is a configurable parameter — allowing it to be set based on the actual characteristics of each specific installation rather than a generic default.
Why These Features Matter for Legionella Risk Management
Inconsistent chemical dosing is not just an efficiency issue. In the context of cooling tower water treatment, it is a Legionella risk factor.
Legionella growth is most effectively controlled by maintaining a consistent biocide residual within the treatment target range. If dosing events are occurring at wrong times — triggered by transient conductivity readings rather than genuine system state — the biocide residual fluctuates. Periods of under-treatment create windows during which Legionella can proliferate before the next effective dose arrives.
Both pre-bleed lockout and lockout period help maintain the dosing consistency that underpins effective microbial control. They ensure that the chemical programme responds to actual system conditions, not to transient readings caused by startup dynamics or post-bleed dilution.
The Broader Point About Controller Intelligence
Basic conductivity controllers operate on a simple threshold logic: conductivity above setpoint, open bleed valve; conductivity below setpoint, close bleed valve; dose chemicals on a timer. This is functional under steady-state conditions, but it produces predictable errors at the edges — startup, shutdown, and post-bleed transients.
The Aquarius Ultima is designed around the recognition that a cooling tower does not always operate at steady state, and that a controller which cannot handle the transient conditions correctly will produce inconsistent results even with a well-designed chemical programme.
Pre-bleed lockout and lockout period are two of the features that distinguish a controller capable of managing real operating conditions from one that only works well when the system cooperates. In practice, cooling towers spend a meaningful portion of their operating life in transitional states — and how the controller handles those states determines how well the treatment programme actually performs.
Autoflo Technology is the distributor for the Aquarius Ultima cooling tower water treatment controller in Malaysia. For more information on controller features and capabilities, contact us at info@autoflotechnology.com.