Fouled conductivity probe
The Controller Said 1,000.The Water Said 1,500.
A commercial cooling tower. The conductivity probe fouled and kept reporting setpoint. No alarm fired, because from the controller’s point of view nothing was wrong. Blowdown never triggered. Scale formed on the tower fill for a week before anyone saw it.
What happened
A probe that fails at setpoint raises no alarm
Conductivity controllers alarm on readings that go out of range. This probe did not go out of range. It fouled while sitting at the setpoint it was supposed to hold, and then stayed there. Every automated check the system had was satisfied: the number was the number it was meant to be.
Meanwhile the actual conductivity in the loop climbed past 1,500 µS/cm. Because the controller believed it was already at setpoint, blowdown never opened. The water kept concentrating, and calcium came out of solution onto the fill.
The old way
One probe and a paper log
- The fouled probe keeps reporting setpoint. No alarm ever fires.
- An operator hand-tests, gets 1,500, writes it on the log sheet. Nobody reads it.
- The log and the controller are never compared. The drift runs for a week.
- The rep finds the fouled probe at the next visit. By then there is scale on the fill.
With e-Sens and the Hub
A second, independent reading
- The e-Sens analyzer runs automated wet tests independently of the controller.
- The Hub compares controller conductivity against e-Sens conductivity, continuously.
- The readings diverge. The Hub alerts the operator and the rep the same day.
- Probe cleaned. No scale.
- The whole sequence is documented in the service report.
A fouled probe reads fine right up until the scale shows. A second, independent reading catches the lie in hours. The Hub cross-checks every controller against e-Sens wet chemistry and alerts people, not paper.
Frequently asked questions
Questions about probe drift
Why doesn’t a fouled conductivity probe trigger an alarm?
Controllers alarm on readings outside a configured range. A probe that fouls while sitting at setpoint keeps reporting a value inside that range, so every check the controller performs is satisfied. The failure is invisible to the only instrument watching for it.
How long can conductivity drift go unnoticed?
Until someone compares an independent measurement against the controller. On a monthly service route that can be weeks. In this case it ran about a week before the divergence was found, and scale had already formed on the tower fill.
Doesn’t a hand test on the log sheet catch this?
Only if someone compares the log against the controller, and that comparison is the step that gets skipped. The number was written down correctly. Nothing read it against what the controller believed, so nothing flagged a 500 µS/cm disagreement.
What actually catches a fouled probe?
A second measurement taken by something that does not share the probe’s failure mode. The e-Sens analyzer runs reagent wet chemistry rather than reading a probe, so when the two disagree the disagreement itself is the alarm.
Download
Fouled conductivity probe
One page: the divergence chart, what the old way looks like, and what changes with e-Sens and the Hub. PDF, 1.5 MB. No form to fill in.
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