How conductivity calibration with standard solutions exposed a costly error – and why accredited calibration instruments are needed.
Dorte Krüger Molin |
The setting
Conductivity is one of the most important key measurements used to control water chemistry in a steam-cycle power plant. Almost anything that generates electricity from steam depends on ultra-low-conductivity water in the boiler circuit – if that water quality is not held, the plant does not survive. In a large thermal power plant, conductivity is monitored at dozens of points; in this case, 95 measuring points across the site. Several of them feed directly into the plant's chemistry control, including the ammonium hydroxide dosing that keeps feed-water pH in range.
The water-steam cycle depends on extremely high-purity water, and even small amounts of ionic contamination can lead to corrosion, deposits and damage to boilers, turbines and associated equipment.
Therefore, many of the measurements are brought together in the plant's Steam and Water Analysis System (SWAS). The SWAS takes representative samples from the water-steam cycle, conditions them to safe and stable pressure and temperature, and provides the controlled sample conditions required for reliable online analysis. Depending on the plant and sampling point, a SWAS may include measurements such as specific conductivity, CACE, pH, dissolved oxygen, sodium and silica.
Different conductivity measurements provide different information. Specific conductivity reflects the total ionic content of the sample and is influenced by the ammonia added to the feedwater for pH control. When ammonia, NH₃, dissolves in water, part of it reacts to form ammonium, NH₄⁺, and hydroxide, OH⁻, increasing both pH and conductivity.
Conductivity After Cation Exchange (CACE) is used to distinguish this intentional chemistry from unwanted contamination. By passing the conditioned SWAS sample through a strongly acidic cation exchanger before the conductivity measurement, cations such as ammonium are replaced by hydrogen ions. This suppresses the conductivity contribution from the ammonia treatment and makes the measurement highly sensitive to unwanted ionic contamination, including chloride and sulphate.
Together, the SWAS measurements provide a continuous picture of the condition of the water-steam cycle. Specific conductivity, CACE and pH can be used to monitor the ammonia-treated feedwater chemistry, while parameters such as dissolved oxygen, sodium and silica provide additional warning of corrosion risk, condenser leakage, carryover or contamination. This makes the SWAS a central part of both chemistry control and early fault detection in a steam-cycle power plant.
What went wrong
During a routine visit, the contractor calibrated the conductivity instruments using low-conductivity standard solutions – and then adjusted each sensor to match.
That is where it went wrong. Low-conductivity standard solutions are notoriously unreliable. At low conductivity levels, a nominal 5 µS/cm solution can drift to 7, 8, even 14 µS/cm simply through CO2 absorption and handling. Calibrating – and worse, adjusting – an instrument against a moving target does not improve accuracy. It introduces an error into a measurement that had been correct.
Lets's look at the consequence.
The consequence
Because the plant controls its ammonium hydroxide dosing on conductivity, the corrupted readings drove the chemistry off course. Operators, trusting their instruments, dosed against false numbers. Within roughly two months the plant had consumed close to two years' worth of dosing chemical – and came uncomfortably close to serious damage to the boiler, the heart of the plant.
Adjusting a stable sensor to match an unstable standard is exactly how a good measurement gets broken.
The intervention
When a specialist was brought in to troubleshoot, the first step was not another bottle of standard solution. It was an accredited calibrated conductivity instrument used as a trusted reference – an Insacal® Conductivity Calibrator. Placed alongside the plant's sensors under real operating conditions, the instrument read the true value while the freshly “calibrated” meter read far higher. This made it clear how to restore the correct control of the chemistry.
Why comparison calibration with an instrument is the right method
The lesson here is about knowing your method. In a power plant, conductivity sensors sit in clean water. They do not drift. Many operate for two decades without ever needing adjustment. The correct approach is therefore to verify each unit against an accredited reference instrument – not a standard solution.
This is the principle behind Insacal® Conductivity Calibrator.
The Insacal Conductivity approach
- Comparison against an accredited reference.
- Insacal Conductivity Calibrator brings the method to the field as a mobile, on-site calibration tool.
- Fast workflow, low uncertainty from accredited calibration delivers traceable documentation.
The takeaway
What happened here is not rare. Across the industry, conductivity is calibrated by well-meaning technicians, not educated enough within metrology, using standard solutions – and it goes wrong. Every power plant, in every country, utilizes conductivity, meaning they are exposed to the same risk. The fix is not a bottle of standard solution. It is a trusted, accredited reference instrument – an Insacal® Conductivity Calibrator.
