pH measurement plays a vital role in industrial applications, supporting stable chemical dosing, efficient treatment performance, and consistent regulatory compliance. While the measured medium can be demanding, understanding common measurement conditions and practicing good practice in maintenance can help the operators obtain reliable and long-term results.
Sensor Fouling / Coatings
Extended exposure to chemical residues can result in the formation and accumulation of deposits, coatings, or fouling on the pH sensor’s surface and the pH electrode’s glass membrane. Buildup interferes with the sensor’s ability to respond quickly and accurately to changes in pH, often resulting in slow response times and unreliable readings.
Implementing a regular cleaning routine helps maintain sensor performance, while retractable holders allow safe in-process cleaning or replacement without interrupting plant operation.
Sensor Installation Locations
When a pH combination electrode is installed in areas with stagnant flow, excessive turbulence, or air entrainment, the sensor may be exposed to non-representative samples, leading to fluctuating or misleading measurements.
Placement too close to chemical dosing points can also result in locally extreme pH values that do not reflect actual process conditions, causing incorrect control actions. To ensure accurate and stable measurement, pH sensors should be installed in locations with stable flow and good mixing, where the sample is representative of the overall process.
Reference Junction Clogging
In industrial water or wastewater applications, suspended solids, chemical precipitation and biological growth can block the reference junction and restrict electrolyte flow. When this occurs, the reference potential becomes unstable, leading to drifting or noisy pH readings that reduce measurement reliability and process control accuracy. Opting for industrial-grade pH sensors reduces the risk of leakage and clogging compared to liquid-filled laboratory electrodes.
pH Measurement Drift
pH sensors are consumables and electrochemical that will degrade over time. This can be corrected with calibration to establish the slope and zero point of the electrode. The frequency of calibration depends on the application, with some applications requiring daily calibration while others may require only weekly or monthly calibration. More frequent calibration is recommended when measuring in heavily contaminated, low-ion, acidic, and high temperature liquid solutions.
- 3-point calibrations are typically performed when the sample pH conditions are not well understood. It is performed in the according order: pH 7.0, pH 4.0 and pH 10.0
- Use fresh, unused buffer solution for calibration. Used buffer solution are contaminated, leading to slow pH electrode response or the inability to calibrate.
- Expired buffer solutions should not be used. Carbon dioxide in the air can change the pH of basic buffer solutions.
Temperature Effect
Changes in process temperature directly influence both the chemical equilibrium of the solution and the electrode’s measurement slope. Temperature fluctuations can cause pH readings to shift, even when the actual process pH remains unchanged, leading to inaccurate control decisions. Automatic temperature compensation (ATC) in the pH transmitter allows the system to correct pH readings in real time, maintaining accuracy across varying temperatures. Additionally, verifying temperature accuracy during calibration ensures that both the pH and temperature signals are aligned.
Electrical Noise and Signal Instability
Pumps and other electrical equipment operating nearby can cause electrical interference that distorts the low-level pH signal, resulting in fluctuating or noisy readings that complicate process control. It is important to apply proper grounding and shielding practices throughout the measurement loop, including sensor cables and transmitters, to minimize interference. Long cable distances may also degrade the signal.
