Ultrapure water (UPW) is free of trace impurities and degassed down to parts-per-billion levels. This is done through filtration, deionization, and degassing methods. UPW has a high solvency potential making it difficult to maintain purity while in use and in storage. Accurately measuring minute changes in UPW pH is critical to ensuring system integrity, product quality, and regulatory compliance.

From our field experience, we have found that the top three issues when measuring ultrapure water reliably are:

• pH sensor technology
• Measurement drift due to noise
• Access to diagnostics for predictive maintenance

Let’s look at each of these in further detail.

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pH Sensor Technology For Ultrapure Water

Conventional UPW pH technologies commonly experience drift, slow response, and degradation resulting in insufficient accuracy, unstable readings, and frequent maintenance. The potentials of the pH measurement chain within a sensor can be adversely affected by:

• Inadequate relationship between the reference system and the glass membrane
• Improper glass impedance
• Stray currents and ground loop potentials
• Degradation of the reference electrode
• Relationship between the sensor and the meter; e.g., cable length, degraded connections.
• Improper temperature compensation and correction

A pH sensor operates on the sum of several potentials including the outer glass membrane and reference junction diffusion potentials. The relationship between these two potentials is critical and especially challenged by the high electrical resistance of UPW. If not addressed, these challenges will result in measurement noise and deviations. Proper sensors ensure the appropriate impedance, maximize the proximity of the two potentials, and provide adequate KCl through the junction.

The Knick SE 558 pH sensor ensures stability and accuracy using advanced engineering and manufacturing to address these UPW challenges. The hydrogen sensitive glass provides excellent sensitivity and fast response in low ionic activity applications. Its KCl storage rings ensure stable delivery of the reference gel through three ceramic junctions. Furthermore, the patented SILAMID reference system prevents degradation of the Ag/AgCl electrode. These design advancements provide superior performance in the process as well as extended sensor life.

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Measurement Drift Due to Noise

Stray currents, ground-loops, and noise induced through coax or unshielded cabling, result in unstable readings, drift, and sensor degradation. 

Memosens technology employs an inductive coupling between the sensor and the Memosens cable. This patented connection is free of metallic contacts and provides perfect galvanic isolation to protect the reference electrode. This digital protocol utilizes RS-485 cabling that is not subject to outside interference. Memosens completely ensures the integrity of pH, ORP, conductivity and dissolved oxygen measurements by eliminating these challenges.

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Diagnostics for Predictive Maintenance in UPW

Analog technologies provide limited diagnostics and rely on failure or standard operating procedures (SOP) to determine maintenance requirements.

Memosens smart digital technology monitors zero, slope, speed of response, wear, glass impedance, and calibration time. Total operating hours, operating hours under extreme process conditions and calibration history are also recorded. The sensor network diagram and lifetime calculator provide intuitive insights for data driven predictive maintenance enhancements to existing maintenance practices. The Portavo portable meters, MemoView mobile tool, and MemoSuite software provide industry leading tools expounding on the benefits of offline calibration and maintenance to execute true asset management.

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Summary

In short, Memosens smart digital technology enhances pH, ORP, conductivity, resistivity, and dissolved oxygen measurements. This advancement in analyzing technology will resolve the shortfalls of antiquated designs that are susceptible to the rigors of ultrapure water analysis. If your goals are achieving faster start-up, maintaining system integrity, ensuring product quality or regulatory compliance, Memosens is the answer.