Self-diagnosing pH sensors predict their own replacement

By Bob Sperber

Smart-transmitting sensors installed in real-time process control systems  can not only transmit self-diagnostic data, they also can play an important role in multivariable condition-monitoring efforts. The Ingold line of pH and dissolved oxygen sensors from Mettler-Toledo (www.mettlertoledo.com) adds a smart chip in the sensor tip itself, not just the transmitter, and this year went all digital to reduce wiring complexity. As part of a process plant’s distributed control system (DCS), it can send pH and dissolved oxygen data as well as indications of signal quality based on real-time impedance measurements, and comparisons of calibration trends to spot sensor degradation.

Some plants, such as wastewater, are likely to use pH sensors in run-to-fail mode. But in a refinery, loss of pH control can damage process pipes; in biotech, off-spec can kill active cells and reduce productivity.

The sensor-and-software solution includes a “dynamic life indicator” based on actual conditions, “so you go into a predictive cycle, where the sensor tells you it’s time to recalibrate or replace,” says Roger Goavert, engineering, procurement and construction projects manager with Mettler-Toledo Ingold.

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