Deep-Sea DuraFET: A Pressure Tolerant pH Sensor Designed for Global Sensor Networks

Area of Science:

• Oceanography
• Environmental Science
• Sensor Technology

Background:

• Rising atmospheric carbon dioxide causes ocean acidification, decreasing ocean pH.
• Ocean pH variability impacts marine ecosystems and records metabolic processes.
• Current pH monitoring is limited to few locations due to the need for ship-based, high-precision measurements.

Purpose of the Study:

• To develop and validate a robust, long-term underwater pH sensor system.
• To enable continuous, high-accuracy ocean pH measurements independent of research vessels.
• To facilitate the establishment of a global ocean observing system for pH.

Main Methods:

• Utilized a pressure-tolerant Ion Sensitive Field Effect Transistor (ISFET) pH sensor (Honeywell Durafet die).
• Integrated the ISFET with a silver-silver chloride (AgCl) pseudoreference sensor for seawater immersion.
• Developed a calibration scheme for accurate pH derivation from HCl activity across ocean pressures and temperatures.
• Employed vertical profiling platforms for in-situ self-calibration in stable deep-water environments.

Main Results:

• The sensor system demonstrated multi-year operational capability on autonomous platforms.
• Achieved high accuracy (±0.01 pH units) and precision (±0.005 pH units) over extended periods.
• Successfully derived calibrated pH measurements from sensor data under varying ocean conditions.

Conclusions:

• The developed ISFET-based sensor system provides a reliable method for long-term ocean pH monitoring.
• This technology overcomes previous limitations of ship-based observations, enabling broader spatial and temporal coverage.
• The system is poised to support a comprehensive global ocean observing network for pH, crucial for understanding and mitigating ocean acidification impacts.