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www.sea-technology.com February 2017 / st 15 T he development of autonomous vehicles capable of performing hundreds of profiles in the ocean from depths of 1 to 2 km has led to a dramatic shift in the collec- tion of deep-sea data from shipboard, often hands-on mea- surements to remote robotic collection. Dissolved oxygen is the first chemical parameter to see this transformation. The potential for pH to be the second chemical param- eter to see this shift is considerable as high-resolution in-situ measurements of pH in the ocean are now possible with innovative adaptations of ion sensitive field-effect transis- tors (ISFET). Oceanographic instruments based on the IS- FET technology have been in development for more than a decade, but, as with most technologies, have had signifi- cant hurdles to overcome. Current instruments have depth limitations or significant drift and environmental constraints that have limited their adoption. Here, we present field and laboratory research results with the Sea-Bird Scientific Deep SeaFET, an ISFET-based pH sensor that demonstrates the technology has reached the stability and robustness neces- sary for a similar transformation in the source of quality pH measurements. The ocean is buffered with respect to pH over long time scales, which for most of the history of oceanog- raphy meant that the fact that the measurement tools for pH are either relatively unstable (glass bulb) or laborious (dye spectrophotometry) didn't significantly impact research and monitoring. Recently, the importance of pH in the ocean has been recognized because, on short time scales relevant to biological productivity, the food chain and carbon fluxes, pH is dynamic and potentially controlling across critical scales. Given that atmo- spheric loading of carbon dioxide from anthropo- genic sources will feedback as a decrease in oceanic pH, the need for robust and reli- able in-situ, oceanographic- quality pH sensors is urgent and has led to many devel- opment paths spurred by the obvious need and encour- aged by global competitions such as the Wendy Schmidt Ocean Health XPRIZE in 2015 (http://oceanhealth. xprize.org). The Technology The Sea-Bird Scientific Deep SeaFET is based on the Honeywell Durafet, an ISFET that was originally de- veloped to replace the glass electrode in industrial applica- tions where chemical robustness and dependable long-term measurements are necessary. The chief concern in the de- Reliably Measuring pH In the Ocean Deep SeaFET Maintains Accuracy over Broad Temp, Pressure Ranges By Charles W. Branham • David J. Murphy • Ian D. Walsh (Top) The Deep SeaFET. (Bottom) Schematic repre- sentation of the ISFET, counter electrode, reference electrode and operating components.