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16 st / February 2017 www.sea-technology.com velopment of the deep-ocean-capable Durafet is the maintenance of the accuracy of the sensor over sufficiently broad ocean temperature and pressure ranges. To reach this goal, the liquid/gel Ag/AgCl reference electrode used in the original Durafet design has been replaced with a solid state Ag/AgCl reference electrode. The use of a solid state reference electrode and a custom-de- signed pressure-tolerant housing for the Durafet chip extends the pressure rating of the sensor from 100 to 3,000 psi. The sensing element of the Deep SeaFET is an ISFET pH sensor, which is a type of metal ox- ide semiconductor field-effect transistor (MOS- FET) where the gate terminal is an ion-sensitive material exposed to the analyte of interest, in this case, the hydronium ions in seawa- ter. The ISFET pH sensor has three major components: the counter electrode, the pH-sensitive material of the ISFET and the reference electrode. For the sensor to func- tion properly, all three of these components need to be exposed to seawater. Once ex- posed to seawater, these three components can be considered as two half cells of a solid state electrochemical cell. One half cell is the combination of the counter electrode and pH-sensitive material of the ISFET. The counter electrode applies a small electric field to maintain a constant current across the ISFET, and the pH-sensitive material is used to detect the hydronium ions in the sea- water. Since chloride ions are relatively con- stant in deep seawater, a solid state chloride ion selective electrode was chosen to be the reference electrode and the other half of the electrochemical cell. The chloride ion selec- tive electrode is an Ag/AgCl puck that has an electrical potential proportional to the concentration of chloride ions in seawater. When these two half cells are combined, the voltage that forms on the chloride ion selective electrode is proportional to the pH in seawater and is the signal that is measured by the sensor. The solid state elec- trochemical cell exhibits a Nernstian response to pH, but presents the added complexity of being sensi- tive to chloride activity. Design, Calibration and Stability Considerations The Deepsea DuraFET's specified operational rang- es for temperature and pressure are 5 to 35o C and 50 to 3,000 psi. The original Du- rafet was developed for low-pressure industrial conditions and could not achieve this pressure range. All three of the (Top) The stability of the Deep SeaFET in filtered seawater over a four-month period in Sea-Bird Scientific's pH laboratory. The blue line is the pH measured by the Deep SeaFET, and the or- ange circles are the pH valida- tion samples measured using dye spectrophotometry. The red dash lines indicate when the seawater was exchanged from the system. (Middle) Participants in the Wendy Schmidt Ocean Health XPRIZE test cruise off Hawaii, watching the deployment of the instruments on the rosette during sea trials May 2015. (Photo Credit: XPRIZE) (Bottom) A profile of pH measured by the Deep SeaFET from the final side-by-side XPRIZE competition in 2015 off Hawaii. The red asterisks are the pH measured from bottle samples by NOAA's PMEL Laboratory. The y-axis is depth in meters.