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

Abstract

Increasing atmospheric carbon dioxide is driving a long-term decrease in ocean pH which is superimposed on daily to seasonal variability. These changes impact ecosystem processes, and they serve as a record of ecosystem metabolism. However, the temporal variability in pH is observed at only a few locations in the ocean because a ship is required to support pH observations of sufficient precision and accuracy. This paper describes a pressure tolerant Ion Sensitive Field Effect Transistor pH sensor that is based on the Honeywell Durafet ISFET die. When combined with a AgCl pseudoreference sensor that is immersed directly in seawater, the system is capable of operating for years at a time on platforms that cycle from depths of several km to the surface. The paper also describes the calibration scheme developed to allow calibrated pH measurements to be derived from the activity of HCl reported by the sensor system over the range of ocean pressure and temperature. Deployments on vertical profiling platforms enable self-calibration in deep waters where pH values are stable. Measurements with the sensor indicate that it is capable of reporting pH with an accuracy of 0.01 or better on the total proton scale and a precision over multiyear periods of 0.005. This system enables a global ocean observing system for ocean pH.

Utilizing the Drake Passage Time-series to understand variability and change in subpolar Southern Ocean pCO<sub>2</sub>

“…Johnson et al, 2016;. In addition, empirical algorithms for alkalinity must be used to estimate pCO 2 from float-based pH [Carter et al 2016;Williams et al 2016].…”

Measuring pH variability using an experimental sensor on an underwater glider

“…Temperature and salinity were obtained from the Sea-Bird CTD sensor on the ship rosette sampler, and the seawater equilibrium constants presented by Mehrbach et al (1973) were used as refitted by Dickson and Millero (1987), which has been recommended by previous studies (e.g. the CARINA data synthesis project) for the Mediterranean Sea (Álvarez et al, 2014;Key et al, 2010). The sulfate constant described by Dickson (1990) and the parameterisation of total borate presented by Uppström (1974) were used.…”

“…It is known that temperature and pressure have an effect on E * (strong linear relationship) and that the Nernst slope is a function of temperature. Studies have also shown that it is possible for ISFET sensors to experience some form of hysteresis as a result of changing T and pressure Bresnahan et al, 2014;Johnson et al, 2016). The first step in correcting pH g aimed to reduce the measured extent of variability to within the measured limits of pH s .…”

“…11). Johnson et al (2016) ran a series of temperature and pressure cycling experiments when testing an ISFET pH sensor based on the Honeywell Durafet ISFET. They found a temperature sensitivity of around ∂pH / ∂T = −0.018 K −1 , which is similar to our calculated slope, and a pressure hysteresis of 0.5 mV (pH of around 0.01) at maximum compression (2000 dbar).…”

“…This is equivalent to a pressure sensitivity of roughly ∂pH / ∂P = 5 × 10 −6 dbar −1 , which is an order of magnitude smaller than in this study. This difference in pressure sensitivity could be related to the different housing materials used, as Johnson et al (2016) used polyether ether ketone (PEEK), whereas acrylic resin was used for our sensor.…”