Optimization of a robust and reliable ISFET sensor for measuring pH in the deep ocean

Long-term in situ pH measurements in the extreme conditions of the deep ocean remain a challenge for many oceanographers. Until recently pH in the deep ocean was measured by traveling to a specific location on the ocean surface, obtaining a seawater sample at a specific depth and analyzing the sample using pH indicator dye spectrophotometry. This process is laborious, time-consuming, expensive, and is incapable of providing the spatiotemporal pH data chemical oceanographers need to understand the processes that are contributing to ocean acidification. As such, the need for a robust and reliable in-situ deep ocean pH sensor is urgent. The Honeywell Durafet, an Ion Sensitive Field-Effect Transistor (ISFET), offers many advantages over current pH monitoring technologies and has been recently adapted for deep ocean monitoring on carousels, profiling floats and moorings. The chief concern in the development of the deep ocean capable Durafet is to maintain the accuracy of the sensor over the temperature and pressure range found in the deep ocean. The calibration of the Durafet will be described and field data will be presented.