Assessment of a pH optode for oceanographic moored and profiling applications

As global ocean monitoring programs and marine carbon dioxide removal methods expand, so does the need for scalable biogeochemical sensors. Currently, pH sensors are widely used to measure the ocean carbonate system on a variety of autonomous platforms. This paper assesses a commercially available optical pH sensor (optode) distributed by PyroScience GmbH for oceanographic applications. Results from this study show that the small, solid-state pH optode demonstrates a precision of 0.001 pH and relative accuracy of 0.01 pH using an improved calibration routine outlined in the manuscript. A consistent pressure coefficient of 0.029 pH/1000 dbar is observed across multiple pH optodes tested in this study. The response time is investigated for standard and fast-response versions over a range of temperatures and flow rates. Field deployments include direct comparison to ISFET-based pH sensor packages for both moored and profiling platforms where the pH optodes experience sensor-specific drift rates up to 0.006 pH d⁻¹. In its current state, the pH optode potentially offers a viable and scalable option for short-term field deployments and laboratory mesocosm studies, but not for long term deployments with no possibility for recalibration like on profiling floats.