The D-2 Hybrid CTD Sensor: Description and Evaluation

Abstract A recently developed oceanographic sensor from D-2, Inc.—named the Hybrid CTD for its novel, freely flushing, fully internal-field conductivity sensor and conceived in part to improve the resilience of ocean observing—is described and performance assessments are discussed based on static and dynamic laboratory calibration work. These are complemented by ocean observations obtained by piggybacking sensors on research cruises to the Arctic in late summer of 2024 and 2025 and offshore Bermuda in spring/summer 2025. Building on an evaluation/development program initiated in 2018, the focus here is on the CTD technology established in spring 2024. The ocean observations were implemented using self-contained controller/datalogger/battery units developed for this project; this subsystem is also documented. Based on this study, the Hybrid CTD is judged ready to join the pool of operational sensors used for ocean observation.