A low-cost, miniature CTD for animal-borne ocean measurements

The study of fine-scale linkages between animal behavior and the physical microstructure of the marine habitat is essential for understanding the ecology of many marine animals. Animal-borne salinity data has the potential to define the importance of physical water mass features to the ecology of marine animals. Recently CTD (conductivity, temperature and depth) data loggers mounted on large marine mammals (pinnipeds and cetaceans) have been able to capture direct qualitative information on the physical microstructure of the foraging environment and microhabitat. In order to understand the physical environment of smaller marine animals (penguins, fish and reptiles) a miniature, inexpensive CTD biologger is being developed. The biologger circuit boards are of a modular design so that several prototypes for different marine species can be developed. Currently two designs (A and B) have been fabricated and they measure 50 × 25 × 25 mm and 85 × 25 × 15 mm (unpotted), respectively. The biologger has additional internal sensors that are managed by a low-power microcontroller. The complete multisensor system measures conductivity, temperature, pressure, light, three-axis acceleration, three-axis magnetic fields, wet/dry and GPS. CTD measurements are used to calculate salinity and must be in close proximity to one another and the seawater. Therefore a novel CTD board was fabricated. The conductivity sensor was fabricated using printed circuit board (PCB) techniques and integrated with MEMS (micromechanoelectrical system) sensors, a thermistor and piezoresitive pressure module, on a liquid crystal polymer substrate (LCP). A four-electrode conductivity circuit that measures electrical resistance was designed. In this paper the biologger initial design is presented along with the conductivity cell circuit and preliminary CTD characterization data.