Low Power Instrumentation To Measure Barotropic Fluctuations

At periods longer than about one day, the oceanic horizontal electric field (HEF) is proportional to the vertically integrated, spatially averaged water velocity. Similarly, sea floor pressure fluctuations are proportional to large-scale barotropic fluctuations. Both types of measurements have found applications to studies of large-scale oceanic phenomena like western boundary currents. This paper will describe two low power instruments designed to make these HEF and pressure fluctuation measurements on the sea floor for extended deployment periods. The pressure instrument measures small pressure fluctuations at full ocean depth. The sensor uses a Bourdon tube to rotate a mirror in an optically centered, electro-optical feedback arrangement with a resolution of about .1 millimeter of water depth. Precise temperature is also recorded to remove temperature-induced errors on the pressure signal. The second instrument measures the horizontal electric field with a sensitivity of .01 uV/Meter. Electrode sensor drift is removed by alternately swapping the electrode contacts to opposite ends of a three-meter salt bridge. Data from an internal compass with pitch and roll information is recorded to orient the instrument on the sea floor. Both instruments use low power processors from Microchip to control instrument functions and provide a simple user interface for testing and deployment. Data is stored internally on non-volatile FLASH memory. Precise timing is achieved with a real time clock driven by a crystal optimized for use at ocean bottom temperatures. All instrument electronics, power source, and acoustic release are housed in a 17" glass sphere. The internal battery power is sufficient for multi-year deployments. The compact design (1 in high and < 40 kg) of these instruments provides case of deployment and recovery at sea.