Power, packaging, and multiplexing considerations in an acoustic travel-time current meter

For users of a current meter, ease of operation is very important, second only to its accuracy of flow measurement. Simplicity of deployment is one measure of this ease of operation. A current meter that can be deployed for an extended period of time, that includes power source and logger in a single case, and that is small and lightweight makes deployment and recovery easy. If the current meter can also accommodate ancillary sensors without extra housings the deployment becomes even simpler. Small size is generally compromised by the need for long deployments. In a recent redesign of the Modular Acoustic Velocity Sensor (MAVS), power management has improved battery consumption by a factor of 50. Power switching alone was unable to provide the benefit required. In the new design, the controller (Onset Tattletale 8 microcomputer) is shut down between measurements and the switching regulators are turned off except during the actual analog conditioning and digitization of signals. Multiplexing of acoustic signals reduces circuit board real estate and allows ports for analog and digital signals from some sensors and from conditioning circuits attached to other sensors. This in turn reduces the total size of the measurement suite, easing the handling of the mooring. A deep sea MAVS with temperature sensing array and high resolution pressure sensor has been built for monitoring heat flux in a hydrothermal vent field in a pressure case 95 mm (3.75") in diameter and 430 mm (17") long. It samples for 270 days on 35 WH of internal battery, storing a sample every minute. A single sample takes 45 milliwatt-seconds (mJ) of energy for a total measurement energy budget of 5 WH.