Pipe MAVS, a deep-ocean flowmeter

An acoustic differential travel-time current sensor with 6000-m depth capability has been adapted to measure flow in a pipe, initially for hydrothermal vent flow studies. The acoustic measurement path, 9.8 cm long, is inclined 18 degrees to the axis of a 20.3-cm inside diameter stainless steel pipe. The integrated component of flow along the acoustic axis is resolved to 0.04 cm/s with a standard deviation noise level near zero flow corresponding to 0.09 cm/s. that makes a threshold of detection of 0.45 cc/s or 0.03 liters/minute. The upper limit of flow that can be measured is 60 liters/minute. Uniform weighting of the measurement of velocity across the diameter of the pipe means that the annulus of fluid near the wall is underrepresented compared to the core of the fluid near the center. Therefore the radial profile of velocity enters into the calibration of the flowmeter. Furthermore, the profile depends on Reynolds number and the roughness of the boundary layer in the pipe. Flow measurements are important when a fluid source is diffuse yet the total volume and rate of flow of the source is needed. In the case of hydrothermal vents, the heat output of a diffuse vent requires both the temperature anomaly and the volume of fluid to be measured. In Pipe MAVS, three thermistors are provided: one to measure the external ambient temperature, and two inside the pipe to measure the temperature at the inlet and the outlet. A collecting structure covering the diffuse source concentrates the flow to be measured by Pipe MAVS. Since measurements are made rapidly, fluctuations in total flow are resolved and can be integrated for any period of interest to remove artifacts of the collector yet reveal variability in the flow. Momentarily closing the pipe establishes the zero offset in situ. Applications of Pipe MAVS to measure shallow water sources of fresh water in marshes and in coastal regions and of flow in either direction through a porous sea-floor are possible within the limits of the zero point resolution. Amplification of flow by increasing collection area is a better way to increase sensitivity than to decrease cross sectional area in the pipe. Pipe MAVS, with its 6000-m depth rating, can supplement current and temperature measurements in monitoring hydrothermal vent energetics.