Pitot Tube Flow Sensor for Use in Black Smoker Chimneys


DOEI Project Funded: 2001

Proposed Research

We propose to build a flow sensor capable of measuring velocity within black smoker chimneys that can be calibrated easily and used routinely from a submersible. Black smoker chimneys form at seafloor hydrothermal systems, and vent hot (~350?C), metal-rich, Mg- and SO4-poor fluids into the ocean at velocities of ~1 to 5 m/s. These seafloor hydrothermal systems support a complex ecosystem on and beneath the seafloor, and it has been suggested that these ecosystems may represent a model for the origin of life on Earth, and for exploration of life on other planets. Accurate measurement of the velocities within chimneys are needed 1) to quantify fluxes of heat and mass from the ocean crust and lithosphere to the oceans, 2) to examine how flow rates vary over time, and 3) to calculate the pressure within chimney structures. The latter can then be used to calculate the rates at which fluid flows across chimney walls and within the porous areas of vent structures where a diversity of microorganisms are known to reside. Over the past twenty years there have been numerous attempts at measuring the velocity within and at the exit of black smoker chimneys, but there is still no tried and true method for doing so. We propose to measure velocities within black smoker conduits using a pitot tube with a custom differential pressure sensor. A pitot tube consists of an inner tube open at the end, and an outer tube that is sealed at the end but contains several small holes. The velocity of flow within a pipe or conduit is determined by inserting the pitot tube and measuring the difference in pressure at the tip of the small tube from that within the outer tube. Based on past experience using other sensor probes, we do not anticipate serious problems with clogging or corrosion as long as the pitot tube can be inserted ~5 to 15 cm into the chimney conduit. We will make the actual pitot sensor using titanium tubes to withstand the high temperature low pH hydrothermal environment in the chimney. We will build a suitable differential pressure gauge with a flat field replaceable stainless diaphragm and a one sided capacity sensor that is pressure tolerant. A nearby electrode will sense the capacity change as the diaphragm flexes. Once the flow sensor is constructed, we will critically evaluate its performance under different conditions. Our goal is to build a version suitable for use with the submersible Alvin and to test it on an Atlantis cruise of opportunity. Eventually, we hope to refine the design so that the pitot tube and gauge can be left in place to get the time variability of the flow.