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For the measurement of the velocities of fluids, in 1732, Henri de Pitot invented the pitot tube. Pitot tubes detect the flowing velocity at a single point (standard), at several points that lead into an averaging probe (multiported), or at many points across the cross section of a pipe or duct (area-averaging). Their advantages are low cost, low permanent pressure loss, and the capability of inserting the probe-type sensors into the process pipes while the system is under pressure (wet- or hot-tapping). The disadvantages of pitot tube-type sensors are low accuracy, low rangeability, and the limitation of being suitable only for clean liquid, gas, or vapor service unless purged