4000, blunted flow profile) Flowmeters based on a Venturi tube (circular tube with a gradual contraction and expansion in diameter, ex. those used in aircraft) maintain laminar flow at normal flow rates - since conservation of mass and energy is assumed to apply, restriction of the diameter leads to increased speed of gas flow and a corresponding decrease in the pressure on the walls The floating bobbin rotameter that is familiar to anesthesiologists is known as the Thorpe tube - the sudden restriction caused by the bobbin leads to a propoional increase in velocity and the Bernoulli equation does not apply (flow is propoionate to the square root of the pressure drop). Because the pressure drop across the orifice is inversely propoional to orifice area squared, the pressure change across the bobbin will decrease as the bobbin rises (and the area of the tube increases). The bobbin reaches a steady state when the pressure drop exactly opposes the gravitational force on the bobbin - at this steady state the pressure drop is always the same (because gravitational forces are constant), however as flow rates increase, the bobbin will move higher because this critical pressure drop will occur over a larger cross-sectional area (since velocity is higher)">