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Venturi Tubes and Human Ventilation

In July, 2013, I watched an episode of Airplane Crash Investigations. In this episode, a commercial jet that had been left idle in a hanger in a tropical country for 3 months prior to the tragic flight.  During this flight, the plane exhibited incorrect data flight measurements of air velocity.  The pilots were reading excessive flight velocities, and therefore controlled the plane by slowing the engines to bring velocity back down to desired instrument values for their flight plan.

What the pilots did not know was that during the storage of the plane, maintenance staff did not cover all external instruments.  Unfortunately, this allowed a species of wasp to nest in the Venturi tube mounted on the external fusilage, causing a reduction in the diameter of the small section (throat) of the tube, which raised air flow velocity through this tube section for a given air flow condition, raising differential pressure measurements from the Venturi tube (lowering the throat pressure reading), and incorrectly sensed higher than real air velocity of the plane.  The pilots eventually caused the plane to stall as they increased altitude while flying at a low velocity, and the plane literally fell from the sky.

I wondered, surely we can apply the science of the Venturi tube to the measurement of human ventilation during indirect calorimetry!  This study was the result of this realization, and it has been a wonderful exercise in experimental science to pursue and apply this important topic to exercise physiology.  I used this study to support of an Honours student in 2014, and we are currently completing the manuscript for submission to peer review. In short, the Venturi tube is incredibly sensitive to increasing air flow, but has trouble with low flow conditions.  We demonstrated that the Venturi tube has even higher accuracy (agreement) compared to an impellar turbine.  See the image below for the voltage output from a differential pressure transducer and an impellar turbine for 3 L ATPS air flow conditions from a calibration syringe at a) low, b) moderate and c) high flow rates.

We hope to have this manuscript published by mid-2015.