The Pitot (pronounced pea-tow) tube is an eighteenth- century invention still flying high amid twenty-first- century technology. Designed by French astronomer, engineer, and mathematician Henri Pilot (1695-1771), this deceptively simple device is essentially a differential pressure gauge and can be used for a variety of flow-rate or speed-measuring purposes.
Pitot's pet interest was water flow, and his personal research led him to conclude that much of the accepted wisdom of the day was incorrect. He would not accept, for example, the prevailing theory that, other things being equal, the speed of flowing water increased with depth. His tube, demonstrated at the French Academy of Sciences in 1732, would show that he was right: it does not.
As well as being used in a fixed position to determine the flow rate of a liquid or gas the L-shaped tube may be attached to a boat or airplane to measure the craft's forward speed. In all cases, the tube functions by registering the difference between the ambient pressure surrounding it and the pressure created by the flow into it (the "impact" pressure), which will increase with speed. The resulting comparative measurements can be displayed via suitable instrumentation. Various improvements to the basic design have been made over the years, although Henri Darcy's 1858 design is more or less the one still in use today.
Today, variations of, and uses for, Pilot's amazing little tube continue to grow; multiport versions exist to enable the measurement of the impact and static pressures at different points. They can be seen on Formula 1 racing cars and spacecraft. They have even been configured to perform an additional role as an aircraft antenna.
