In late years some hydraulic engineers have come to distrust the pitot tube as an accurate instrument for measuring the flow of water in pipes. It has been found that some pitot tubes do not have the same coefficient for all conditions of flow. There is, therefore, general disagreement as to the coefficient which must be applied for a particular set of conditions to obtain the velocity within 1 per cent or less of the true value. The following experiments were made in an effort to find the reasons for this variation of the coefficient.
Tests made on an 84-ft rotating boom in still water showed the coefficient of the impact tip to be unity. The pitot deflection was balanced against a centrifugal head.
A method of measuring the angularity of turbulent flow in pipes by use of the law of probability was devised. The measurements made in pipes 12, 40, and 78 in. in diameter check the Prandtl and von Kármán theories of turbulent flow closely and show the mean angularity to be about 2 deg. The effect of the pitot support rod on the reading of the pressure-head piezometers and impact tip was determined. Tests were also made to determine the effect of pulsations.
Tests made to determine the effect of the pitot support rod showed that the support rod did not cause an error in the impact-tip reading, but did affect the pressure measured by the wall piezometer. The rod was also found to affect the reading of the pressure orifice of the pitot-static tube. It follows that any form of combined type of pitot tube in which the error could be determined by calibration of the instrument alone would give accurate results.
The average magnitude of pulsations was found to be too small to cause an error in reading of the pitot tube.
The conclusions drawn from these tests were that the impact tip correctly registered the true dynamic head whether the flow was smooth or turbulent, but that the pressure piezometers were likely to be in error. Since the pressure piezometer of the pitot-static tube was a part of the instrument and calibrated with it, this instrument was considered more reliable than the simple pitot tube using wall piezometers.