The Speed Wand

The ideal gas law (Ch. 11) may be used to predict the pressure p ) of the air trapped above the liquid in the tube. Since the temperature of the air in the tube before and after a test will be the same, the ideal gas law states that  

The Bernoulli equation [Eq. (5.15)] may be applied to point (B) before and after equilibrium has been established  

Of course, the markings on the tube should be based on a calibration rather than on calculations. However, the calculated values were actually found to be in very good agreement with experimental values, indicating that the foregoing analysis is adequate for initial design of this instrument. 

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