Pitot Tube for 5m Flume

OPERATING THE PITOT TUBE

INTRODUCTION

The Pitot tube and manometer are used for measuring low velocities of water in the flume.  The manometer has Paraffin/Kerosene above each column of water and if used with excessively high velocities, the Paraffin will be pushed out of the manometer into the flexible tubing which may result in Paraffin entering the flume.

REMEMBER:  DO NOT open the valve at the base of the reservoir on the manometer during operation.

Slowly open the flume inlet valve and allow water to flow slowly through the flume.  Carefully open the isolating valves at the base of the manometer and note the difference in levels in the two limbs of the manometer.

The velocity of the water is calculated as follows:

For the Pitot tube                                         v =                  Eqn 1

For the manometer                                            Eqn 2

Where:

 v = Local velocity of water                                                            (m/s) k = Pitot tube coefficient (assumed to be 1.0) pt = Total pressure                                                                          (N/m2) ps = Static pressure                                                                         (N/m2) ρf  = Density operating fluid, water                                             (kg/m3) ρm = Density of manometer fluid, paraffin                                 (kg/m3) h = Difference in levels in manometer                                         (m) g = Gravitational constant                                                              (9.81m/s)

By substituting Eqn 2 into Eqn 1:

v =

density of paraffin is 784 kg/m3

v =              (m/s)

For defining the position of the Pitot tube relative to the flume, the following convention can be used:

xp  =  dist. along the flume (scale marked on the side of flume)                      (m)

yp =  location across flume                                                                                     (m)

zp  =  height above bed of flume (vertical level gauge)                                     (m)

These dimensions can be tabulated with the other results obtained.

The Pitot tube assembly is used where velocities are required.  The velocity profile in the flume can be obtained by moving the Pitot tube vertically and horizontally across the flume at different sections, noting the readings on the manometer at each position and converting these readings to a series of velocity profiles.