Water Hydraulics - Wastewater Operators - page 94

To tal pressure tap

Static pressure port

figure 2.27 Pitot tube.

2.13.4.2.4 Pitot Tube

A pitot tube is a point velocity-measuring device (see Figure 2.27). It
has an impact port; as fluid hits the port, its velocity is reduced to zero,
and kinetic energy (velocity) is converted to potential energy (pressure
head). The pressure at the impact port is the sum of the static pressure
and the velocity head. The pressure at the impact port is also known as
stagnation pressure or total pressure . The pressure difference between
the impact pressure and the static pressure measured at the same point is
the velocity head. The flow rate is the product of the measured velocity and
the cross-sectional area at the point of measurement. Note that the pitot
tube has negligible permanent pressure drop in the line, but the impact
port must be located in the pipe where the measured velocity is equal to
the average velocity of the flowing water through the cross-section.

2.13.5 Magnetic flow Meters *

Magnetic flow meters are relatively new to the wastewater industry.
They are volumetric flow devices designed to measure the flow of elec-
trically conductive liquids in a closed pipe. They measure the flow rate
based on the voltage created between two electrodes (in accordance with
Faraday's law of electromagnetic induction) as the water passes through
an electromagnetic field (see Figure 2.28). Induced voltage is proportional
to flow rate. Voltage depends on magnetic field strength (constant), dis-
tance between electrodes (constant), and velocity of flowing water (vari-
able). Properties of the magnetic flow meter include: (1) minimal head
loss (no obstruction with line size meter); (2) no effect on flow profile; (3)
suitable for sizes ranging from 0.1 to 120 in.; (4) accuracy rating of 0.5 to
2% of flow rate; and (5) measurement of forward or reverse flow.

The advantages of magnetic flow meters include:

• Obstructionless flow•

• Minimal head loss

* This section is adapted from Water and Wastewater ITA and USEPA, flow Instrumentation:
a Practical Workshop on making Them Work , Water and Wastewater Instrumentation
Testing Association and U.S. Environmental Protection Agency, Sacramento, CA, 1991.