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 Area-Velocity
Flow Measurement:
An
Area Velocity Flow Meter continuously measures both Level (head)
and Velocity to calculate flow volume in an open channel or
pipe. Calibration is simple: enter the pipe ID or channel width
and the AVFM automatically computes flow volume and displays
the flow rate. A submerged pressure transmitter or ultrasonic
sensor measures the liquid level. The sensor is installed at
the bottom of a pipe or channel. To measure water level the
ultrasonic sensor transmits ultrasonic pulses that travel through
the water and reflect off the liquid surface. The instrument
precisely measures the time it takes for echoes to return to
the sensor. Based on the speed of sound in water, the level
is measured with accuracy of ±0.25%. In case of a pressure
transmitter, the liquid level is measured by measuring the pressure
exerted by a column (or head) of liquid on the sensor. The sensor
converts Hydrostatic Level Pressure into current signals proportional
to the liquid level. A semiconductor transducer containing oil
and a metal or ceramic sealing diaphragm is employed to achieve
accuracy and durability. The Hydrostatic Level Pressure being
exerted depends on the height of the liquid column as well as
its specific gravity.
Flow velocity is measured with an ultrasonic Doppler signal
continuously injected into the water. This high frequency sound
(640 kHz) is reflected back to the sensor from particles or
bubbles suspended in the liquid. If the fluid is in motion,
the echoes return at an altered frequency proportionate to flow
velocity. With this technique the instrument measures flow velocity
with accuracy of ±2%.
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