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Flow
Rate
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Flow
Rate Sensor
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Venturi Valves
A Venturi valve reduces the cross section
of a pipe to create a pressure differential
from the normal pipe diameter. The pressure
differential increases with the velocity
of the flow to aid in determining the flow
rate.
Transit-Time Flow Measurement Principle
A transit-time flowmeter measures the
effect of a liquid's flow velocity on bi-directional
acoustical signals. An upstream transducer
(T1) sends a signal to a downstream transducer
(T2) that in turn sends a signal back. When
there is no flow, the time to go from the
T1 to T2 is the same as the time going from
T2 to T1. However, when there is flow, the
effect of the liquid's flow velocity on
the acoustical signal is to assist the signal
in the up to downstream direction and hinder
the signal in the down to upstream direction.
This creates the time difference by which
the liquid's flow velocity, and ultimately
the flowrate, is determined.
Pitot Tubes
Pitot tubes have been used in flow measurement
for years. Conventional pitot tubes sense
velocity pressure at only one point in the
flowing stream. Therefore, a series of measurements
must be taken across the stream to obtain
a meaningful average flow rate.
Flow Transducers
Fluid flowing through the sensor spins
a magnetic rotor to induce a voltage in
a coil. An electronic circuit measures the
frequency of the electrical pulses generated
and computes the flow rate. This rate is
converted to a 0-5 VDC or 0-20 MA output
proportional to the flow rate and also used
to control a relay. The relay trip point
may be preset at the factory or adjusted
by the user by turning a potentiometer.
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