Quantcast Vibrating Meters

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3.2.5 Acoustical Meters.  Two types of ultrasonic meters are used to measure
flow rate., Doppler meters and time-of-travel meters. The Doppler meter is
based on a transducer signal of known frequency sent into the fluid. A pulse
r e f l e c t e d off of solids, bubbles, or any discontinuity is received by another
t r a n s d u c e r in the form of a frequency shift. The frequency shift is
p r o p o r t i o n a l to the velocity of the liquid. Time-of-travel meters have
transducers mounted at 45 to the direction of flow.  Since the speed of the
signal between the transducers varies, a t i m e - d i f f e r e n t i a l r e l a t i o n s h i p
p r o p o r t i o n a l to the flow is obtained by transmitting the signal alternately in
b o t h directions.  T o minimize signal scatter and absorption in time-of-travel
m e t e r s , the liquid must be relatively free of entrained gas or solids.
3 . 2 . 6 Vibrating Meters.  Mass flow can be measured by taking advantage of the
C o r i o l i s force.  One type of meter uses a sensor housing with a U-shaped
tube.  The flow tube is vibrated up and down at its natural frequency by a
magnetic device contained inside the tube.  The resistance to change in
d i r e c t i o n by the liquid flowing in and out of the tube during the vibration
cycle causes the tube to twist.  T h e amount of twist is directly proportional
to the mass flow rate.  Attached sensors feed this data for processing to an
e l e c t r o n i c conversion unit.  A voltage proportional to mass flow rate results
i n a digital readout.


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