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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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