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What is the difference between Transit Time Technology and CROSSFLOW Systems Technology?
Non-intrusive flow meters using ultrasonics have gained significant industrial acceptance during the last decade.
There are several different methods of using ultrasound to measure flow velocity, such as the transit time system
and the cross correlation system used by CROSSFLOW.
The transit time systems direct ultrasonic beams diagonally across the pipe and detects the difference in the time
of pulse transmission with and against the flow. Since the beam has equal sensitivity to the local flow velocity
along all of its length, a reasonably accurate correction for the effect of velocity profile can be calculated.
The major limitation on this system of measurement is that it can be very sensitive to temperature non-uniformity
which affects the path taken by the beam across the pipe, sensitivity to speed of sound in the liquid that must be
known to determine flow velocity, and the fact that the velocity is derived from a small difference between two
relatively large quantities (speeds of sound).
CROSSFLOW Systems involves using four ultrasonic transducers (UTs ) on each bracket with typically one bracket being
mounted on each pipe. On each bracket there is one upstream and one downstream transducer station, each station
consisting of one transmitting and one receiving transducer.
It has been proven that during turbulent flow, vortexes (also known as eddies) are formed within the flow/fluid.
These eddies are due to turbulent fluctuations in the flow and affect the frequency, amplitude and phase of ultrasonic
waves propagated across the pipe.
The turbulent eddies in the pipe form a spatially random pattern that moves with the flow and changes along with time.
This random pattern is akin to cloud movement in the sky. You can pick out distinct cloud patterns and follow these
patterns for a period of time as the cloud moves across the sky. Eventually the clouds dissipate. However, for a
finite period of time, the clouds were visually detectable and their movement traceable.
Similarly, the random pattern (of turbulent eddies) remains recognizable over a certain distance and cross-correlation
system detects the eddies.
The Turbulence signatures in the flow that are detected by each of the UT pairs are sent to a signal processing unit.
Signatures from the upstream and downstream transducer stations are compared, using the cross-correlation technique
to obtain a highly accurate measurement.
Crossflow does not directly measure the bulk velocity of the liquid. It actually measures the velocity of eddies that cross the upstream and downstream ultrasonic paths. To measure flow rate, a Flow Correction Factor is required to obtain the liquid bulk velocity from the velocity of eddies.
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