®
Instruction manual
NivuSonic Clamp on
Page 14
4.2
Flow velocity detection
The flow velocity is determined by using the ultrasonic transit time principle.
1 Sensor 1
2 Sensor 2
Fig. 4-2
One-path transit time measurement principle
This measurement principle is based on directly measuring the transit time of
acoustic signals between two ultrasonic sensors, the so-called hydro-acoustic
converters.
A short ultrasonic impulse with a defined frequency sent towards the medium
flow direction in a defined angle requires a longer transit time than another im-
pulse being sent with the medium flow direction in an inverse angle.
The difference between the transit times is proportional to the average flow ve-
locity within the measurement path.
Flow determination is possible as soon as both cross-sectional area as well as
flow geometry of pipe or channel are known.
Presuming that C>>
υ
1-2
and the flow direction is known, it is possible to ap-
proach the transit time (
∆
t) applying
2
2
1
2
1
2
c
L
t
−
−
⋅
=
∆
υ
Whereat:
L
1-2
length of acoustic measurement path between sensors 1 and 2
C
velocity of sound in water
υ
1-2
average flow velocity between sensors 1 and 2 along the measure-
ment path
Using two paths (instead of one as used for transit time measurement) fixed on
the outside of the pipe allows to collect more information on flow velocity and
flow can therefore be determined more accurate.
Summary of Contents for NivuSonic Clamp On
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