Functional Description
page 29
NF 650 - rev. 01 / 29.06.2020
19 Functional Principle
19.1 Flow velocity detection
The flow velocity is determined by using the ultrasonic transit time principle.
1
Sensor 1
2
Sensor 2
a
Defined angle
t
1
Time of the impulse
towards
direction of flow
t
2
Time of the impulse
with
direction of flow
L
Path length
Fig. 19-1
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.
The transit time difference method does not determine the average path velocity, but the
effective velocity of sound propagation upstream (decelerated due to flow) and downstream
(accelerated due to flow).
Two sound impulses are transmitted consecutively and the different transit times between
transmitter and receiver are measured.
The upstream impulse needs a time t
1
.
The downstream impulse needs a shorter time t
2
.
Sound heading downstream will reach the receiver within a shorter period than sound heading
upstream. The difference between the transit times is proportional to the average flow velocity
within the measurement path.
There is no transit time difference when both sensors receive the transmitted ultrasonic
impulses simultaneously. There is no measurable flow available.
The NivuFlow 650 works with both clamp-on sensors and wetted sensors. The clamp-on sen-
sors are installed on the outside of the pipe. In this case the transit time through the pipe wall
is calculated and considered.
In order to determine the flow rate, the cross section as well as the flow geometry of the pipe,
the canal or the water body must be known.
Here the formula below is used:
v =
L
·
1
-
1
2
t
2
t
1
given:
• L
= length of acoustic measurement path between sensors 1 and 2
• v = average flow velocity between sensors 1 and 2 along the measurement path
