Nivus osm-f, Operation Instruction Manual

Page: 15 / 103

Instruction Manual
OCM F
OCM F - Rev. 04 as of 18.02.2014 Page 15
5.3 Flow Velocity Detection
The flow velocity sensor, type “KDA“ operates according to the continuous Dop-
pler principle (CW-Doppler) using 2 built-in piezo crystals with a slope of 45°.
The crystal surfaces are arranged parallel to the slope of the flow velocity sen-
sor. One of the crystals continuously operates as ultrasonic transmitter, the other
one as receiver detecting the reflected ultrasonic signal.
The sensor enclosure enables acoustic coupling of the high-frequency ultrasonic
signal between piezo crystal/enclosure and enclosure/medium. Due to that rea-
son an ultrasonic signal with an angle of 45° is sent permanently against the flow
direction of the medium to be measured. As soon as the signal hits dirt particles,
gas bubbles or similar a portion of the sonic energy is reflected, being converted
into an electric signal by the receiving crystal subsequently.
Caused by the movement of the reflecting particles in relation to the acoustic
source the frequency of the ultrasonic signal is shifted. The resulting frequency
shift is directly proportional to the particles’ movement within the medium and
hence represents the flow velocity.
The sensor processes the received reflection signal, converting it to be sent to
the transmitter.
Due to varying velocities within the flow profile, vorticity, rotation of single reflect-
ing particles, surface waves etc. a frequency mixture is emerging. This mixture is
evaluated directly within the KDA sensor regarding statistic considerations relat-
ed to average flow velocity. The frequency mixture is indicated on the transmitter
display in >I/O / v-Histogram< (see chapter 9.5.6).
It is recommended to verify the measurement if conditions are disadvantageous
from a hydraulic standpoint. Verification should not be based on the CW-Doppler
method since in this case it is not possible to spatially allocate recorded flow ve-
locities.
In this case the VDI/VDE Directive 2640 is very helpful and important.
NIVUS recommends the portable meter Type >PVM/PD< or >PCM Pro< as cali-
bration measurement or the NIVUS initial start-up service.