Nivus OCM F, Instruction Manual

Page: 18 / 110

Instruction Manual
OCM F
page 18 OCM F - rev. 05 / 06.04.2018
12 Functional Principle
12.1 In General
The OCM F is a permanent measurement system for flow measurement and flow control.
The device is designed to be used primarily in slight to heavy polluted media with various
compositions. It can be operated in partial and permanent filled channels and pipes with vari-
ous shapes and dimensions.
Important note
The measurement method is based on the ultrasound Doppler principle. Hence, it is indis-
pensable for the system’s capability to work that the water contains particles which are able
to reflect the ultrasonic signal sent by the sensor (dirt particles, gas bubbles or similar).
OCM F transmitters utilise a compact active Doppler sensor (KDA sensor). The KDA wedge
sensor is available as flow velocity and as combi sensor. KDA combi sensors simultaneously
determine the flow level along with the flow velocity by using a built-in pressure measurement
cell. KDA pipe sensors, however, are available only as flow velocity sensors. Detailed infor-
mation on the KDA sensors can be found in the Technical Description for Doppler Sensors.
12.2 Level Measurement using Pressure
The combi sensor type “KDA” additionally contains a hydrostatic level measurement via inte-
grated pressure measurement cell. The piezo-resistive pressure sensor operates according
to the relative pressure principle; i. e. the pressure of the standing water column above the
sensor is direct proportional to the flow level. During initial start-up procedure, the pressure
sensor is going to be adjusted by entering a manually investigated reference value.
12.3 Flow Velocity Detection
The flow velocity sensor, type “KDA” operates according to the continuous Doppler 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 sensor. One of the crystals continuously op-
erates 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 be-
tween piezo crystal/enclosure and enclosure/medium. Due to that reason 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 sub-
sequently.
Caused by the movement of the reflecting particles in relation to the acoustic source the fre-
quency 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 transmit-
ter. Due to varying velocities within the flow profile, vorticity, rotation of single reflecting parti-
cles, surface waves etc. a frequency mixture is emerging. This mixture is evaluated directly
within the KDA sensor regarding statistic considerations related to average flow velocity. The
frequency mixture is indicated on the transmitter display in >I/O / v-Histogram< (see chapter
“24.7 I/O submenu “v-Histogram””.
It is recommended to verify the measurement if conditions are disadvantageous from a hy-
draulic standpoint. Verification should not be based on the CW-Doppler method since in this
case it is not possible to spatially allocate recorded flow velocities.
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 calibration measurement or the NIVUS
initial start-up service.