Instruction manual
OCM Pro CF
®
OCM Pro CF - Rev. 09 as of 07.06.2016
page 25
6.5
Flow Velocity Detection
The piezo crystal which has a slope towards the flow direction operates as a
flow velocity sensor. Here an ultrasonic burst with a defined angle is sent into
the medium. All the particles in the measurement path (air, dirt, suspended sol-
ids) reflect a small amount of the ultrasonic signal. Depending on shape and size
of the particle a particular signal results. Hence, the multitude of the reflected
signals results in a reflection pattern (see Fig. 6-8). This pattern is going to be
received by the piezo crystal again, converted to electric signals and loaded into
a digital signal processor (DSP) which is built in the active sensor.
Fig. 6-8
Situation at first signal reception
After a certain period a second ultrasonic burst is sent into the medium. The
newly generated reflection signal is saved in the DSP too.
In various flow levels there are different flow velocities (flow velocity profile). De-
pending on the level, the reflecting particles’ movement away from the first
measurement point therefore varies. Hence, a distorted reflection pattern results
(see Fig. 6-9). At the same time slightly different reflections occur: some parti-
cles have been turning around and thus have another shape of reflection; some
particles are no longer within the measurement range and others have now
moved into the measurement range.
Fig. 6-9
Situation at second signal detection
The DSP checks both the received reflection patterns for similarities using the
cross correlation method. All existing signal differences are rejected so that two
similar but temporarily offset signal patterns are left for velocity evaluation.
Depending on the flow levels both patterns are subdivided into up to 16 mea-
surement windows. Then, in each measurement window the lag
∆
t of the signal
pattern is investigated (see Fig. 6-10).
