Nivus ocm pro cf Instruction Manual

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Nivus ocm pro cf Instruction Manual Manual pdf 25 page image

Instruction manualOCM Pro CF®OCM Pro CF - Rev. 09 as of 07.06.2016 page 256.5 Flow Velocity DetectionThe piezo crystal which has a slope towards the flow direction operates as aflow velocity sensor. Here an ultrasonic burst with a defined angle is sent intothe 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 sizeof the particle a particular signal results. Hence, the multitude of the reflectedsignals results in a reflection pattern (see Fig. 6-8). This pattern is going to bereceived by the piezo crystal again, converted to electric signals and loaded intoa digital signal processor (DSP) which is built in the active sensor.Fig. 6-8 Situation at first signal receptionAfter a certain period a second ultrasonic burst is sent into the medium. Thenewly 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 firstmeasurement 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; someparticles are no longer within the measurement range and others have nowmoved into the measurement range.Fig. 6-9 Situation at second signal detectionThe DSP checks both the received reflection patterns for similarities using thecross correlation method. All existing signal differences are rejected so that twosimilar 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 signalpattern is investigated (see Fig. 6-10).