Nortek Vector Comprehensive Manual

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Comprehensive Manual18© 2018 Nortek ASwill never see correlations of zero because of noise due to electronics, temperature fluctuations, andother factors that will always correlate above zero.Discarding measurements with correlations lower than a threshold significantly reduce the varianceof the data set, which is generally the goal of most data screening operations. Removing lowcorrelation measurements is still a good idea because correlation is a strong indicator of data qualityin the sense of a valid Doppler phase shift determination. However, high correlation is not alwaysindicative of a valid measurement of the flow. It is also important to note correlation is tied to aspecific beam velocity measurement, not to an orthogonal coordinate direction such as X, and anyscreening should take this into account.In the early days of the acoustic Doppler velocimeter, if a user did some tests and determinedcorrelations were above ~70% then the instrument was considered to be generating good qualitydata. Many users still use this number for screening out bad data, although a generalization to someuniversal value is unwarranted and a close examination of the dataset is the best way to set acorrelation threshold (if any) for discarding bad data points.Figure: Correlation from one Vectrino Profiler beam . This plot corresponds to the SNR figure in theprevious section. Note how the boundary show s up in this plot1.9 Distance Measurement / Bottom CheckThe distance check (also referred to as Bottom Check) is used to measure the distance to the bedfrom the central transducer. The return from this pulse is listened to over some time to obtain anamplitude profile. The amplitude profile is checked for peaks, and an interpolation scheme is used tolocate the peak location based on the largest amplitudes recorded.The Distance Check function is available in the Vectrino Plus firmware, the Vector and the VectrinoProfiler, but not in the Vectrino standard firmware.Vector: The Vector reports the distance to the boundary at the start and end of each burst, or at thestart when running the instrument in Continuous mode. That means that it is possible to do veryshort bursts and get almost simultaneous measurements (requires Firmware V1.20 or higher).Vectrino Plus: The Vectrino+ firmware comes with a separate boundary distance mode (10 Hz),where the distance to the boundary is measured. The distance measurement is not done at thesame time as the velocity measurements. The Boundary Distance functionality is typically used inexperiments where bottom changes as a function of time are needed, or when the Vectrino+ ismounted on traverse systems for bottom topography measurements.Vectrino Profiler: The Vectrino Profiler has an interleaved boundary distance functionality. In otherwords, you get both velocity profiles and distance to the bottom at the same time. The reasons forthis are both that it can be done technically, and also that the problem of weak spots is much morechallenging with the Vectrino Profiler than with a single point instrument. Hence, in order to operatethe Vectrino Profiler properly we depended on interleaving velocity and distance measurements, suchthat the time lag between the two pulses in every pulse pair can be adjusted adaptively.