Intermittent low correlation in one beam or another

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Intermittent low correlation in one beam or another

Posted by Irene Hu at February 03. 2017

Hello,

I have a Vector set up in a not-terribly-large tank and am creating turbulence with a recirculating pump.  The velocities I measure are in the +/- 5 cm/s range (the average is close to 0, the stdev of velocity is around 1 cm/s).

I have this problem where I see periods of low correlation in one beam or another.  Yesterday it was Beam 1; today it is Beam 2.  The correlation values are as low as 20-30, sometimes higher, but rarely above 70.  The other two beams have correlations in the 90s.  The SNRs that correspond to low correlation data points are fine (20s, and no different between low and high correlation points). Noise floor is 47 counts for all beams.

Yesterday I saw this problem (same beam) across different measurements of a few minutes each (I tried it with and without my pump on; it happened in both stagnant water and what I hope was some amount of turbulence).  Then I spent some time doing something else, and when I started measuring again (maybe 10-15 minutes later), my correlations were magically a lot better.  Is there any reason I would see behavior like this?  Would it happen if there are large particles (like wood chips) floating around in the water?  I don't think it is from low SNR because the SNR is fine, and I don't think it is from weak spots because the problem comes and goes while I am not changing the location of the floor.

Thanks!

Irene

Re: Intermittent low correlation in one beam or another

Posted by Cary Troy at November 17. 2017

Has anyone figured out this problem?  I have one beam on my Vectrino consistently with correlation 95+, one at 50, one at 70, etc.  No changes in seeding or configuration settings alleviate the discrepancy, and applying the usual correlation threshold filter to the data removes most points due to the low beams.  It would be helpful to know whether this indicates a hardware problem, i.e. return the instrument to Nortek for servigin, or whether there is some other issue.

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