Nominal velocity range for Vector.

Hi,

I am trying to set the "Nominal velocity range" for my first Vector experiment.  I'm wondering whether I should set this value to +/-0.1 m/s or +/-0.3 m/s.

Typical vertical velocity in our study site is +/- 1 cm/s, but can be as fast as +/- 3-4 cm/s.  Typical horizontal velocity is less than +/- 10 cm/s, and sometime reaches to 30 cm/sec.  If I were to set the nominal velocity range to +/- 0.1 m/s, would it be too close to the maximum horizontal velocity (35 cm/s) the Vector can measure?  On the other hand, if I set the nominal velocity range to +/- 0.3 m/s, the signal might be too close to the noise level.  I would appreciate any suggestion.

Thanks,
Mei

Hi Mei,

     You'll want to balance your need for a lower noise level with the potential for the velocity to phase wrap (something you'll need to handle in post-processing). If the horizontal velocities regularly in the 30 cm/s range I would recommend going with the 30 cm/s nominal velocity range. If it's only occasionally the velocities get this high than you could go with the lower range and just hope any velocity wrappings that occur are easy to correct.

Personally, I would go with the 30 cm/s nominal velocity range as phase wrapping can be a pain. You'll have fairly strong flow and the Vector should easily measure turbulence if there are enough scatterers (and for the ocean/tidal areas you're in this shouldn't be a problem).

P.J.

Hi Peter,

Thanks for your advice. I also have the following questions: 

(1) If memory fills up before the battery depletes, does Vector stop recording or overwrite the data by erasing the previously recorded data?  How can I make sure that Vector will not overwrite the data and will stop recording when memory is full?

(2) Advanced settings: I am trying to decide values for sampling volume and transmit length.  By changing the transmit length, available sampling volume sizes and measurement load are also changed.  Why is that?
     I want to increase the resolution of eddy size Vector can resolve, by setting smaller sampling volume.  By decreasing the sampling volume to 3.5 mm and transmit length to 2.0 mm,  will the precision of the measured velocity be decreased too much?  Is there any reference value of SNR or correlation that I can refer to, in order to decide the value for each parameter?

Thanks,
Mei

The Vector will simply stop recording data when the recorder fills up unless you tell it to wrap recording and keep the newest data. I think the only way to enable file wrapping is to turn on the analog outputs in the advanced tab. By default, file wrapping is not enabled.

I would personally leave the the sample volume and transmit length at their default values. A shorter transmit length is going to drop the SNR, and setting it to the lowest value and shrinking the sample volume to it's smallest will really drop the SNR.

Transmit length and sample volume are coupled because the receive window for the sample volume is defined relative to the transmit length.

I've done some basic testing with these settings in the lab with a Vectrino. I don't have the plots on my laptop, but I do remember their being more noise with the smallest transmit length/sample volume and the data generally not looking as good as with the default settings. I'm not really sure what you mean by reference value for SNR and correlation. The Vector typically has very high SNR (> 20 dB) and when properly configured, most velocimeters will have correlations in the high 90s.

If you do decide to change these settings, I recommend doing some basic tests in a controlled or repeatable flow and seeing if you actually improve your measurements. I don't think you will see much of an improvement by tuning these parameters and that ultimately the nominal velocity range is going to be the main way you'll want to change the instrument setup.

P.J.