Configuration

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Symptoms

1. The signal-to-noise ratio (SNR) is very low.

SNR should be around 30 at the center of the profile and above 20 at the ends, with a parabolic shape peaking at the sweet spot of 0.05 m range. SNR at ranges greater than 0.06 m will drop sharply so attention should be paid to this region of the profile if used.

Is low SNR is encountered, try adding scatterers to the water column such as the seeding material provided with the Vectrino II. Add a little at a time and see if SNR improves.

If there are enough scatterers in the water, check the Power Level in the Vectrino-II Configuration dialog and make sure it is on high.

If the Power Level is on High and scatterer concentration is high enough, but SNR is still low, please contact support@nortekusa.com.

2. The velocity time series look spiky and correlation is low or varies widely.

Here is an example of a spiky velocity time series and correlation from one range cell:

Correlation should be very high, not widely scattered as seen above. Correlation profiles should have the same parabolic shape as SNR. Try and achieve correlations greater than 90% throughout the profile. The highest correlations will be at the sweet spot at a range of 0.05 m, with correlations at ranges greater than 0.06 m usually dropping off sharply.

To improve correlation, first make sure SNR is high (see 1 above). Then, check the Velocity Range and Ping Algorithm in the Vectrino-II Configuration dialog. The actual Horizontal range and Vertical range are reported in the configuration dialog just below where the Velocity Range is set. These need to be appropriate for your flow. These ranges are also reported in the Text Data area in the leftmost pane of the Vectrino-II software window. Click on the text data button if this pane is not visible.

If your velocity range isn't what you expected, check which ping algorithm is selected. Reported velocity ranges will depend on the algorithm. From Page 20 of the Vectrino II SW User Guide here are descriptions:

Max interval: Produces the longest possible ping interval to achieve desired ambiguity velocity and sampling range. This is best used for smooth flow conditions.

Min interval: Produces the smallest possible ping interval to achieve the sampling range and, at minimum, the desired ambiguity velocity. This is best used for turbulent measurements.

Adaptive: (If licensed) Collects a long profile and examines the return echoes to determine where acoustic interference is occurring. The ping rate is then calculated to achieve the desired ambiguity velocity and sampling range while minimizing / removing acoustic interference. The adaptive ping interval selection is carried out with every configuration performed so the ping intervals selected may vary when the beam return signals change. This is the best general choice.

An appropriate Velocity Range for a flow will typically produce a Horizontal Range (m/s) and Vertical Range (m/s) which is larger, but not significantly so, than expected velocities. Horizontal refers to the x and y instrument velocity components, Vertical to the instrument z component. Most users will want to match the Horizontal Range to flow conditions. If you expect large velocities aligned with the Vectrino II's z velocity component, make sure the reported Vertical Range is large enough.

Highly turbulent flows (e.g. a turbulent jet) will typically need a larger Velocity Range than mean velocities suggest.

When adjusting the Velocity Range to improve correlation, try moving to a slightly higher value first. In most circumstances a too large Velocity Range is better than a too small Velocity Range.

3. The Velocity Range is set appropriately, but there are low correlation regions

in the profile.

These are due to pulse interference, also called weak spots, where a past acoustic pulse's boundary echo is interfering with the current pulse's processing.

Weak spots have fairly distinct symptoms. There will be a region of increased amplitude (and SNR) accompanied by low correlations, while velocities will be very spikey and noisy (similar to the above figure). Here is an example of weak spot affected velocity, SNR and correlation profiles.

The simplest method to deal with weak spots is to use the Adaptive ping interval algorithm mentioned above. Adjusting the velocity range manually will move the weak spot around. If the Adaptive ping algorithm can't eliminate the weak spot, manually adjusting its position to the least important part of the profile is a good option.

Set the Adaptive check: interval to an appropriate value based on measurement conditions. Over a hard boundary where the Vectrino II position is fixed Once will work well. In moving probe or boundary measurements, select an interval reflecting expected time scales for changes in the probe o rboundary position.

4. The Bottom Distance is reporting incorrect values or is noisy.

In the Bottom Check tab of the configuration dialog:

Check the Minimum depth (mm): and Maximum depth (mm): values and make sure they span the expected distance to the bottom. Check the Cell size (mm): and make sure it is also set to the desired resolution. Larger cells will create coarser estimates in the bottom position.

If the bottom distance flips between two different values like this

A second echo is being identified as the bottom. Check the Center Beam tab in the main window display to see the amplitude profile from the bottom check ping and look for additional echos at roughly integer multiples of the expected bottom distance. Adjust the Minimum depth (mm): and Maximum depth (mm): values appropriately so only the true bottom echo is in the expected range.

Hard bottoms, in particular metal surfaces, can generate strong echos which overwhelm the receivers and create a very broad bottom peak. This will increase uncertainty in the bottom location. Set the Gain reduction (dB) to a value which results in the bottom peak having an amplitude of approximately -10 dB in the Center Beam tab of the main window display. Here is an example of a saturated peak and the reduced gain version of this peak.

If available, a physical measurement with a ruler can confirm the bottom distance measurement from Vectrino II.

If measuring over a rough boundary, be aware the return echo will be fairly complex and the bottom distance will be subject to increased uncertainty. Alternative methods of determing bottom distance will be needed.

5. Where's Probe Check?

Longtime users of acoustic velocimeters will undoubtedly miss the Probe Check function in the Vectrino II software.

Don't panic.

The functionality of Probe Check is available for profiling and non-profiling versions of the Vectrino II.

For non-profiling versions, Probe Check has been renamed Beam Check and is accessed through the Doppler tab of the configuration dialog. The display will be a tab labelled Beam Check in the main display window, displaying a static image of a 35 cm profile of beam amplitudes. See Page 20 of the Vectrino II SW User Guide for a complete description.

For profiling Vectrino IIs, enable the Adaptive ping alogorithm and select an update interval. The amplitude data will be displayed in a Beam Check tab in the main window, updated at the rate specified in the Adaptive check: interval.

6. Beam Check data doesn't look right. One or more of the beams has a higher amplitude.

All four beams should show roughly the same amplitude and profile shape.

Check for an obstruction in the beam path. These may be fairly obvious such as a piece of vegetation or debris. If measuring near objects, such as in the wake of a cylinder, echos from vertical boundaries may appear in the profiles.

Multiple echos from the bottom reflecting off the surface can also appear in the Beam Check data. These will typically be at roughly integer multiples of the actual bottom check distance. Depending on the acoustic path length, an echo closer than the actual boundary position may result.

If there are no obstructions and multiple bottom echos have been ruled out, there may be damage to the probe head. If there are any signs of damage to your Vectrino II head, such as a bent receiver arm, a bent probe stem for rigid probes, a damaged cable for cabled probes, or damage at the bulkhead connector on either end of the electronics case, please contact support@nortekusa.com