If you are having problems with Distance Check there are a few things to try. First, please make sure you are using the most recent firmware appropriate for your instrument. There have been improvements made in the Distance Check algorithm and it is generally quite robust. You can download the most recent firmware here: https://www.nortekgroup.com/software
Everything is referenced to the central transmitter for range. Be aware you won’t be able to measure right above the bed. You should be able to measure from 5–10 mm away from the boundary depending on specific experimental conditions such as the boundary type. Configuring to profile from 50 to 60 mm means you will have velocity cells centered at 50, 51, 52, …, 60 mm range from the central transducer.
You could check your distances with a physical measurement and against the Probe Check output. This gives you a couple of ways to gauge the quality of your measurement. The “Quality” field when running a Distance Check should be over 100 and typically will be several hundred.
If you do not believe the measurement or there is erratic behavior, start a Probe Check and see if there is anything odd going on in the received signal. Odd echoes, grounding problems, and several other things can result in problems with the Distance Check. It is possible for the algorithm to detect a second peak, and when the second peak is more distinct than the first one that may be what is throwing the distance check off. It is just searching for the highest peak in this case and unfortunately reporting an erroneous reading. You should see a clear, sharp peak in the return signal strength at the boundary in all four beams. The type of boundary will also have an effect.
Vectrino Profiler: 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 the plot below, a second echo is being identified as the bottom.
Measuring a wrong distance will not affect the velocity measurements since the Velocity Range setting in the Configuration window sets the lag between pulses and not the distance to the bed. Just be aware of this problem, consider whether the measurement reported by the instrument is believable, and always check your Probe Check plot to verify the distance measurement if it’s in doubt.
Example: Vectrino Profiler. The yellow line represents the raw amplitude return. The green line is the bottom peak location estimate and should only vary in y location. The red line represents the real-time fitting process. The problem here is that the bottom echo is quite loud and saturating the receiver. The bottom peak is broad; what it should look like is a fairly sharp Gaussian bump. If you look in the software manual and search for “gain reduction” there is a bit more information on this setting. In the Bottom Distance configuration tab, there is a setting called “Gain Reduction” which takes a value in decibels. Set this so the bottom peak is below -10 dB (yellow line in the screenshot below). Setting the gain reduction to be too large will mean the bottom peak is lost in the background noise and won’t be as reliably detected. Because different bottom surfaces will provide different echo intensities, there can be range attenuation of the signal, etc. The receiver is also only linear within a certain range, so aiming for the middle of the receiver’s range should yield the best results.
Example of a saturated peak and the reduced gain version of this peak.
One thing to note when operating in a flume is that the bottom material can have a huge effect on the data quality. Surfaces that are acoustically highly reflective will cause very large return signals that pollute the water column in such a way that obtaining reliable data is not possible. We’ve seen metal-bottomed flumes that have very large returns from the bottom – so large, in fact, that they caused large areas of interference in profiles that were stepped up from the bottom. You can tell how large the return is by looking at the signal return from the bottom check. If you have to use a large gain reduction value (30 dB or more) to reduce the intensity of the bottom check return to an acceptable value, then there’s a good chance that you’ll find that significant portions of the stepped profiles will have areas of interference in them. If this is the case, you can try adding a thin layer of acrylic or rubber to the bottom to provide some acoustic dampening to the return. This could readily explain the problems that you’re seeing in the stepped profiles. For the interested reader visit the manuals page for a document called “Near boundary measurements with a profiling acoustic Doppler velocimeter”, presenting near boundary measurements and the effect of different boundary materials.