Re: Advise: concentration, bent arm, echoing & side-looking
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Re: Advise: concentration, bent arm, echoing & side-looking
Posted by
Sergi Capapé
at
August 01. 2012
Dear Vectrino users and experts,
I’m planning to use my ADV Vectrino for measuring flow velocity and turbulence in a flow with high sediment concentration (~50 g/l) of very fine sediment (4 microns). Do you think I’ll be able to get correct measurements? I’m worried about the presence of too many particles that could ‘block’ the signal.
On the other hand, I suspect my Vectrino is not working properly. My guess is that its receiver arms are not in the right position (bent). That’s what I conclude from the Probe Check -see figure 1 (up) and 2 with the ‘down looking Vectrino’ perpendicular to the flume bed. Do you agree? The echoing at 280mm is referred in the next paragraph. The hydraulic conditions for the Probe Check are: flow depth = 35cm, width = 37cm, receiver arm at 27.5cm from the bottom, flow velocity < 5cm/s, centered measures.
Finally, we were told that Vectrinos used in flumes made of steel and glass materials can experience ‘echoing issues’. I believe that it’s happening to my Vectrino. I observe the chaotic noise starting at 280mm in Figure 1. In order to solve it, since we are using a down looking probe, the use of a side looking probe could be the solution. Do you think it would work? My flume is 37 cm wide and my flow depth in the real experiments around 10cm. Figure 3 shows the Probe check when the ‘down looking Vectrino’ is placed parallel to the bed flume, i.e. with the transmitter in the same direction as the streamlines (Fig.1 (down)) (I’m aware that it is different from the side-looking probe, but I consider it as the only way to surrogate it and to assess the ability of the side-looking probe to get correct measurements). The hydraulic conditions are the same than for Figure 1.
Many thanks in advance.
Sergi
Re: Advise: concentration, bent arm, echoing & side-looking
Posted by
P.J. Rusello
at
August 01. 2012
Hi Sergi,
based on your probe checks I think your probe head is damaged. The peaks for the sample volume in each beam don't line up suggesting the arms are bent on at least a couple of them. You can verify this visually by placing the head on a flat surface and seeing if all arms touch at the same time. I suspect they don't. If this is the case, the only option is to purchase a new head as we don't repair the Vectrino probes because of their fragility and small size.
We've found a metal surface tends to be the worst material to try and make measurements over. The echo from a metal boundary seems to cause the the most persistent echoes leading to severe interference with measurements. Glass should be less of a problem but can cause some issues closer to the boundary.
The high sediment concentration may cause issues. This is approaching the upper limit of where we expect the instruments to work well, but the main issue will be the return signal. Please see this thread for the main issue with high suspended sediment concentrations: http://www.nortek-as.com/en/knowledge-center/forum/velocimeters/974943093
Finally, I think the spike at 280 mm is the bottom echo.
P.J.
Re: Advise: concentration, bent arm, echoing & side-looking
Posted by
Sergi Capapé
at
October 05. 2012
Thanks for the reply.
I was able to get a side-looking probe to test it in the same conditions as the first 'failed' experiment.
The results are quite different but the side wall (glass) does indeed cause interferences (Figure 1, starting at ~18cm), as well as the metal bottom when measuring close to the boundary (mostly in beams 3 & 4) (Figure 2).
Nevertheless, correlations are high and quality cell shows high values...
Can I rely on these results?
Many thanks,
Sergi
Re: Advise: concentration, bent arm, echoing & side-looking
Posted by
P.J. Rusello
at
October 05. 2012
Hi Sergi,
The "interference" at 180 mm is just the bottom echo. I don't see anything in the probe check around the sample volume at 50 mm range to suggest interference. To clarify, when I say interference I'm thinking more along the lines of a weak spot (search for "weak spot" to see a lot of discussion of this). This will show up during velocity data collection as erratic and or low correlation when SNR is generally quite high (15-20 dB range typically).
In the second probe check, there is a strong echo, but again nothing in the sample volume looks to be particularly problematic. If you search for "weak spot" you'll find a table and can determine if for your velocity range and boundary distance there's a possibility of interference.
Finally, we try to not to say too much about specific data being reliable or not as there's quite a bit more that goes into this assessment. I don't see anything problematic in the probe checks, so you should be fine, but ultimately you'll need to perform the quality control on your data.
P.J.
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