Calibrating a Vector ADV for SSC measurements

Hello

First of all I am sorry ot start an new conversation for such an old topic! Just trying to increase chances that my voice will be heard...

I just finished some intercomparisons of turbidity sensors in the lab (cylindrical water container, stirrers, etc) and I have included in my tests also an ADV. We did something similar in the past for mud and we've got good, almost linear response as a function of SSC. However this time we tried to do the same, using also sands and in some cases the trend was positive (signal = f(SSC)) up to a concentration, above which the signal started to decrease (almost linearly) as the SSC increased further. (I have attached also a figure)

This means that the signal was dumped due to suspended material? Any way this could be useful?

Thank you very much

Michalis

Hi Michalis,

We discussed your question with Dr. Yong Kim at Horn Point Environmental Lab (ykim@hpl.umces.edu), and he had the following reply:

In your test experiment, the backscatter signal received by a Vector show a positive relationship in low
concentration zone (i.e., <0.5 g/ltr), and the signal saturated at concentration level of 0.5-1.5 g/ltr. Then
it shows an inverse relationship at concentration higher than 1.5 g/ltr, whereby backscatter signal
decreases with increasing concentration. As you expected, this inverse relationship might be due to
absorption of acoustic signal by high concentration of sediments. Similar inverse relationship was
reported by Kineke and Sternberg (1992) in deployment of Optical Backscatter Sensors (OBS) in high
concentration conditions (>36 g/ltr).

In addition, the reason why you only find this inverse relationship in the experiments with sand and not
occurred in the mud experiment, is that the volume scattering strength is not a function only of
concentration but also grain size (see Nortek technical note #3 – would be better if we may put a link
for this). In short, the peak sensitivity occurs when k*a = 1 (where k is the acoustic wave number of
Vector signal and a is the particle radius), which implies the circumference of the particle is equal to the
acoustic wave length. For example, sensitivity for a 6MHz Vector reaches the maximum value at particle
size of 80 microns (fine sands). Since sensitivity for particles of k*a < 1 is proportional to the radius of
particles to the fourth power, the same instrument hits saturation point at significant higher
concentration in the experiment with mud than sand. Thus you may find similar inverse relationship in
>10 g/ltr in mud experiment.