Seeding material and more

Hi again. Two questions regarding 10 mhz 3D NDV.

First, I have problems with the SNR because, after some time, the particles floating in the water sediment and then the SNR decreases. I've used titanium dioxide as seeding material but again after some time the signal isn't good enough. When i run the wavemaker in the flume the waves are not strong enough to move the material so, only after running a current and for a limited time I have a good signal. Is there any solution for this? Maybe another seeding material?

Second. The Advs work fine when I use them separately. When I plugg them together I get strange behaviors. For instance, I had a double length sinusoidal signal (in the time axis). Just stopping the data aqusition and running it again I can get the same problem in two of them. Just playing, I changed the sampling rate form 25 Hz to 10 Hz and the problem appears in other probes, or in the ones that already had it and also in some other. The more tests I do the more it looks to me to be a random behavior. I've read some people had the same problem but nobody had a solution for this!

I hope you can help me. Thank you in advance,

PT