YSI ADV6600 - Manual (Page 175)

Appendix D. Frequently Asked Questions
ADV6600
Y S I Environmental
Page 165
• The sensor must be totally submerged to successfully measure the water velocity. If the ADV
sensor comes out of the water during a measurement, the ADV will collect invalid data even
though there will be no damage to the system.
• The sensor measures the velocity at a distance approximately 6.8 cm in front of the sensor head.
There must be no objects or boundaries (water surface, river bottom) blocking either the
transmitter or receivers. Intermittent blockage of the transmitter/receivers can certainly occur in
field or lab environments. This blockage can be caused by ropes, cables, weeds, or similar
extraneous objects, moving back and forth in front of the ADV sensor and within the system’s
measurement area. Users should be aware that invalid or biased data will be collected during
periods of blockage.
Since the acoustic energy from the ADV is reflected from particles in the water, will variations
in the turbidity of the water cause inaccuracy in the velocity readings? Is there a minimum
level of turbidity (or suspended particles) for the ADV sensor to function properly?
Doppler current meters will not work in a body of water where no particles (other than the water
molecules themselves) are present. Fortunately, natural streams almost always have some sort of
particulate matter present, even if it is just tiny air bubbles. Doppler technology is such that even a
very small amount of particles in the water is enough for good measurements. As such, Doppler
technology is almost certain to function properly at most environmental sites.
Because the scattering ability of particles is dependent not only on their number, but also on their
shape, distribution, and type, YSI cannot give an exact turbidity value in NTUs that is a minimum
requirement for acceptable Doppler performance. However, the “beam check” procedure (see later),
which should be performed before the initial deployment of the ADV6600 at every site, will
confirm that the water contains enough particles for acceptable Doppler measurements.
Will biological fouling of the Doppler emitter and receivers compromise performance?
Moderate fouling of the Doppler emitter and receivers will have no significant effect on the velocity
data (other then flow disturbance that may be caused by such fouling). For sites where the fouling is
extreme (e.g., estuarine applications where barnacles form readily), anti-fouling paint may be used
on the ADV sensor without affecting the quality of velocity measurements. Note, however, that
long algal or plant fragments which have been caught on the Doppler sensor arms during
deployment and which can move back and forth into the beam path in a random motion will likely
cause the velocity data to be grossly compromised.