YSI SonTek ADVField, Technical Documentation Manual

Page: 101 / 228

SonTek/YSI
ADVField/Hydra Operation Manual (September 1, 2001) 89
5.11.3. Excessive Noise in Velocity Data
Some random variation (noise) is expected in ADV velocity measurements. Generally, under
good operating conditions, the noise for data output at 25 Hz (50 Hz for MicroADVs) should be
about 1% of the velocity range setting (i.e.;
±
1 cm/s when using the
±
100 cm/s velocity range).
The variations in velocity are a combination of instrument noise and real variations in the flow. It
is often helpful to operate the probe in a small, quiet tank of water with a small amount of seed-
ing material for diagnostics. A “known” velocity can then be generated by moving the probe.
Below is a list of some basic checks if ADV velocity data seems particularly noisy.
•
Make sure the probe is submerged and all cables are securely connected.
•
Add seeding if the SNR is low (less than 5-10 dB).
•
Make sure the velocity range is larger than the actual velocities (see
ADV Principles of
Operation
).
•
Run
ADFCHECK
to look for potential hardware problems.
If in doubt as to whether the velocity fluctuations are real or are generated by the ADV, collect a
time series and calculate the power spectrum. Instrument-generated noise is purely random
(white), and its signature is a flat power spectrum.
5.11.4. Low Signal-to-Noise Ratio (SNR)
A low SNR, typically considered less than 5-10 dB, can cause increased noise in the velocity
data. SNR is a function of the amount of scattering material in the water. In very clear water, the
ADV will not be able to make accurate velocity measurements. If the SNR is low, check the fol-
lowing areas.
•
Clean the faces of the acoustic sensors.
•
Add seeding material.
•
Check the probe using
ADFCHECK
to look for hardware problems.
See the
ADV Principles of Operation
for more details.
5.11.5. Low Correlation
Correlation data is a direct measure of the quality of ADV velocity data. Low correlation data in-
dicates something is affecting the ADV’s ability to accurately measure velocity. For moderate
flows with low turbulence, correlation will typically be 90% or greater. High turbulence, low
SNR values, and a few other factors will decrease correlation values. In general, correlation val-
ues above 70% are desired for good velocity data. In some situations, correlation values as low as
30-40% will still yield good results (although with a higher noise level). If the SNR seems low,
check the following areas.
•
Clean the transducers for accumulated bubbles.
•
Check for low SNR values.
•
Check probe using
ADFCHECK
to look for hardware problems.
Low correlation values can occur without consequence for data quality if the ADV is being used
only for mean velocity measurements. See the
ADV Principles of Operation
for more details.