SonTek – a Xylem brand
FlowTracker2 User’s Manual (January 2019)
86
• For 2D systems, if the SNR of either beam is low, this will affect all velocity
data even if the other beam shows higher SNR values.
• For 2D/3D systems, if only the SNR of Beam 3 is low, vertical velocity data
(Vz) are affected; the horizontal velocity data (Vx and Vy) may still be valid.
This can occur if Beam 3 is out of the water in very shallow water.
iii). Approach Low SNR. If the SNR is between 4 and 7 dB, “Approach Low
SNR” alert will be given at the end of point and section measurement.
iv). Beam SNRs Not Similar. When the difference between any two beams is
greater than SNR Threshold,
• This may indicate interference from an underwater obstacle or a potential
problem with the probe,
• At the first alert, repeat the measurement (perhaps after moving probe
location),
• If the problem persists, run Beam Check to evaluate FlowTracker2
operation in more detail.
v). Large SNR Variation. When the standard deviation of the SNR of each beam
during the measurement is greater than a fixed threshold of 5 dB,
• This may indicate interference from an underwater obstacle, a highly
turbulent environment, or highly aerated water,
• At the first alert, repeat the measurement (perhaps after moving probe
location),
• If the problem persists, evaluate the measurement environment. In some
cases, large variations may be unavoidable and may not impact the quality
of velocity data.
vi). SNR Threshold Variation. When SNR for a new measurement differs from
the mean of three or more completed stations in this measurement by more
than SNR Threshold,
• This may indicate interference from an underwater obstacle or some other
dramatic change in stream conditions,
• At the first alert, repeat the measurement (perhaps after moving probe
location),
• If the problem persists, evaluate the measurement environment to look for
any potential cause for the change in SNR.
vii). SNR Threshold is checked with the completion of a point velocity
measurement and measurement section (see Timing of Warning Messages).
• User is notified of any stations that exceed the above criteria.
• If desired, user can delete suspect stations and repeat the measurements.
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