Velocity Spike Filter - Xylem SonTek FlowTracker2 User Manual

• User is prompted to consider moving the probe to avoid this interference.
• If the probe is moved, repeat the boundary test prior to data collection.
• If repeated Boundary QC tests do not give improved results, user can
ii). The Boundary Interference is checked before a point velocity measurement
is made and if the condition is not "Best", then the Boundary Interference
warning is given (see
e) This criterion can be adjusted or disabled under
6.1.4

Velocity Spike Filter

The Velocity Spike Filter quality control is applied to each point velocity measurements
in both General mode and Discharge mode. Spikes in velocity data occur with any
acoustic Doppler velocity sensor such as the FlowTracker. Spikes may have a variety of
causes – large particles, air bubbles, or acoustic anomalies.
a) The FlowTracker2 spike filter is a variation on a method called "Tukey's Outlier.
Filter
Just like in Tukey's method, the data are sorted in ascending order, then divided
Method
into 4 quartiles.
i). A histogram of each velocity component is calculated.
ii). The FlowTracker2 determines the 1st Quartile (Q1) velocity value where 25%
of the samples are less than this value. The 3rd Quartile (Q3) is the velocity
value where 75% of the samples are smaller than this value. The Interquartile
Range (IQR) is defined as the spread between the 1st Quartile and the 3rd
Quartile (IQR = Q3-Q1).
iii). If the IQR is less than 0.015 m/s, IQR is set to 0.015 m/s.
iv). Any value less than (Q1–2*IQR) or greater than (Q3+2*IQR) is considered a
spike and is not used for mean velocity calculations.
b) Spikes are filtered based on all velocity components (Vx, Vy, and Vz). If any
velocity component falls outside the above limits, that sample is not used for the
mean velocity, standard deviation or velocity standard error calculation.
c) No other filtering or editing is done to FlowTracker2 velocity data.
Spike d) The number of spikes is displayed and recorded at the end of each
Data measurement.
Display
e) Raw, one-second velocity data are recorded with each measurement station. This
allows you to evaluate unedited velocity data for each station.
f) The Velocity Spike Filter is applied to each point velocity measurements in both
Spike
General mode and Discharge mode, but the filter is not attempted unless there
Data
are at least 10 samples in a measurement. The Spike Threshold (default 10%)
Evaluate
is used as follows to determine the number of velocity spikes present in the data.
FlowTracker2 User's Manual (January 2019)
proceed with the measurement, but should carefully evaluate velocity data.
Timing of Warning
SonTek – a Xylem brand
Messages).
Quality Control
90
Settings.