Xylem SonTek FlowTracker2, User Manual

Page: 234 / 308

SonTek – a Xylem brand
FlowTracker2 User’s Manual (January 2019) 233
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We must assume that the movement of the particles in the water is
representative of the movement of the water itself,
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If there are no particles (other than the water molecules themselves) present in
the water, Doppler current meters will not work. Natural streams almost always
have something else other than just “water” (even if just tiny air bubbles), and the
technology is such that even a small amount of particles in the water is usually
enough for good measurements.
Visual inspection is not an acceptable method for determining the amount of particles in
the water and no simple relationship exists with turbidity.
•
We cannot say that for a given turbidity level your water is too clear for a
Doppler-based measurement.
•
The amount of particles in the water is not the only factor affecting signal
amplitudes. Particle size, shape, distribution, and type also control these values,
and these conditions vary from site to site.
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Therefore, one stream may meet these conditions, but another with the same
turbidity may not.
If the site conditions seems unsuitable for FlowTracker2 measurement due to lack of
particles in the water it is recommended to perform an Automated Beam Check. If the
results are within the instrument specifications and quality control parameters user can
start with velocity measurements. The FlowTracker2 can return reliable measurements
down to about 3 dB (in SNR), although we recommend 10 dB and above for best
operating conditions.
13.11. Flow Interference
The FlowTracker measures velocity in a small sampling volume nominally located 10
cm (4 in) from the tip of the probe (see
Sampling Volume Definition ). The placement of
the sampling volume relative to other structures in the water must be considered when
measuring velocity to avoid flow interference.
Structures 13.11.1
a) The FlowTracker probe should be located away from any underwater structures
or obstacles, particularly those upstream.
b) Consider the size and location of nearby structures and obstacles, and the
probable magnitude and direction of flow, when choosing a measurement
location.
Probe orientation relative to flow 13.11.2
a) The FlowTracker should be oriented so the axis of the transmit transducer is
roughly perpendicular to the expected direction of flow (see Figure 13:6 ).
b) For side-looking probes (2D and 2D/3D), the probe should be oriented looking
across the expected direction of flow (so the X-axis aligns with the expected
flow).