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11.4. Seeding
11.4.1
Scattering Material
If FlowTracker2 velocity data appears "noisy", the most common cause is a lack of
scattering material in the water.
a) Acoustic Doppler Velocimeters require scattering material for velocity
measurements (see
b) A lack of scattering material can increase noise in velocity data. If insufficient
scattering material is present, the FlowTracker2 will not be able to accurately
measure velocity,
c) In most field applications, there is sufficient scattering material naturally present,
d) Large, quiet laboratory tanks often have insufficient natural scattering material,
e) To evaluate seeding requirements, use the raw data display from the handheld
interface,
i). For ideal operating conditions, the SNR should be above 10 dB,
ii). The FlowTracker2 can operate reliably with SNR as low as 3-4 dB.
f) A lack of scattering material can be remedied by adding seeding.
11.4.2
Field Applications
For field applications, seeding can be introduced by stirring the bottom (e.g., walking
across the river upstream of the measurement location). In some situations, seeding
material will need to be introduced. An ideal seeding material should have the following
qualities.
• Neutrally buoyant (to remain in suspension for a long period),
• Mean particle diameter of 10-20 µm (for peak sensitivity of the acoustic signal),
• Inexpensive, readily available, with no adverse effects on the operating
environment.
The best seeding material we have found (from an acoustics point of view) are hollow
glass spheres with a mean diameter about 10 µm and a mean density close to that of
water.
• Small quantities of this material are available from SonTek,
• Larger quantities can be purchased from the manufacturer at the address below,
Potters Industries
Valley Forge PA USA
Phone: +1-610-651-4700
Internet: www.pottersbeads.com
Part name: Potters Sphericel
FlowTracker2 User's Manual (January 2019)
Principle of
Operations),
190
SonTek – a Xylem brand
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