Dynamic Pressure (Pressure Sensor) - Xylem SonTek FlowTracker2 User Manual

13.12. Dynamic Pressure
Dynamic pressure is the kinetic energy per unit volume of fluid and affects the pressure
sensor measurements performed by the FlowTracker2 ADV internal pressure sensor in
flowing water. The extent of the dynamic pressure affecting the pressure sensor
measurements is directly related to the water velocity. The correction for dynamic
pressure on FlowTracker2 pressure sensor measurements is described in this section.
In fluid dynamics, the Total Pressure (P0) is defined as the sum of the Static Pressure
(P) and the Dynamic Pressure (Pd),
�� = �� + ��
Equation 13:1 – Total Pressure
0 ��
The internal pressure sensor of the FlowTracker2 provides an accurate measurement of
the Total Pressure (P
(P) to be known. For still or slow moving water the Dynamic Pressure (Pd) is negligible
and thus the Static Pressure (P) and the Total Pressure (P
Total Pressure (P ) as measured by the pressure sensor instead of the Static Pressure
0
(P) in these cases provides a good conversion for all practical purposes.
However, for faster flowing water, ignoring the Dynamic Pressures (Pd) is not an option
and would result in a significant error in the pressure-to-depth conversion. Correcting
for Dynamic Pressure is essential to ensure a proper depth computation.
Dynamic Pressure (Pd) is defined as,
�� = 0.5 × ������������������������ × ��������������������
Equation 13:2 – Dynamic Pressure
��
All parameters required in
• WaterDensity can be approximated or in the case of the FlowTracker2 is precisely
computed from Salinity (user entry), Latitude and Altitude (from the integrated
GPS),
• WaterSpeed is provided by the ADV (the main function of the FlowTracker2 is to
measure water velocity). To produce a precise WaterSpeed, the AdvWaterSpeed
(measured water speed) is corrected by a constant "head drag coefficient" (a)
which describes the flow disturbance around the pressure sensor introduced by
the ADV head and is different for different head shapes (e.g. 2D or 2D/3D).
�������������������� = �������������������������� × ��
Equation 13:3 – Corrected AdvWaterSpeed
FlowTracker2 User's Manual (January 2019)
(Pressure Sensor)
). Calculating depth from pressure requires the Static Pressure
0
2
are available to the FlowTracker2
Equation 13:2
SonTek – a Xylem brand
) are very close. Using the
0
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