YSI ADV6600 - Manual (Page 174)

Appendix D. Frequently Asked Questions
ADV6600
Y S I Environmental
Page 164
D-2. What does the system measure?
What is the difference between velocity and speed?
Speed is the distance traveled by an object (such as a suspended particulate in water moving down a
river) divided by the time it took to cover that distance. Velocity is the measurement of how fast this
same object is going, but also includes the direction the particulate is traveling. In other words, there
is a directional component for velocity but not for speed.
As mentioned above, the ADV6600 measures the velocity of particles in the water. However, the
user may also elect to display speed and direction separately if these parameters are preferred.
What is the difference between velocity and flow?
For water, velocity (as described above) is the speed and direction of travel Flow is the volume or
quantity of water divided by the time it took for the volume to pass a specific point. Flow is
sometimes referred to as flux and can be computed as the mean velocity multiplied by the area the
water is moving through.
Therefore, water velocity is measured in meters per second (m/s; a speed unit). The water flow is
measured in cubic meters per second (m
3 /s; a volume and speed unit).
How does the ADV sensor measure the velocity of water?
Doppler current meters, such as the ADV sensor in the ADV6600, do not measure movement of
water, but actually the movement of particles or “scatterers” (such as suspended sediment,
biological matter, or bubbles) in the water. It is assumed that the movement of the particles in the
water is representative of the movement of the water itself, and this assumption turns out to be valid
in virtually all cases.
Acoustic pulses transmitted from the ADV sensor impact these particles and, using a physical
principle known as the Doppler Shift, reflections of the Doppler radiation from the particles in the
water column are used to determine the water velocity. The different orientation of each of the
receivers (one on each arm of the ADV sensor) allows the system to calculate the velocity of the
water in 3 dimensions.
Critical points on water velocity measurement: