What is the minimum velocity a velocimeter can measure?Follow
All Nortek Velocimeters use pulse-coherent processing techniques. An important benefit of this technique is that the system can be configured to measure velocities as small as 1 mm/s, or even smaller. This is quite useful in applications where the water motions are minimal, such as in settling tanks, bore holes and in some lakes.
Measuring velocities of a few mm/s or smaller has a few inherent problems that should be considered while planning the measurements:
- All Velocimeters are designed to output as much power as possible. This is done to ensure a strong echo (high SNR) and good quality data in a broad range of applications. However, the high output power actually generates a non-linear pressure wave that accelerates the water below the probe. If the horizontal water velocity is small (<10 mm/s), the pressure generates a (negative) vertical velocity of 1-15 mm/s in the sampling volume. In the 10 MHz NDV, this problem can be minimized by using the lowest velocity setting or requesting a special probe configuration file. In the Vector, the problem has been taken care of by having two different power setting and using the lower setting while measuring low flows. In the Vectrino, the problem has been solved the same way as in the Vector.
- Low flow means low turbulence levels, which may mean clear water. In general, all Velocimeters have problems measuring in clear water because of lack of scatterers (there is no echo). Experience shows that unpumped boreholes are difficult while settling tanks usually have enough scatterers in suspension.
- Thermal flows can superimposed on the net transport flow. In some case, the parameter of interest is not really the point velocity but the net transport rates. If the liquid is not thermally balanced (e.g, daytime heating) thermal convection cells can be generated. This velocity can be quite strong (several mm/s) and it can be difficult to detect the lower, net background transport velocity.
An alternative for low flow measurements are high-resolution current profilers. This instrument does not provide true point measurements but it is quite powerful for low flow applications in the field (for example, quiet lakes).
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