Framework conditions for a reliable measurement
The installation location can have an impact on the quality of the measurement results. Gradient drops, vertical drops, baffles, bends, or junctions can cause turbulent flow and distort the velocity profile. For optimum accuracy, install the VEL‑R‑5
- at a location with constant flow
- in the centre of a channel that runs straight for a long distance
- parallel to the flow
- against the direction of flow (upstream)
- at an angle of 45° to the water surface
- with the "top marking" facing upwards
For the surface velocity to be determined correctly, waves with a minimum height of 2 mm must be present. If, under normal conditions, there are no surface waves of sufficient height in the sensor's detection range, either an alternative installation location should be selected or waves should be generated deliberately. Ensure that the measures taken do not distort the velocity profile. The detection range of the sensor is elliptical in shape. The dimensions of this detection range are determined by the distance to the water surface and the angle of attack of the sensor and are calculated as follows:
Rm = H / sin (α) W = 2 x H x tan (4°/2) L = 2 x Rm x tan (4°/2) |
The following table shows the dimensions of the sensor's detection range at different mounting heights. The calculations are based on the recommended angle of attack of 45°.
| D [mm] | Rm [mm] | W [mm] | L [mm] |
|---|---|---|---|
| 1000 | 1414,2 | 69,8 | 98,8 |
| 1500 | 2121,3 | 104,8 | 148,2 |
| 2000 | 2828,4 | 139,7 | 197,5 |
| 2500 | 3535,5 | 174,6 | 246,9 |
| 3000 | 4242,6 | 209,5 | 296,3 |
| 3500 | 4949,7 | 244,4 | 345,7 |
| 4000 | 5656,9 | 279,4 | 395,1 |
| 4500 | 6364 | 314,3 | 444,5 |
| 5000 | 7071,1 | 349,2 | 493,9 |
When chosing a suitable installation location, the following conditions must also be avoided:
Vibrations: Select a stable, vibration-free mounting site. Vibrations can be caused by traffic or wind, for example.
Turbulent currents: Select a mounting site at a suitable distance from drop sections, vertical drops, baffles, bends or junctions, as these affect the velocity profile (see Guidelines for choosing the installation location).
Accumulation of foam: Foam can significantly impair measurements due to its uneven, air-filled surface, as the radar signal is both scattered and attenuated.
Flooding: Always install the VEL‑R‑5 above the maximum expected water level. The block distance (0...30 cm ) should also be taken into account. This indicates the minimum required distance to the water surface along the extended sensor axis. The minimum permissible vertical distance to the water surface depends on the angle of attack of the sensor and can be calculated as follows:
hmin = Blocking distance x sin (angle of attack)
The VEL‑R‑5 can withstand short-term flooding, but can be damaged by foreign matter in the wastewater. When installed correctly, the Ceiling mount for pipe (400417) offers a certain degree of protection against foreign substances and can be used in channels that are temporarily filled to capacity. However, as long as the VEL‑R‑5 is flooded or the water level is within the blocking distance (0...30 cm ), the measurement will not provide valid values.
Wind: Select an installation location where the water surface cannot be affected by wind. Crosswinds, for example, can cause backflow or side currents.
Rain: Rain can significantly affect measurements, as raindrops reflect and scatter the radar signal. The reflections from large drops are difficult to distinguish from those on the water surface. In addition, the kinetic energy of the falling drops creates concentric waves that further disturb the water surface. At speeds below 1.5 m/s, these can significantly distort the measurement of surface velocity. At higher speeds, the influence of rain is negligible, as the surface waves generated by the flow superimpose the disturbances caused by rain.
Standing waves: Do not install the VEL‑R‑5 in duct sections where standing waves (also known as stationary waves) can occur. Standing waves are caused by the superposition of two opposing waves, triggered, for example, by reflections from obstacles, sudden cross-sectional changes, backwater or countercurrents. The fixed crests and troughs of stationary waves cause significant interference with surface velocity measurements.
Obstructive objects: There must not be any obstacles between the VEL‑R‑5 and the water surface. This applies to the entire space between the sensor and the water surface covered by the radar cone (see figure Sensor detection range).
Sedimentation: Install the VEL‑R‑5 in channel sections where the bottom is as smooth as possible and free of sediments, as these can affect the velocity profile and hydraulic cross-section.
