The OTT ADC (acoustic digital current meter)

The OTT ADC (acoustic digital current meter) is an acoustic flow measuring instrument for the measurement of point velocities in open waters (eg, rivers, small streams, irrigation canals and so on). It calculates discharge measurements according to EN ISO 748 and USGS standards. It combines the plumb-line method with innovative technologies for the measurement of flow velocity and water depth. Approved acoustic measuring principles combined with a high-end signal processor provide precise and reliable data.

The unit consists of two components: the sensor and the display unit. They are connected with a sensor connection cable and attached to the wading rod with flexible adapters. The sensor is the basic element of the instrument. It contains all important components which are needed for the measurement of flow velocity, water depth and temperature.

With OTT ADC it is possible to measure flow velocities of -20 cm/s to 2.40 m/s with an accuracy of 1% of the measured value ±2.5 mm/s. The independent Dutch research institute WL <0x007C> Delft Hydraulics has confirmed and certified the compliance with these high-accuracy requirements over the whole measurement range by testing according to ISO 3455 [4] in their own rating tank.

Velocity measurement takes place in a sampling volume in front of the probe at a distance of 10 cm. Two ultrasonic bursts are emitted simultaneously against the direction of flow. They will be reflected by suspended particles in the water body (eg, suspended materials, air bubbles, plankton and so on). Depending on particle size and shape, an echo image returns and is processed in the system’s DSP (digital signal processor) as an echo image. After a short interval the instrument emits a second burst and a second echo image will be digitised. Following the measurement phase, the DSP checks both received reflection images for similarities using a cross correlation method. The result of this mathematical calculation allows a conclusion about the time shift between the two images which is then used to calculate the water velocity. Various internal quality inspections ensure reliability of data at any time. Furthermore, a built-in pressure sensor provides automatic depth readings for the determination of both, individual vertical depths and sensor immersion within a measuring range of 0 to 5 m.

All essential measured parameters are presented on the lightweight, weatherproof, ergonomically designed display unit. All relevant functions can be displayed during the measurement (eg, graphical visualisation of sensor position and movement, position of the actual measurement vertical, quality of data and so on).

The device is suitable for use with standard wading rods and relocating devices. It is available with three different cable lengths (2.5, 6 and 10 m), thus enabling measurement in free profiles and from bridges. The compact design enables the measurement close to the water surface (2 cm below water level) as well as measurements close to the ground (2 cm above river bed). Regular recalibration of the sensor is not required.