General

The EasyQ is mounted in a cyclindrical housing and has a length of 60 cm, a diameter of 7,5 cm and is 0,7 kg buoyant in water. The housing makes it very easy to mount the unit at the side of the river by means of a clamp on a pole. The front of the unit houses the four transducers, X/Y tilt, temperature and pressure sensors. Furthermore the system has a standard recording capacity of 8 MB. The unit is powered by the internal alkaline battery pack or externally. With the software functions as deployment planning, range and stage checks, data retrieval, ASCII conversion, on line data collection and graphical display, the EasyQ is complete and ready to go.

Transducers

As already mentioned the EasyQ is equipped with four transducers as follows:

* Two transducers for the measurement of horizontal current velocity mounted under an angle of 25° (see velocity).

* One transducer looking upwards for the measurement of water-depth to the surface (see stage).

* Onze transducer for river bed monitoring mounted under an angle of 45° down (see diagnostics).

Figure 1. EasyQ beam orientations and numbering.

Transducer 1 and 2: Velocity

The EasyQ measures velocity in three cells with the two horizontal Doppler beams looking sideways into the flow. It is possible to adjust the position of the measurement cells to place them close to the sensor or our further in the river. It averages velocity over an adjustable duration, ranging from a few seconds to an hour. Its carefully implemented Doppler processing gives you better than 1% measurement uncertainty (see Figure 2).

Figure 2. EasyQ measurement cell location. With a cell size of 75 cm and 35 cm blanking, the mid point of the first cell is situated 35 + 75 = 110 cm away from the sensor head. The second cell is 35 + 2 x 75 = 185 cm, and so forth. Blanking an cell size can be adjusted depending on the width of the river and the ideal location for the measurement cell.

Transducer 3: Diagnostics

Transducer number 3 looks down at 45 degrees from the vertical and detects changes in the bottom associated with scour and siltation. The EasyQ includes diagnostic software that enable you to monitor changes in the river bed. The diagnostic measurements consist of a series of Range Check profiles on all four of the EasyQ's beams.

Figure 3. Examples of the diagnostic mode for the downward looking Beam 3. The upper graph shows color coded signal strength in range cells of 0.20 m for more than 200 record in time. At record 80 an event occurs which quite quickly leads to a rise of bed level. The lower graph shows echo amplitude in 0.2 m range cells before the even (blue) and after the event (green).

Transducer 4: Stage

Transducer number 4 is used for the stage measurement. Stage and velocity measurements are different and they require different processing. This section describes how an EasyQ's stage measurement works.

An EasyQ measures stage by measuring the travel time between the transducer and the surface. The water surface is typically smoother than the bottom and it often looks like a mirror. A mirror-like reflector means that the echo returning to the EasyQ will look like the signal it transmits. The difficulty of making water level measurements acoustically is that the sensor can be confused bij echoes from debris in the water or from out-of-range echoes. Our-of-range echoes often arise from multiple bouncing between the EasyQ and the surface. An echo returning from the surface bounces back to the surface from the flat, mirro-like EasyQ transducer. Because of variations in sound propagation and electronics response, it is possible for second and even third bounces to appear stronger than the first bounce.

The EasyQ uses several strategies to identify the direct echo from the surface. These include the following:

* A matched filter process enhances the echo from the surface relative to echoes from debris. A matched filter enhances echoes that match the transit pulse, relative to echoes that have changed. The mirror-like echo from the surface looks more like the transmit pulse than does an echo form debris in the water, so the matched filter responds more strongly to the surface echo.

* The EasyQ uses a pressure measurement to place a window around the surface, rejecting echoes that fall outside the window.

Figure 4 illustrates both of the above. Note how the signal Quality (the output of the matched filter) produces a sharper response than the original signal strength. Peak a rises above the quality treshold and it lies within the pressure window, so it qualifies as a good surface measurement. Peak B is disqualified on both counts. If both peaks qualify, the EasyQ picks the peak closest to the EasyQ.

Figure 4. Schematic of Signal Strength and Quality vs. Elevation above the EasyQ. Quality is the output from a matched filter.

The EasyQ makes many pings to find the surface. It averages 23 pings each second before it evaluates the echo against the quality threshold and pressure window. If the 1-s average echo passes, it is counted as a good echo. The EasyQ computes the distance to the surface using an algoritm from Visbeck and Fischer (1989). It repeats this process for very second during the measurement interval, then computes the average of all of the good echoes. The result is a measurement of stage that averages out water level fluctuations from waves and other surface disturbances, making stilling wells unnecessary.

Custom transducer heads

We can produce custom head geometries especially for your requirements and we can do this cost-effectively (data sheet with various head configurations available).