HWM's hi-tech solution makes perfect SonicSens

In remote areas, monitoring water rates can be a challenge. But, as Scottish Water has discovered through its Water Framework Directive responsibilities, the selection of versatile ultrasonic level sensors has proved a canny choice.

Scottish Water is using extensively Halma Water Management's (HWM's) SonicSens ultrasonic level sensor, to monitor flow rates in a wide variety of situations - siphon chambers, Combined Sewer Overflows (CSOs), overflow and storm tanks, weirs, reservoirs and compensation channels. It is a non-contact, highly accurate level sensor and flow meter.

The technology behind ultrasonic level sensors is founded on a simple principle: an inaudibly high-frequency sound wave is emitted, directed towards the surface to be measured. This surface reflects the wave back, and the time taken for the reflected wave to return is in direct proportion to the intervening distance.

So, with a known total distance between the sensor and the bottom of the vessel or channel, the depth of the water is found by simple subtraction. Flow rates can then be calculated easily if the volumetric dimensions of the channel are known, faster flow raising the fluid level accordingly.

Water directive

Gerard O'Hara, project engineer for Scottish Water, says: "The design team here are very happy with the performance of the instruments, and the SonicSens system has been used many times as a part of our award-winning Water Framework Directive contract."

The WFD was introduced in 2000, and made into Scots Law in 2003; its purpose is to maintain, protect and improve the quality and management of all Scotland's water resources.

According HWM, a major benefit of ultrasonic measurement is that it is non-contact, and so non-contaminating and low-maintenance. The SonicSens builds on this with a package that includes millimetre resolution, a long (five-year) battery life and the facility for versatile remote telemetry options such as GSM, GPRS, radio or satellite communications.

The sensor itself connects to a separate datalogger via a choice of standard, ATEX-rated, wired and wireless combinations, with or without a Barrier box. These features make the product well suited to 'fit and forget' operations in hard to access locations - once it is installed, it will continue to function as programmed without local maintenance or interaction.

Because of this, Scottish Water is using the product for a wide range of applications across its wide area of responsibility, which includes places beyond the reach of telephone lines, cellular reception or even normal access by anything less than a dedicated off-roader.

A recent installation at a compensation weir in a secluded valley presented potential problems with signal reception. The sensor itself is suspended over the weir - with a known width for flow calculation - and connected to a short-range TRX20 relay unit safely raised above.

The datalogger was installed high up on top of the dam, where the GSM signal reception is strong enough for the long-range data telemetry.

CSOs are common installation sites for the Scottish Water's SonicSens systems, as they must be monitored for spill events where heavy rainfall will raise the water level above the storm screen. When this happens, the SonicSens will alert Scottish Water that a storm condition has occurred, and that the screen will need cleaning.

All ultrasonic level sensors have a 'blanking' distance - this is a minimum distance between the sensor unit and the measuring surface. This is required for the device to receive a clean reflected sound wave, one that will be unaffected by the vibrations caused by emitting the sound in the first place.

With another CSO application, there was not enough room to site the SonicSens in the same chamber as the overflow screen. A new chamber was built over the pipe itself, to measure the water levels from there - but there was still not much space, and the 300mm blanking distance would not be cleared if the water level were raised by a storm condition.

In a creative workaround, the sensor unit was installed on its side, along with a reflector plate mounted at a 45º angle, situated far enough away to account for the dead-band. Although an uncommon situation, it is a simple and effective solution that further highlights the versatility of such measurement technology. n

www.hwm-water.com

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