Scottish Water Selects Sonicsens Remote Monitor

Scottish Water is using the Halma Water Management (HWM) Sonicsens ultrasonic level sensor to monitor flow rates in a variety of situations. These include siphon chambers, combined sewer overflows (CSOs), overflow and storm tanks, weirs, reservoirs and compensation channels. Sonicsens is a non-contact, highly accurate level sensor and flowmeter. Ultrasonic level sensors are based on a simple principle: an inaudible high-frequency sound wave is emitted and 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. With a known total distance between the sensor and the bottom of the vessel or channel, subtraction reveals the depth of the water. Flow rates can then be calculated easily if the volumetric dimensions of the channel are known, faster flow raising the fluid level accordingly. Ultrasonic measurement is non-contact and therefore non-contaminating and low-maintenance.

The Sonicsens builds on this with a package that includes millimetre resolution, five-year battery life and the facility for versatile remote telemetry options such as GSM, GPRS, radio or satellite communications. The sensor 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 suited to 'fit and forget' operations in hard-to-access locations - once it is installed, it will continue to function as programmed without the need for local maintenance or interaction.

Because of this, Scottish Water is using the product for a 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. An installation at a compensation weir in a secluded valley presented potential problems with signal reception. The sensor was suspended over the weir - with a known width for flow calculation - and connected to a short-range TRX20 relay unit (IS approved) safely raised above.

The datalogger was installed 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.

In 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 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 was 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 45-degree angle, situated far enough away to account for the dead-band.

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