Sensor protects Cheddar Gorge

Sensors from a UK instrumentation specialist, Gems Sensors, are helping to protect the ecology of the River Yeo as it flows from the historic Cheddar Gorge in south-west England. Colin Lussenden, product manager at Gems explains the importance of accuracy and reliability in this environmentally sensitive region.

The Yeo is Britain’s biggest-flowing underground river, travelling up to 16km from the Mendip hills before rising to the surface in Cheddar Gorge. The Gorge is one of the four wonders of England.

First recorded in the will of Alfred the Great in 880AD, it has been a tourist attraction since the 19th century. The flow of the Yeo was ideal for watermills, and, in the 18th century, there were as many as 15 mills within a mile of the riverhead.

Watermill ponds near the surface source of the river are still used by Bristol Water today. A water-supply pipe is used to top up the nearby Axbridge Cheddar reservoir.

To preserve the ecology of the river and the fish and other wildlife it supports, the flow has to be at least 11,000m3/d, which limits the amount that can be diverted to the reservoir. In times of low water reserves and drought, as the Yeo is now experiencing, this becomes a very delicate balancing act.

The flow of the river is measured by the height of the water over a weir. The level needs to be a minimum of 125mm to protect the Yeo’s ecology; over this, water can safely be diverted to the reservoir, which supplies drinking water to the surrounding area.

To maximise the resource, the water level needs to be measured to an accuracy of 1mm. This is not easy in itself, but is rendered even more difficult by wave-formation on the surface, caused by wind, and by changing temperatures, which affect the specific gravity of the water and therefore the pressure measured by the sensor for any specific level..

In the past, Bristol Water has had problems finding a level sensor which produced the required accuracy and reliability. Now it is using the new Gems Sensors DCL 9300 series pressure sensor.

This unit features a built-in temperature sensor and internal signal processing via an onboard microprocessor. It is unique in being able to compensate automatically for changes in water temperature.

During the internal signal processing, a filter can be applied to average out transient changes caused by weather or waves. The DCL 9300 is the only sensor to offer this and has been approved after rigorous trials with the UK Environment Agency.

As well as high levels of accuracy, the sensor is very robust and reliable. It is triple-sealed to ensure no water ingress, and contains components to protect it from lightning surges.

The unit’s microprocessor continually monitors the temperature and compensates for changes in the resistance in the Wheatstone bridge four-active-arm pressure sensor. It corrects for any repeatable errors such as nonlinearity by learning the sensor’s characteristics when the unit is tested and calibrated in manufacture.

This, combined with correction for the changes in specific gravity, allows the sensor to measure level to within 1mm. Shaft encoders would normally have to be used to achieve this accuracy, but they have a higher cost to install and greater maintenance cost compared to the DCL 9300, which has a 4-20 mA output against level and is designed for quick and easy installation and very low maintenance.

The DCL 9300 and DCL 9500 sensor series were originally designed at the request of international water authorities, who recognise the importance of accurately monitoring the groundwater levels to predict how this precious resource is standing up to extraction. The DCL 9500 is now routinely used to log water levels from year to year; using its SDI -12 digital interface to feed the data to loggers for analysis.

Other applications such as reservoir or borehole level measurement have also become routine for the DCL 9300. Its 4-20 mA output can be configured before or after installation to the specific level to be measured.

It can reverse this measurement for keeping track of drawdown. The readings can also be configured to take into consideration zero offset caused by a head of water, for example, when measuring level in a reservoir which has an associated water tower.

The microprocessor stores the original calibration data to allow the user to check it against its original calibration and reconfigure it, if it is necessary to move it to another location with different settings.

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