Easy to use – and ever so accurate

Launched 10 years ago, Flowline's Flo-Dar system has proved itself a reliable - and rugged - general purpose flow meter. And, as Mark Davis reports, it's still evolving.

ALTHOUGH OPEN channel flow meters have existed for hundreds of years none were able to offer the promise of zero maintenance and long-term reliability, combined with ease of installation and use. Since its introduction in 1999, the Flo-Dar instrument from Flowline has remained the only non-contact area velocity flow meter available, and has now evolved into several different systems that cover a wide range of applications and industries.

Initially developed in the US and launched by manufacturer Marsh-McBirney, the system was a response to the problems associated with “conventional” wetted type flow sensors, which, until that time, were the principal method used in area / velocity metering. Wetted sensors of all technologies; ultrasonic, electromagnetic, all require on-going maintenance.

When dealing with sewage and effluent flows, channels will silt up, ragging and fat will accumulate on anything in contact with the flow medium. The issue now is that maintenance has become a lower priority. Removing the sensor from the flow eliminates the problem.

Conventional non-contact open channel flow meters are available, but they only measure level and rely on a primary device such as a flume or calculate flow based on level measurements and inferred velocity.


Marsh McBirney’s solution was to develop a radar/ultrasonic-based system, measuring both the speed (radar) and the level (ultrasonic) of the flow that enables it to be installed in existing channels and pipes, without the need for a primary device. This greatly simplifies installation and usually eliminates civil costs, thereby significantly reducing capital outlay.

The system comprises a single field mounted sensor with the sensing head mounted above the flow. Ideally, it is mounted in a location where possible surcharged or peak flows would not reach it, although surcharged flows can also be measured.

Flow velocity is measured using a Doppler radar and the radar itself uses low power and requires no user licence. Level is measured using an ultrasonic sensor and the data from the sensor is transmitted in digital format to the control unit.

The remote mounted control unit combines the sensor data with the pre-programmed site data and calculates a volumetric flow reading, which is displayed, logged and/or transmitted.

With a ten-year history and more than 4,000 installed systems, it is natural that Flo-Dar has evolved over time. The first models were only available as fixed AC-powered units with a small local display and no data logging capability. A portable battery-powered version was launched in 2001, with the first systems being sold to Scottish Water for survey use on small remote treatment works.

Most Flo-Dar systems have been installed in existing chambers and channels, often as part of a plant upgrade but also to allow users to conform to environmental regulations. This retrofitting ability has seen the system used in challenging applications including:

  • Dewatering discharge from a quarry, installed into a 900mm dia pipe placed into a small river channel, flow has a PH of 1
  • Hot brine discharge from salt processing plant, atmosphere attacks all metals including stainless steel
  • Effluent discharge from chemical plant, as well as being classified as ATEX Zone 1 the atmosphere in the chambers is toxic with entry into the area prohibited

The user base for the system is split 50:50 between water companies and industrial dischargers. The water companies mainly use the meter for control purposes, whereas consent monitoring is the typical application for industrial users.

It is in the area of sewer flow measurement and control that the benefit of maintenance-free flow meters can really be appreciated. The main spur to develop the Flo-Dar was to measure raw sewage flows and it is for this application that the system has been most widely appreciated.

Until the arrival of radar-based technology the only area velocity meters available for sewer flow monitoring and surveying used a wetted “mouse”, usually placed in the invert of the pipe. The sensors used ultrasonic velocity measurement, either Doppler or correlation type, with both types using ultrasonic beams that reflected from particles suspended in the flow.

When first launched, these systems benefited survey and water companies, sewer design consultants and regulatory bodies. Although still using wetted sensors, the biggest advances seen with these systems has been with the electronics, most notably signal processing, which has expanded their usefulness and allowed them to be used in previously unsuitable applications.

Some systems use multiple sensors that can help to compensate for the effects of ragging, silting and fat build up and be used where sewers surcharge. The downside of using wetted sensors for survey work is maintenance. Typically, a survey team will have to visit a site every two weeks to download data and check the sensor has not become “ragged up”. If a sensor has become fouled then velocity data will be lost, the flow data will then have to be “reconstructed” from just the level readings combined with periods where the velocity data was available.

Non-contact sensors never rag up, which means the flow data does not require any processing prior to releasing to a customer. Customers can directly access their own data, which brings us to the next benefit of maintenance free meters, remote monitoring.


Data transfer to the internet via the GSM/GPRS network is now an off-the-shelf solution. Because of the need to “clean up” data, this technology was not easily applied to sewer monitors. With Flo-Dar this has now become a reality.

The new ATEX-certified internet enabled systems log data and transfer it at periods to suit requirements. This data is displayed on a web page and can be downloaded in spreadsheet format. The website itself can show data from more than 30 different flow meters on a single page. This ability to measure real-time data benefits users who need to control sewer flows automatically, and many systems have been installed and work in a closed loop control function to automatically regulate flows in sewers and CSOs. With new housing schemes putting additional pressure on existing networks, automatic control of flows will become more of a necessity, as will automatic measurement and billing of discharges into sewer networks.

The primary benefit of non-contact technology is long-term reliability. At a sewage treatment works in Fazakerley, Liverpool, the Flo-Dar meter has required no maintenance during ten years of service.

The Flo-Dar system is a rugged, general-purpose flow meter for use in most open channels such as sanitary sewers, stormwater sewers and other man-made channels including aqueducts and also ‘natural’ small streams.

It measures open channel velocity and depth by non-contact means, virtually eliminating the need to periodically clean the sensors – as required by all submerged type sensors.

Data recovery from Flo-Dar deployments ranges between 98 to 100% even under site conditions that render most submerged systems inoperable. The accuracy of the system is more than adequate for the most demanding of metering applications, including open channel billing applications.

Mark Davis is director at Flowline. T: 020 8207 6565

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