The CSA Group*, the provider of the Environment Agency’s MCERTS scheme, recently released Bulletin 27, an update on how area velocity (AV) flow measurement systems should be installed and used.

The use of AV technology can cause confusion – particularly as new rules on flow to full treatment (FFT) are due to come into force. Our guide aims to help you make a more informed decision when choosing your wastewater system. 

What does AV mean?

Area velocity (AV) flow meters are used for measuring flow without a flume or weir. They operate on the “area velocity principle” – a probe is inserted into the water and continuously transmits ultrasonic pulses.  when particles pass through it registers the shift in area velocity and uses this to calculate the flow.

AV meters are popular because they are quick and easy to install – simply fitting them to a pipe or open-channel. They also tend to be more affordable than alternatives, such as electromagnetic flow meters.

However, a downside to AV meters is that it is difficult during MCERTS inspections to verify the accuracy of the results they are producing.   This means that although they may seem like a cost-effective solutions, in reality they can often end up costing more in the long run.   As MCERTS inspectors we would only recommend AV meters over other meters in certain situations, such as a temporary option or where other solutions aren’t viable.

Soon, the Environment Agency will require wastewater treatment works to comply with new regulations in the way they measure flow to full treatment (FFT).   This may mean that many sites will opt for AV meters as a quick and affordable solution to meet requirements, but it is important that you make yourself aware of potential limitations before committing.   In all instances you should seek the advice of MCERTS-qualified advisors.

What is Bulletin 27?

The CSA Group occasionally shares updates and additional guidance relating to the existing Environment Agency MCERTS guidelines, known as bulletins.

Bulletin 27 – “Installation and use of area-velocity flowmeters in free surface flows.” is the most recent update and provides guidance on how area velocity flow meters should be used in order to pass MCERTS inspections.

The key things it covers are:

  • Location and installation
  • Maintenance
  • Verification

What does Bulletin 27 mean for AV?

Bulletin 27 clarifies some areas of concern raised on how the accuracy of area velocity flow meters can be verified.

One of the key things it confirms is that there must be a “defined measurement section”. This is described as:

“a straight, stable length of channel or pipe with a constant cross section and with a length which is sufficient to create a fully developed flow profile at all expected flowrates.

Note: This will normally require at least 20 channel widths/pipe diameters upstream and 5 channel widths/pipe diameters downstream of the flowmeter.”

The additional note is important – although it acts as a recommendation it gives important guidance as to how we can accurately verify the results the area velocity flow meter provides. Bulletin 27 also explains that if these conditions can’t be met the site can still be MCERTS certified, so long as the inspector “is satisfied that that the flow measurement uncertainty conforms to the MCERTS requirements at all expected flow rates”. 

Another important issue addressed is maintenance. Area velocity flow meters measure the area of the channel – and that means that any build-up of material within the channel will skew the results. It states that “the sensors and the measurement section shall be kept free from sediment and other fouling material.” In other words, the channel needs to be kept cleaned and well-maintained in order to confirm accuracy of results.

Our view

Area velocity technology should be carefully considered as a long term solution, weighing up issues of potential accuracy and verification before focusing on cost and speed of installation.  Despite the relatively low initial outlay for an AV meter, inaccurate results and the issues presented by them over a long period could ultimately cost businesses and water companies more money. Electromagnetic flow meters or flumes can remove the typical issues found with AV meters and their data accuracy and, even though more costly for initial installation, can prove more cost-effective in the longer term.

It is encouraging to see further clarification on the use of AV meters from CSA Group and we believe that if these guidelines are followed – particularly the recommended installation location – then it will go some way in improving the accuracy of this technology.

Talk to the experts

To discuss your requirements, get in touch with our friendly team on 0191 5131313, or email hello@siris.co.uk.

*Confused about the role of the CSA Group? Read our guide to the key wastewater industry players.