What is a weir tank? In this guide we dive into the world of weir tanks – what they are, how they work and why they can be a good flexible flow measurement option. 

what is a weir tank

What is a weir tank?

Weir tanks are a cost-effective and easy-to-install flow measurement solution suitable for both above and below ground applications. Combined with an open channel flow meter, they provide a high accuracy flow system that can measure a wide range of flows.

How do weir tanks work?

Weir tanks work by placing a fixed obstruction (i.e. a thin plate weir) within the flow of effluent.

The weir is located within a chamber of sufficient capacity to enable calm and tranquil conditions within the measurement area. The length width and depth of the chamber are governed by BS/ISO standards and depend upon the maximum flow rate that needs to be measured.

The weir is mounted onto the outlet wall of the chamber and the flow spills over the weir. By measuring the upstream depth of liquid discharging over the weir the flow rate can be calculated.

Why use a weir tank?

Weir tanks can be useful in applications where flow is above ground and the flow of water is piped above ground. They provide a flexible option that can accurately measure very low flows being discharged from smaller sits.

Weir tank systems are not suitable for use where there is a high solids content in the effluent as there is settlement within the measurement chamber requiring maintenance/cleaning, deposits on the weir plate profile can also impact measurement and flow.

Case study: Food manufacturer 

This is a great example of a weir tank providing a cost-effective flow measurement solution.

The site’s existing wastewater monitoring system wasn’t fit for purpose – it was incorrectly designed for the flow rates being discharged and was costing the manufacturer a great deal of money due to the system reading a flow rate greater than actual.

We assessed the site and following discussions with the client on the various options available we recommended an above ground weir system of sufficient capacity to accurately measure the flow rates being discharged. This resulted larger weir chamber giving calm and tranquil flow conditions enabling high accuracy readings.

The customer recovered the cost of the installation in reduced charges in less than a year.

Read more about this project.

SIRIS weir tanks

Our standard weir flow measurement systems cover a wide range of flow rates and can be designed to suit most discharge applications.

SIRIS weir tanks are manufactured in rigid PVC and reinforced for maximum rigidity and durability, including two solid external support bands to prevent bowing and distortion, with the inlet and outlet pipes able to be fabricated to suit customer requirements.

The system includes provision for a data logging flow meter housed in a weather-proof kiosk, a critical laser-machined thin plate weir, an upstream baffle and a maintenance drain-down valve. In addition, the outlet weir plate has additional supports so it remains free from any bowing or skew.

Our weir tanks are pre-calibrated by our experts at our specialist facility, guaranteeing compliance with both BS/ISO 1438 and MCERTS. On-site installation is as simple as connecting the inlet and outlet pipes, powering up and switching on – making it an ideal off-the-shelf solution.

Weir tank sizes

weir tank sizes

Measurements represent internal dimensions. Bespoke weir tanks can be designed to specific client requirements.

*Flow ranges for the Siris M & L tanks are dependant upon type of weir plate installed.

Talk to the experts

Our team of experts have can offer advice and guidance on the best type of flow measurement system for your requirements, including weir tank sizing. We encourage you to get in touch to find out how we can help on hello@siris.co.uk.

Additional resources