For more than 100 years, flumes and weirs have been used for measuring flow: they are still the simplest, most reliable means of accurate measurement.

The ISO Standards define several contraction geometries and provide their flow/head, Q(h), relationships.

Hymetrics has manufacturing systems, available under license, for the production of ISO Standard weirs and flumes.

The current range includes:

• ISO 1438 Thin Plate weirs, including the V-notch
• ISO 4359 Flumes of three contraction geometries: 
     a) rectangular
     b) trapezoidal
     c) half-round
• ISO 4360 Triangle Profile Weirs.

High turbulence and steep velocity gradients cause significant errors when measuring flow.  To illustrate the problem, the diagram shows  a typical velocity profile in open channels and drains.

Flumes avoid this by causing the flow to accelerate through a contraction to greatly reduce turbulence and gradient effects.

Hymetrics, through its hydraulics ISO 17025 calibration facility, has gained a unique understanding of the interaction between natural channel flow and the performance of weirs and flumes. 

This has enabled Hymetrics to develop high-performance, compact measurement systems.

The ISO 1438 V-notch requires a weir tank.  For this reason, the V-notch is rated only for use in 'clear' sediment-free water: refer to ISO 8369 - 'Guide to the selection of structure'.

The V-notch is therefore not suited for use with industrial wastewater.

The Hymetrics ISO 17025 calibration facility was used to develop the V-flume as an alternative to the V-notch.  A V-Flume fits directly into the channel, there is no weir tank.  The V-Flume can be used to measure industrial process waters and wastewater where high solids levels may exist.

V-flumes are available through Hymetrics' licensees.

The diagram shows how a flow-computer determines the flow measurement using an ultrasonic pulse-echo sensor for level measurement.

The flow-computer triggers the sensor and receives an echo from the water surface after a period dt (typically 0.001 to 0.006 seconds) which enables the computer to compute the water depth, h, from a knowledge of the sonic velocity.  The computer uses this value h to compute a flow rate.

The graph in the middle shows the sensor characteristic h(dt) which is defined when the sensor elevation, E, has been determined during a commissioning process. 

The graph on the right shows a Q(h) curve which is pre-programmed into the flow computer – usually as a sequence of discrete Q and h values.