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BGN Metal Tube Variable Area

The BGN range of metal tube variable area flowmeters has been designed for those applications where conventional glass tube instruments may not be used on account of high pressures or liquid opacity, or for safety considerations when metering hazardous chemicals.

As opposed to the 'fixed float diameter tapered tube' principle used in conventional variable area meters, the BGN utilises a fixed sharp edged orifice and tapered profiled float, the float profile being machined to exacting standards to ensure a linear scaling.

More Information

Operating pressures are largely limited only by the flange ratings, and elevated temperature operation is possible by extending the indicator coupling to remove the indicator and associated electrical or pneumatic transmitters from the source of heat.

Large-scale production and exacting machining specifications enable all parts of the flowmeter to be mechanically interchangeable within a specific range size, and range conversions and replacement of indicators can be carried out by any skilled engineer without difficulty.

Electrical transmitting system or high and low alarm contact devices may be fitted in minutes within the indicator housing.

Operating Principle

The measuring system comprises a sharp-edged fixed orifice ring and a tapered float.

When the fluid flows vertically upwards, the float rises until a point of equilibrium is reached where
the weight of the float is balanced by the buoyancy force 'A'.

This results in an annulus 'S' the area of which is directly proportional to the flow rate. The vertical movement of the float is transmitted to an external indicator through a magnetic coupling system such that glands and seals are avoided.

BGN Metal Tube Variable Area




  • Variable area flowmeter for Liquids, Gasses and Steam
  • Robust construction
  • Versions suitable for harsh environments
  • 316SS, with plasatic and Hastelloy/Titanium versions
  • Vertical installation or, spring restrained horizontal versions
  • Custom Scales Alarm, 4-20mA and HART options