Float measuring principle

Basics

Variable area flowmeters feature an upright tapered measuring tube, wider end up, in which a specially shaped float moves freely up and down.

The medium flows through the tube from bottom to top. In so doing, it raises the float until there is an annular gap between the wall of the tube and the float and equilibrium of the forces applied to the float has been achieved.

Three main forces act on the float

  • The buoyancy A, which is dependent on the density of the medium and the volume of the float. It is constant (at constant density).
  • The weight G, which is dependent on the mass of the float. Floats can be manufactured using stainless steel, aluminum, titanium or hard rubber, for example.
  • The flow force S: The flow force changes transitionally with a change in the flow until a new state of equilibrium has been achieved.

Different measurement ranges are attained through variations in the nominal width of the cones (e.g. DN 15, DN 25…) 

  • Cone shapes (pitch, length etc.)
  • Float shapes (form drag)
  • Float materials (mass)

Every flow value corresponds to a defined annular gap resulting from the conical form of the measuring tube and the specific position of the float. With glass cones, the flow value can be read directly from a scale at the level of the float reading line. With metal cones, the float position is transmitted to an indicator by magnetic means. 

The VDI/VDE 3513 Bl.1 guideline describes the procedure for converting the scales of variable area flowmeters. It takes into account all material and flow parameters including. density, viscosity, pressure and temperature.

This method can also be used for scale conversions to accommodate changed operating conditions.

KROHNE provides you with its own software for this 

  • KROHNE Variable Area Selection KROVASEL
  • KROHNE Variable Area Calculation KROVACAL

Accuracy of variable area flowmeters

With the 2008 version of directive VDI/VDE 3513 Sheet the definition of accuracy for variable area flowmeters has been revised. The accuracy of variable area flowmeters is no longer specified by accuracy classes but by 2 new parameters G und qG.

 

  • Permissible error G: Constant, permissible error in % of measured value applicable above the linearity limit qG
  • Linearity limit qG: Flow limit value in % of full scale. Above this limit, the permissible, relative error is constant.
  • Below the limit value qG the permissible error increases towards lower flow rates inversely proportional

With given parameters G and qG the maximum permissible, relative error according to VDI/VDE 3513 Sheet 2 (08/2008) can be determined as follows:

KROHNE measuring devices are calibrated with water or air. The operating data is then converted and the devices are scaled accordingly.

Examples for different permissible errors G with linearity limit qG = 50%