Variable Area Flowmeter - Calculation

Calculation of a scale at new conditions

The scale of the variable area flowmeters is designed in the factory for a specific fluid with properties (density, viscosity) and the operating conditions (pressure, temperature) at which the the device is used.

Most manufacturers provide design software for variable area flowmeter-conversion of a scale at new conditions, which can be used to specify the optimum device for the measurement task

Example for calculation software: http://sizing.heinrichs.eu/programs/schwebekoerper/de

The most common task for the user is to adapt the existing scale to the new conditions when operating conditions change or a different fluid is used. or when using a different fluid, to adapt the existing scale to the new conditions. Changes in the material properties and operating conditions of gases or liquids have different influences.

• Gases:
  Gases are compressible, i.e. they change their volume and thus density when the pressure and temperature conditions change. Because of the low gas density and low viscosity, a change in density and viscosity with different gases usually has little influence on the measurement.
• Liquids:
  Liquids are normally non-compressible, i.e. they do not or only slightly change their volume and thus their density with changes in pressure and temperature.
  A change in pressure and temperature does not normally have to be taken into account for the same fluid. However, other viscosity or density values for different liquids have a great influence on the measurement. It may also be necessary to take into account changes in the viscosity of the liquid caused by a change in temperature.

The following rules of thumb can be used to calculate a factor with which an existing measuring scale can be converted to new operating conditions or material properties.

• Factor calculation for gases: Calibrated scale in standard volume (e.g. Nm³/h)

  New scale values = factor x calibrated scale values



p _cal = Absolute pressure of calibrated scale, p _new = Absolute pressure of new scale, T _cal = Temperature of the calibrated scale in Kelvin, T _new = Temperature of the new scale in Kelvin, d _cal= Density of the gas of the calibrated scale, d _new= Density of the gas of the new scale
Kelvin = 273 + °C

• Factor calculation for gases: Calibrated scale in operating volume (e.g. m³/h)

  New scale values = factor x calibrated scale values



p_cal = Absolute pressure of calibrated scale, p_new = Absolute pressure of new scale, T_cal = Temperature of the calibrated scale in Kelvin, T_new = Temperature of the new scale in Kelvin, d_cal= Density of the gas of the calibrated scale, d_new= Density of the gas of the new scale
Kelvin = 273 + °C

Example of an Nm³/h air scale calibrated at 2 bar_abs.. New operating pressure 8 bar_abs.