Variable area flowmeters (Rotameters) theoretically involve indication errors when the operating condition deviate from the design condition. However, these errors can be compensated by calculations. The calculation procedure by fluid types are shown below.


IN CASE OF LIQUID MEASUREMENT
The liquid density change from the design condition produces indication error.

Actual Flow rate = Indicated flow rate × Correction coefficient Density correction coefficient Design density (g/cm3) to be referred to Approval drawing Density of the measuring liquid (g/cm3) the density of actual liquid Density of float (g/cm3) differs by material, Refer to the following table. Float material Density of float * Stainless steel 7.7 PVC 2.7 * Figures may have deviations due to final design.

< Compensation Calculation Example >

Actual flow rate of Alcohol = 10 × 1.134 = 11.34 m³/h

　

IN CASE OF GAS FLOW MEASUREMENT

Actual Flow rate = Indicated flow rate × Correction coefficient ·Density compensation Density correction coefficient Design density [kg/m3(Normal)] to be referred to Approval drawing Density of the gas to be measured [kg/m3(Normal)] ·Pressure compensation Pressure correction coefficient Design pressure (MPa) in Gauge pressure,to be referred to Approval drawing Actual operating pressure (MPa) in Gauge pressure ·Temperature correction Temperature correction coefficient Design temperature(°C) to be referred to Approval drawing Actual operating temperature(°C)

< Compensation Calculation Example >

Actual flow rate of CO₂ gas = 5 × 1.010 = 5.05 m³/h(nor)

　

IN CASE OF (SATURATED) STEAM FLOW MEASUREMENT
Correction calculation is done by the density of the steam to be measured

Actual Flow rate = Indicated flow rate × Correction coefficient Density correction coefficient Density of actual steam (kg/m3) Design density (kg/m3)

< Compensation Calculation Example >

Actual Steam Flow rate at 150°C = 10 t/h × 0.703 = 7.03 t/h