Rotameter Correction Calculator
Correct a rotameter (variable area flowmeter) reading when the fluid or operating conditions differ from those the scale was calibrated for.
- F = √[(ρf − ρact) × ρcal ÷ ((ρf − ρcal) × ρact)] = 1.0951
- Qtrue = 100 × 1.0951 = 109.51
Rotameter Correction Formulas
Gas, standard units: Qtrue = Qread × √[(SG1 × P2 × T1) ÷ (SG2 × P1 × T2)]
ρf is the float density, ρcal and ρact the calibration and actual liquid densities. For gas, 1 is the calibration condition and 2 the actual condition at the meter; P is absolute pressure, T absolute temperature and SG the gas gravity relative to air. Viscosity effects are ignored, so use the vendor sizing program for viscous liquids.
Worked example
A stainless float (8020 kg/m³) scaled for water reads 100 on oil of 850 kg/m³: true flow = 100 × 1.0951 = 109.5. A gas meter calibrated for air at 1.013 bar a and 20 °C, run on nitrogen (SG 0.967) at 3 bar a and 30 °C, reads 100: true standard flow ≈ 172.
Frequently Asked Questions
Why does a rotameter need correction?
The float balances weight against drag and buoyancy, which depend on fluid density. A different fluid or gas pressure moves the float to a different position for the same flow.
Does viscosity matter?
Yes for viscous liquids. The correction here assumes viscosity effects are negligible, which holds for water like liquids and most floats.
Where should gas pressure be measured?
At the meter outlet for meters with an inlet valve, since that is the pressure inside the tube.
Learn more on the blog
Types of flow meters
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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