Flow Measurement · Glossary

What is a Rotameter?

variable area flow meterFlow Measurement

A rotameter is a variable area flow meter where a float rises in a tapered tube until drag balances its weight.

Rotameter Explained

Flow lifts the float until the annular gap is large enough that drag and buoyancy balance the float weight. The float height is read on a scale or by a magnetic follower.

Rotameters are simple and need no power, but they are calibrated for one fluid density. A different fluid, pressure or temperature needs a correction.

Rotameter Formula

Q₂ = Q₁ × √[(ρ_f − ρ₂) ρ₁ ÷ ((ρ_f − ρ₁) ρ₂)]

Q₁, ρ₁	calibration flow and density
Q₂, ρ₂	actual flow and density
ρ_f	float density

Note: For gases, where ρ_f is much larger, this reduces to Q₂ = Q₁ × √(ρ₁ ÷ ρ₂).

Calculators for Rotameter

CalculatorRotameter Correction CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is a Rotameter?

A rotameter is a variable area flow meter where a float rises in a tapered tube until drag balances its weight.

Must a rotameter be vertical?

Yes, with flow upwards, because gravity on the float is part of the measuring principle.

Where is the reading taken?

At the reference edge marked by the maker, usually the widest part of the float.

Learn more on the blog

Types of flow meters

A practical guide on instrumentationblog.in with more detail on rotameter and how it is applied on real plants.

Read Full Article

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