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A rotameter is a variable area flowmeter: a float rides up a vertical, upward-widening tube, and the height of the float on a printed scale tells you the flow of a gas or liquid. This guide is for lab staff, core-facility users and equipment buyers who need to understand the instrument, read it correctly, and apply the manufacturer’s corrections. It is built mainly on ABB’s variable area flowmeter fundamentals manual, which follows the German VDI/VDE 3513 guideline.
What is a rotameter?
A rotameter is a flow-measuring instrument for liquids and gases in pipelines. ABB, quoting VDI/VDE 3513, describes the design as a vertical tube through which the fluid flows, whose diameter increases from the bottom to the top, and a float that can move vertically in the tube. “Rotameter” is a common name for this variable area (VA) design; “flowmeter” is the generic term for any flow-measuring device, so every rotameter is a flowmeter but many flowmeters (see flowmeter types) work on other principles.
Who uses it: anyone who needs a direct, local indication of gas or liquid flow on a line, such as a bench gas supply or a process line. The simplified ball-float flow meters read on gas outlets work on the same principle (see how to read an oxygen flow meter).
How does a rotameter work?
As flow increases, the float moves to a higher position until its resistance to the flow is balanced by the float’s buoyed weight in the fluid. That buoyed weight is constant and independent of the flow rate, so the float’s position is a measure of the flow rate, read against a scale. The fluid must flow upward through a vertical mounting; Yokogawa’s short-tube Rotameter datasheet specifies installation in a vertical pipeline with the flow direction upwards.
Parts of a rotameter (diagram described)
ABB’s basic-construction figure for a glass-tube meter numbers the parts. Read it as a labelled drawing from the bottom of the instrument to the top:
- Float, the moving element.
- Meter tube, tapered so the flow area grows upward.
- Scale, applied directly on the tube for glass-tube meters.
- Float stops at inlet and outlet, which restrict float movement.
- Inlet/outlet fittings, flanged or threaded, for mounting in the pipeline.
- Protective housing around the tube.
- O-rings that provide the seals.
Glass tubes are the simple case. Where glass lacks adequate mechanical, thermal or chemical resistance, metal meter tubes are used, and the float position is shown outside the tube by a transmission system (a magnet in the float in the designs ABB and Yokogawa describe). Converters can turn the reading into a 0/4…20 mA output signal for remote indication, recording or control. Variable area flowmeters generally have a 1:12.5 measuring range, which is 8 to 100 % of the scale.
Float types
| Float type (ABB) | What the manual says |
|---|---|
| Ball float | Used for smaller flow rates, sizes 1/16 in. to 3/8 in.; changing the float material (density) changes the flow range in one tube size. |
| Float with tail guide | Used with beadguide tubes and percent scales as the standard design; largely independent of viscosity effects, within the viscosity limit in the flow range table. |
| Low-pressure-drop float | Developed for gases at low pressure; very low pressure drop; used with beadguide tubes. |
| Pole-guided float | Has a hole through its length axis and is guided by a pole in a plain tapered tube. |
| Tapered float with guide rods | Used in metal designs; a magnet in the float transmits its position to an external indicator. |
Scales and how to read the reading edge
Float drawings in the ABB manual mark a “reading edge” on each float shape. Read the scale at the edge the manufacturer marks for that float, not at an arbitrary part of the float. Beadguide and tri-flat tube designs keep the float position readable even in cloudy fluids. What the markings mean depends on the scale type:
| Scale type | What it shows | Note (ABB) |
|---|---|---|
| Diameter ratio (Dt/Df) | Linear ratio of tube diameter to float diameter | Universal for liquids and gases; a flowrate table converts the reading; standard for tri-flat tubes with ball floats. |
| Percent | Percentage of maximum flow, over 8 to 100 % | Standard for beadguide and plain tapered tubes; you need the maximum flow for your fluid and conditions. |
| Direct reading | Volume or mass flow per unit time (for example l/h hydrogen) | Valid only for exactly defined operating conditions. |
| Millimeter | Float height only | Used mainly to reproduce a fixed flow rather than to read the flow value. |
How to calculate flow rate from a rotameter reading
The method depends on the scale. On a direct-reading scale the printed number is the flow, under the stated conditions. On a percent scale, flow equals the reading divided by 100, multiplied by the maximum flow from the maker’s flow range table for your fluid, float and tube. On a diameter-ratio scale, use the supplied flowrate table.
Then correct for conditions that differ from calibration. ABB’s example: a meter calibrated for air (normal density 1.293 kg/m3) used on nitrogen (1.25 kg/m3) has its indicated flow multiplied by 1.02. The rule of thumb in the same table: if the new gas’s normal density is larger the factor is below 1, if smaller the factor is above 1. Pressure and temperature that deviate from calibration conditions also need correcting. ABB’s published ranges are based on a 304 stainless steel float (8.02 g/cm3) and water at 1 g/cm3 and 1 mPa·s for liquids, and air at 0 °C and 1.013 bar for gases.
Illustrative arithmetic only (hypothetical numbers): a percent-scale meter whose flow range table gives 200 l/h maximum, read at 40 %, indicates 0.40 × 200 = 80 l/h on its calibration gas. Used on a different gas, multiply by the table’s factor.
Viscosity also matters. Floats are designed to be independent of viscosity up to the Viscosity Immunity Ceiling (VIC) listed in the manufacturer’s tables; above it, the scale must be made at the factory for that specific viscosity.
Accuracy class: what the number on the instrument means
Under VDI/VDE 3513 sheet 2, the maximum allowable error is the sum of two parts: three quarters of the class value as a percentage of the reading, plus one quarter as a percentage of full scale. ABB writes this as E = (3/4 M + 1/4 Eend) × K / M, where M is the measured flow, Eend the scale end value and K the class value. ABB lists classes 1, 1.6, 2.5, 4 and 6.
| Flow, % of scale | Class 1 | Class 1.6 | Class 4 |
|---|---|---|---|
| 100 | 1.000 | 1.600 | 4.000 |
| 50 | 1.250 | 2.000 | 5.000 |
| 10 | 3.250 | 5.200 | 13.000 |
Values are total error in percent of reading, from ABB’s table. Check with the formula: class 1.6 at 50 % flow gives (0.75 × 50 + 0.25 × 100) × 1.6 / 50 = 2.0 % of reading. The table shows that error as a share of the reading grows as the float sits lower on the scale,.
Glass tube or metal tube?
| Tube | Scale and indication | When ABB’s manual points to it |
|---|---|---|
| Glass | Scale printed on the tube; float seen directly | Default simple construction; allowable gas operating pressures are limited per series and must not be exceeded. |
| Metal | Float position transmitted externally; optional 0/4…20 mA output | When glass lacks the mechanical, thermal or chemical resistance needed. |
Common mistakes
- Reading a percent scale as if it were litres per minute, without the maximum-flow value for the actual gas and float.
- Using an air-calibrated direct-reading scale on another gas or at a different pressure and temperature without correction.
- Exceeding the allowable operating pressure of a glass tube on gas service.
- Mounting the instrument other than vertically with flow upward.
FAQ
What is the difference between a flowmeter and a rotameter?
Flowmeter is the generic term for any instrument that measures flow. A rotameter is one type, the variable area design with a float in a tapered tube.
Can you provide a diagram of a rotameter?
The parts list above follows ABB’s basic-construction figure (float, tapered meter tube, scale, float stops, fittings, housing, O-rings). ABB’s manual, linked below, contains the original drawings.
What is MPFM in oil and gas?
MPFM usually refers to a multiphase flow meter, a different instrument class from a rotameter and not covered by the sources used here.
How to calculate flow rate from a rotameter?
See the calculation section above: read at the maker’s reading edge, convert using the scale type, then apply the gas or condition correction.
Sources
- ABB, Variable Area Flowmeter: Basic Fundamentals and Descriptions, D184B003U46 (read 2026-11-07).
- Yokogawa (Rota Yokogawa), Short-tube Rotameter datasheet GS 01R01B02-00E (read 2026-11-07).
- VDI/VDE 3513, as cited in the ABB manual above.
