Variable Area Flow Meters
Passive Metal-Tube Rotameter Series, DN15 to DN150
Accuracy ±1.5 % to ±2.5 % of full scale
DN15 to DN150, 304 / 316L / PTFE / Hastelloy wetted
1:10 turndown · vertical mount mandatory
- Magnetic float in tapered metal tube; mechanical pointer follows position with no shaft penetration
- Local pointer build needs no power and no Ex certification (passive, ATEX-by-design)
- Optional 4-20 mA / HART / pulse / RS-485 transmitter with hi-lo alarm and integrated LCD
A rotameter is a passive variable-area instrument: a magnetic float rises inside a vertically mounted tapered metal tube and an external pointer follows the float by magnetic coupling. Single-phase clean fluid only, ±1.5 % of full scale, 1:10 per float, DN15 to DN150, vertical bottom-up mount. The family sits in the passive visual / low-cost / ATEX-by-design niche distinct from the eight electronic-meter families on the site; pick it when no-power and local readout matter more than custody accuracy.
Variable Area Models
Four models cover the rotameter family. Each card targets a distinct fluid + scenario; pick the closest match and the configurator inside each model page returns the build code.
Metal Tube Rotameter Flow Meter
Catch-all rotameter for clean water, process water, light oil, compressed air, and industrial gas. 304 / 316L wetted parts, local pointer or 4-20 mA option, the default pick for general-purpose variable-area service.
View configuration →Chemical Rotameter Flow Meter
Acid, alkali, chloride, and electroplating-bath service where 304 / 316L is not enough. Local mechanical pointer (no electronics in corrosive atmosphere) with material-to-medium decision matrix on the model page.
View configuration →Low Flow Rotameter Flow Meter
Small-bore variable-area for lab benchtop, reactor sample lines, additive dosing skids, and pilot-scale rigs. NPT thread connection, water 0.025–2.5 m³/h or air 0.4–65 Nm³/h across float classes.
View configuration →Gas Rotameter Flow Meter
Compressed-air drop indication, N2 purge of Ex analyser cabinets, O2 manifold sub-lines, and spot-check verification parallel to a primary gas meter. Passive build needs zero electronics inside the purged volume.
View configuration →How variable-area flow meters work
A variable-area flow meter measures volumetric flow by reading the equilibrium height of a magnetic float inside a vertically mounted tapered metal tube. Flow enters at the bottom flange and lifts the float; the tube cross-section widens with height, so the annular gap between float and tube wall grows as the float rises. The float settles at the height where the upward drag force from the fluid balances the downward gravitational force on the float weight; this equilibrium height is a direct linear function of volumetric flow rate at calibration conditions. The relationship is printed on the scale next to the float.
An external magnetic indicator follows the float by magnetic coupling through the metal tube wall; no shaft, no gland, no penetration of the pressure boundary. The local mechanical pointer is purely mechanical and needs no electrical power. An optional 4-20 mA / HART / pulse / RS-485 Modbus transmitter reads the same magnetic coupling and outputs the flow signal for SCADA. Each float calibration delivers 1:10 turndown; multi-float orders or interchangeable floats extend the effective measurement range. Accuracy is ±1.5 % of full scale (code A) or ±2.5 % FS (code B); an indication-grade specification on a passive device, not a custody-grade one.
When to specify a variable-area flow meter
Match the service against the table below. Out-of-range service goes to vortex, electromagnetic, Coriolis, or oval-gear families.
| Service | Variable-Area Pick | Alternative |
|---|---|---|
| By structure (orientation / material / DN / accessory mapping) | Vertical bottom-up + 304 SS + DN25-DN150 + flange → Metal Tube Rotameter. Vertical + PTFE / Hastelloy + DN15-DN50 + flange → Chemical Rotameter. Vertical + 304 SS + DN15 / DN20 + NPT thread → Low Flow Rotameter. Vertical + 304 SS + DN15-DN100 + gas-calibrated float → Gas Rotameter. | Industry-standard mapping convention: pick by orientation, then wetted material, then bore size, then accessory (connection / indicator / jacket). |
| Plant water / process water / boiler feedwater visual indication | Standard pick (Metal Tube, DN25-DN150) | — |
| Compressed air drop / instrument air sub-line | Standard pick (Gas Rotameter, DN15-DN50) | TMF Thermal Mass for direct mass flow output |
| Acid / alkali / chloride dosing | Chemical Rotameter (PTFE-lined or Hastelloy) | — |
| Lab benchtop / reactor sample / additive injection (sub-DN25) | Low Flow Rotameter (DN15 / DN20 NPT) | — |
| Gas-station N2 purge / O2 manifold local indication | Gas Rotameter (passive, no Ex cert needed) | — |
| Slurry / particulate-laden / abrasive liquid | Outside the operating range (float jams) | Electromagnetic (LEA Series) chemical-magflow |
| High-viscosity liquid above 5 cP (HFO / glycerine / syrup) | Outside the operating range (float drag biased) | Coriolis for HFO; oval-gear for syrup (future model) |
| Custody / fiscal billing service | Outside the operating range (±1.5 % FS is indication-grade) | Coriolis (liquid mass) or Electromagnetic (conductive) |
| Line pressure above 2.5 MPa (DN15-DN50) or above 1.6 MPa (DN65+) | Out of range | Coriolis straight-tube or LWGY liquid turbine |
| Steam at high temperature (above 220 °C without jacket) | Outside the operating range | Vortex (LUGB Series) steam-vortex |
| Wet / saturated gas / pulsating compressor outlet | Outside the operating range | Swirl (LUXB) for wet gas; vortex + accumulator for pulsating |
| Cryogenic service (LN2 / LOX / LNG below −40 °C) | Outside the operating range | Cryogenic Coriolis straight-tube |
Application-to-build map.
Four variable-area model pages live under this hub. The map below shows the coverage and which model page is the recommended pick for each service.
| Service range | Build spec | Model page |
|---|---|---|
| General process water / utility / cooling-water visual | DN25-DN150, 304 / 316L, local pointer, flange PN16 | Metal Tube Rotameter |
| Compressed-air drop on packaged skids | DN15-DN50, 304 SS gas-calibrated float, local pointer | Gas Rotameter |
| Acid metering on chemical line / electroplating bath | DN15-DN50, PTFE-lined or Hastelloy, local pointer | Chemical Rotameter |
| Reactor sample line / additive dosing / pilot-scale | DN15 / DN20, 304 SS, NPT thread, local pointer | Low Flow Rotameter |
| N2 purge of Ex analyser cabinet / instrument cabinet | DN15-DN25, 304 SS gas-calibrated, local pointer (passive) | Gas Rotameter |
| Spot-check parallel to primary electronic meter | DN25-DN100, 304 / 316L, local pointer or 4-20 mA | Metal Tube Rotameter |
All variable-area models share ±1.5 % FS accuracy code, 1:10 turndown per float, vertical bottom-up mount, ISO 9001 calibration certificate, optional Ex d IIC T6 Gb on electrical builds, IP65 ingress, EMC 2014/30/EU, 5–7 business day lead time.
Universal install rules
Install rules across all variable-area models. Model-specific variations live on each page.
- Vertical bottom-up mount; required Standard floats rise against gravity in proportion to upward flow. Body axis within ±2° of true vertical (spirit-level on flange). Tilt biases the reading. Horizontal mount needs a factory horizontal build with a re-calibrated float; specify at order entry.
- Upstream filter for clean service Float-to-tube clearances are tight; particulate larger than 100 μm can jam the float or score the tube bore. Install a Y-strainer or basket filter upstream sized for the line. Leave one DN of straight pipe between filter and meter inlet.
- Avoid pulsation source Variable-area integrates over float oscillation; pulsation causes pointer jitter or biased reading. Mount downstream of any pressure regulator or pump pulsation dampener. For reciprocating-pump service, add an accumulator upstream.
- Respect the 5 cP viscosity bound Standard calibration is valid for liquids up to 5 cP. Heavier fluids (HFO, glycerine, syrup) need a viscosity-correction float or a different family. Send >5 cP service to Coriolis or oval-gear (future model).
- Match DN, material, and pressure class to the line Production range covers DN15-DN150. Pressure cap is 2.5 MPa for DN15-DN50, 1.6 MPa for DN65 and above. Material 304 / 316L for clean water and air; PTFE-lined or Hastelloy for corrosive medium; Titanium or Monel for chloride / HF service. Light straight-pipe practice: 5D upstream and 250 mm downstream of any disturbance.
Frequently asked questions
Need help sizing a variable-area flow meter?
Send medium, DN, P/T, flow target, and indicator preference. Reply in 1 day; lead time 5–7 business days.