Low Flow Rotameter Flow Meter

A low flow rotameter is a small-bore variable-area meter: a magnetic float rises inside a vertically-mounted tapered metal tube under upward flow, and the float position is read magnetically by an external indicator. Float equilibrium height is a direct linear function of volumetric flow at calibration conditions. This page covers the metal-tube class on DN15 and DN20 bores; with up to 2.5 MPa working pressure, hazardous-area-eligible via the passive local-pointer build (no electronics = no Ex cert burden), and rugged enough for chemical-injection-skid and reactor-sample service. It is the working pick for lab benchtop water, additive dosing, sample-injection trim, and pilot-scale R&D rig service where DN25 and larger bores would be over-spec. Buyers who need higher pressure, hazardous-area service, or a rugged metal body for a dosing or sample-injection service should specify this metal-tube build.

Water service on DN15 covers four float classes: 0.025 to 0.25 m³/h, 0.04 to 0.4 m³/h, 0.063 to 0.63 m³/h, and 0.1 to 1.0 m³/h. Water service on DN20 covers two classes: 0.16 to 1.6 m³/h and 0.25 to 2.5 m³/h. Air service on DN15 covers four classes from 0.4 Nm³/h Qmin up to 25 Nm³/h Qmax. Air service on DN20 covers two classes from 3.5 Nm³/h Qmin up to 65 Nm³/h Qmax. Each float delivers a 1:10 turndown across its own range. Multi-float orders deliver a wider effective range by swapping the float at re-calibration. For other liquids or other gases, a density-correction factor is applied at order entry. If the service needs DN25 or larger, go to the metal-tube-rotameter parent page.

The local mechanical pointer is a magnet-driven indicator: the magnetic float inside the metal tube couples to a follower needle outside the wetted volume. There are no electronics inside the meter, no power supply needed, no Ex certificate required, no transmitter signal chain to wire, and no DCS point to commission. For a chemical-injection skid with a half-dozen dosing lines, the local pointer build is the lowest total-installed-cost option and gives the operator a direct visual confirmation on plant-walk. The electrical 4-20 mA / HART build is available where a remote signal is needed, but most dosing-skid buyers default to the pointer build and reserve the transmitter build for unmanned lines or DCS-tied trim loops.

Yes; with the right wetted-parts material. The standard build uses 304 stainless steel for clean water and light-chemical service. For mineral acid (HCl, dilute H₂SO₄), strong alkali (NaOH), chlor-alkali, electroplating bath, or aggressive additive dosing, specify the PTFE-lined build (medium temperature −40 to +85 °C) or the Hastelloy C build. 316L stainless steel covers most mid-aggressive service. Match the wetted-parts material to the actual chemical first; engineering will review the medium composition during the sizing reply. For severe-corrosive service the chemical-rotameter counterpart page is the dedicated PTFE / Hastelloy model; this low-flow page covers the broader lab and dosing market across both clean and chemical service.

Accuracy is ±1.5 % FS (full-scale) standard or ±2.5 % FS economy; the ±1.5 % is referenced to the top of the calibrated float scale. In practical terms, the absolute uncertainty stays roughly constant across the range: at the top of the float scale it is ±1.5 % of Qmax; at 50 % of full scale the same absolute uncertainty reads ±3 % of the indicated flow; near Qmin (about 10 % of full scale) it reads ±15 % of the indicated flow. This is the inherent characteristic of variable-area technology; the float scale is the device, absolute uncertainty stays roughly constant across the range, and relative uncertainty grows toward the low end. The buyer plans for this by sizing the float so the working flow range sits in the upper half of the scale; multi-float orders deliver a wider working range without sacrificing the upper-range accuracy. For billing-grade service the rotameter is not the pick; go to Coriolis or to the LQ Roots gas family. For lab indication, additive dosing, sample injection, and visual operator confirmation, ±1.5 % FS is the industry-default working accuracy.

Standard lead time is 5 to 7 business days from order confirmation, factory direct. Each meter ships with the factory K-factor calibration certificate listing the calibrated DN, float-class identifier, medium reference (water at 20 °C or air at 20 °C and 101.325 kPa), tested flow range, and accuracy class (A or B). A mill cert for the wetted-parts material (304 SS / 316L SS / PTFE / Hastelloy / Titanium per selection) and the build-coded serial plate with the model string ship with every meter. The electrical build adds a Ex d IIC T6 Gb certificate when the Ex option is specified; the local mechanical pointer build is passive and needs no Ex certificate at all. Engineering will share build documentation and calibration traceability records to support buyer audit submissions.