Gas Rotameter Flow Meter

A gas rotameter is a metal-tube variable-area meter built with a gas-calibrated float; a lighter float than the liquid build, sized for low gas density. The magnetic float rises inside a vertically-mounted tapered metal tube under upward flow, and the float position is read magnetically by an external indicator at the calibrated scale. Scale reference is air at 20 °C and 101.325 kPa; the same body covers nitrogen, oxygen, argon, and dry methane with a density-correction factor applied at order entry. Real-service pressure changes density and shifts the indicated reading, so the buyer either reads against a known supply pressure or specifies the transmitter build with downstream temperature and pressure compensation. The range covers DN15 through DN100 on gas service; the larger DN125 and DN150 bores are liquid-only in the published flow table.

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 an N2 purge of an analyser cabinet, a flare-stack pilot line, or an instrument-cabinet pressurization (NFPA 496 Type Y / Z / X compatible install), this passive build avoids the cost and audit overhead of an Ex-rated electronic meter. The electrical build is available where a remote signal is needed, but for purge-flow indication in a classified area the pointer build is the standard working pick; no electronics, no ignition source, no Ex certificate to maintain.

The float is the device, and the calibration is the device. A liquid float is heavier and sized to balance the upward drag of a dense fluid against gravity; a gas float is lighter (often lower-density alloy or a hollow build) so the same upward drag of a low-density gas can lift it through the calibrated scale. Specifying the wrong float for the medium gives wrong readings: a liquid float on gas service barely rises and saturates the bottom of the scale; a gas float on liquid service slams to the top of the scale. The factory ships the gas-build with the gas float pre-installed, the scale engraved against the reference gas (air at 20 °C / 101.325 kPa), and the K-factor traceable to that calibration. For nitrogen, oxygen, argon, or dry methane the density-correction factor is applied at order entry and listed on the calibration certificate.

The rotameter alone is not the pick for billing-grade gas accounting. Accuracy is ±1.5 % FS, the float reading is referenced to the calibration pressure (typically atmospheric), and real-service pressure changes density and shifts the indicated value. For dosing, purge, spot-check, and visual operator confirmation service this is acceptable working accuracy and the buyer reads against a known supply pressure. For billing-grade service where invoice settlement depends on flow totalization, go to the LWGQ compressed-air turbine (1:20 turndown), LQ industrial-gas Roots (1:50 turndown, billing-grade), CSUF gas ultrasonic (1:30, non-contact), or TMF compressed-air thermal (1:50, native mass-flow). The rotameter is the indicator and the spot-check parallel meter; the electronic counterparts are the billing primaries. A typical large compressed-air plant runs the electronic primary at the main and the passive rotameter at the drops.

Four buyer angles where the rotameter beats the electronic counterparts: (1) Power not available; analyser-cabinet N2 purge, remote pilot-flame lines, off-grid instrument-air drops. The electronic counterparts all require 24 VDC; the pointer build needs no power at all. (2) Ex-area purge service; the passive build has no electronics inside the purged volume, compatible with NFPA 496 Type Y / Z / X install practice; the electronic counterparts each need an Ex certificate. (3) Local visual operator walkthrough; the pointer is read at 3 m without DCS access, transmitter signal chain, or LCD power, ideal for packaged-skid commissioning and plant-walk audit. (4) Spot-check parallel meter; installed alongside a billing primary (LWGQ turbine or CSUF ultrasonic) as the independent visual cross-check at a fraction of the unit cost. Pick the rotameter for these applications; pick the electronic counterparts for billing-grade service.

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, gas reference (air at 20 °C and 101.325 kPa), density-correction factor for the ordered medium (N2, O2, Ar, dry methane), tested flow range, and accuracy class (A or B). A mill cert for the wetted-parts material (304 SS / 316L SS / Hastelloy C 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 Ex-by-design and needs no Ex certificate at all. Engineering will share build documentation and calibration traceability records to support buyer audit submissions.