Metal Tube Rotameter Flow Meter

A metal-tube rotameter is a variable-area (VA) flow meter. Flow enters at the bottom of a vertical tapered stainless tube and lifts a magnetic float; the float rises until upward flow drag balances its weight, and the equilibrium height is a direct linear function of volumetric flow at calibration conditions. An external pointer reads the float position magnetically through the wall; no shaft, no gland penetration, no electronics inside the wetted line. Each tapered tube is calibrated for water (liquid) or air (gas) at a reference density; other media are corrected by density factor at order entry. The local pointer build is fully passive (no power, no Ex cert required); the electrical build adds a 4-20 mA or HART transmitter on top of the same mechanical primary.

Variable-area meters are scale-based devices, not frequency-counting devices. The float position is read against a printed scale, so the uncertainty spread is fixed in absolute units across the range; it does not scale with the flow rate. The build publishes ±1.5 % of full scale (class A) or ±2.5 % of full scale (class B). At 100 % of range a class A meter reads within ±1.5 % of the actual flow; at 50 % of range that same absolute spec is ±3 % of the indicated value; at the 10 % bottom of a single-float turndown it is ±15 % of indicated. This is why rotameters are specified for indication, dosing, and verification service rather than custody billing; a turbine or Coriolis with a percent-of-reading spec holds the same absolute spec across its full turndown.

Three applications favor the rotameter. First, no-power / no-electronics service; the local mechanical pointer build is fully passive, needs no Ex certification, and is the lowest-total-cost option for hazardous-area visual indication. Second, low flow at small bore; the range covers down to 0.025 m³/h water on DN15, well below the practical minimum of a vortex (Reynolds-limited) or electromagnetic (conductivity-limited) meter. Third, the same DN body supports liquid (water-referenced) or gas (air-referenced) calibration at order entry, no separate tooling required. Vortex wins on steam and dry gas at higher Reynolds; electromagnetic wins on conductive liquid and slurry. The rotameter wins where a positive visual readout, a passive primary, and a low installed cost matter more than absolute precision.

The variable-area principle relies on gravity. The float weight sets the reference force; upward flow drag lifts the float against gravity until the two balance. Horizontal mount removes the gravity reference and the float will not return to a repeatable zero. The factory build is therefore vertical, flow bottom-up; the meter body must be plumb within ±2° of true vertical. A horizontal-mount version exists as a factory build (different float weight and spring assist), but it is a separate order code; not a field-convertible setting on a standard meter. Pipe support and short flexible runs above and below the meter protect the float column from pipe stress and thermal expansion.

Standard lead time is 5 to 7 business days from order confirmation, factory direct. Each meter ships with an ISO 9001 calibration certificate listing the DN, float class, calibrated medium (water or air), and tested flow range; a mill cert for the 304 or 316L stainless wetted parts; the build-coded serial plate with the model string; and the installation drawing showing flange dimensions, total length, and the vertical-mount orientation. Electrical builds add the Ex d IIC T6 Gb certificate and the 4-20 mA / HART loop-test record. Hazardous-area passive builds (local pointer only) need no Ex certificate; there are no electronics inside the meter to certify.

Route corrosive service (HCl, H2SO4, NaOH, chlorides, HF) to the chemical rotameter with PTFE-lined or Hastelloy / Titanium / Monel wetted parts; standard 304 / 316L will corrode. Route slurry, particulate-laden, or abrasive service to the insertion electromagnetic meter; the rotameter float clearance is too tight and will jam. Route high-viscosity service above 5 cP (HFO, glycerine, heavy syrup) to the Coriolis straight-tube family. Route custody / fiscal billing service to Coriolis, electromagnetic, or Roots gas; ±1.5 % of full scale is indication-grade, not billing-grade. Route sub-DN15 lab service to the low-flow rotameter and gas-only service at higher accuracy to the gas rotameter.