Rotor Flowmeter

Product Overview

The rotor flowmeter (also called a variable-area flowmeter, rotameter, or float flowmeter) is the simplest, lowest-cost, and most reliable way to get a local visual indication of flow rate on a chemical injection line. The device consists of a precision-tapered tube (typically borosilicate glass for visual indication, or PSU / PVDF for chemical compatibility with no glass) and a calibrated float that is pushed up the tube by the flow. The float's vertical position, read off a graduated scale, gives the flow rate directly. There are no moving parts other than the float itself, no electrical power required, and no electronics to drift or fail in the field.

Our rotor flowmeter line is designed specifically for the chemical injection duty — the low flow rates (0.5-150 L/h), the corrosive chemistry (NaClO, HCl, NaOH, polymers, antiscalants), and the space constraints of a chemical skid. Three body materials cover the bulk of the application space. PVDF body with PVDF float is the workhorse for water-treatment chemicals at ambient temperature; it tolerates HCl up to 30%, NaOH up to 40%, NaClO up to 15%, and most water-treatment polymers, and is rated to 16 bar at 20°C. PSU (polysulfone) body with PSU float is the clear-tube variant for low-pressure service (up to 6 bar) where the operator wants to see the float and visually verify the chemical color. SS316L body with SS316L float and tempered borosilicate glass tube is the high-pressure / high-temperature variant (40 bar, 200°C) for hot-process service and food/pharma.

The rotor flowmeter is a local-indication device. For plants that require remote indication or SCADA integration, an optional 4-20 mA transmitter is available — a magnet-coupled Hall-effect sensor tracks the float position and outputs a proportional 4-20 mA signal that can be tied into the plant DCS. The transmitter is powered from the 24 VDC loop and adds 2% F.S. to the measurement uncertainty.

Working Principle

Inside the tapered tube, the cross-sectional area increases linearly with height (the taper is typically 1:50 to 1:200, depending on the flow range). As the flow enters at the bottom, it pushes the float upward. The float is buoyant in the fluid and experiences three forces: the upward drag force from the flow (which increases with the annular area between the float and the tube), the downward gravitational force (the float's weight minus buoyancy), and (for guided-rod variants) the spring force from the central guide rod.

At any given flow rate, the float reaches an equilibrium position where the drag force exactly balances the weight. Because the annular area between the float and the tube increases with height, the flow velocity at the float's location decreases as the float rises. The drag force is a function of the velocity squared (in turbulent flow) or the velocity (in laminar flow), so the equilibrium is unique. The result is that the float's height is a monotonic function of the flow rate, and the graduated scale on the tube (or on the housing) is calibrated to read the flow rate directly.

The standard rotor flowmeter is calibrated for water at 20°C as a reference. For other fluids, the indicated flow must be corrected by the fluid's density and viscosity. The correction factor K is read from a per-float curve supplied with the flowmeter. K = √(ρ_water/ρ_fluid) for density correction (low-density solvents read high, high-density brines read low), multiplied by an additional viscosity correction for fluids above 50 cP (viscous fluids read low at low flow rates).

Technical Specifications

Parameter	Value
Measurement Principle	Variable-area (float in tapered tube)
Flow Range (Standard)	0.5-1.5 L/h, 1-5 L/h, 2-10 L/h, 5-25 L/h, 10-50 L/h, 20-100 L/h, 30-150 L/h (water at 20°C)
Flow Range (Custom)	0.1-1,000 L/h on request
Accuracy (Standard Float)	±5% of full scale (water, 20°C)
Accuracy (Calibrated Float)	±2% of full scale (process fluid, factory wet-cal)
Repeatability	±1% of full scale
Turn-Down Ratio	10:1 (standard float) / 20:1 (low-flow float)
Body Material (PVDF)	PVDF (Kynar 740), FKM seals, PVDF float, max 16 bar at 20°C, 10 bar at 80°C
Body Material (PSU Clear)	Polysulfone, EPDM seals, PSU float, max 6 bar at 20°C, 4 bar at 50°C
Body Material (SS316L)	SS316L body, tempered borosilicate glass tube, FKM seals, SS316L float, max 40 bar at 200°C
Float Material Options	SS316L (water/aqueous), PVDF (dilute acid/base), PTFE (most chemicals), Tantalum (high-pressure water), Hastelloy C-276 (hot acid)
Tube Material	Borosilicate glass (clear), PSU (transparent amber), PVDF (opaque white)
Tube Taper	1:50 to 1:200 (float-range dependent)
Process Connections	1/2" NPT (standard) / 3/4" NPT (DN15+) / 1" NPT (high-flow) / Tri-clamp (sanitary) / Flanged DN15 (high-pressure SS316L)
Installation Orientation	Vertical, flow up (standard) / Guided-rod (any orientation, ±8-10% F.S.)
Upstream Straight Pipe	5D minimum (no valves, elbows, or pumps within 5D)
Downstream Straight Pipe	3D minimum
Pressure Drop (Max)	0.05-0.3 bar depending on float and flow range
Temperature Range (PVDF)	-20°C to +100°C
Temperature Range (PSU)	0°C to +80°C
Temperature Range (SS316L)	-40°C to +200°C
Indication Scale	Graduated L/h or %, factory-printed on tube housing
Local Output (Standard)	Visual indication via float position vs. graduated scale
Remote Output (Optional)	4-20 mA loop-powered Hall-effect transmitter, 24 VDC, ±2% F.S. added error
Switching Output (Optional)	Reed-switch or Hall-effect limit switch, programmable setpoints
Housing / Connection Head Protection	IP65 (standard) / IP67 (with sealed conduit entry)
Documentation (Sanitary Variant)	EN 10204 3.1 material cert, surface finish cert, FDA compliance
Net Weight (PVDF, 5-25 L/h)	0.4 kg
Net Weight (SS316L, 10-50 L/h)	2.2 kg

Float & Range Selection

Float selection is driven by the fluid's density and viscosity at operating conditions, and by the target flow range. For water and most dilute aqueous chemicals (ρ ≈ 1.0 g/cm³, μ < 5 cP), the standard SS316L float gives the published flow ranges. For lower-density fluids (organic solvents at 0.8-0.9 g/cm³), the float reads higher than indicated — apply a correction factor of √(1.0/0.85) = 1.08, so a 10 L/h reading on the scale corresponds to 10.8 L/h of actual flow. For higher-density fluids (concentrated brines at 1.2-1.4 g/cm³), apply a correction of 0.85-0.91.

For viscous fluids (> 50 cP), the float behavior is no longer a simple function of flow rate. The float's height is reduced at any given flow because the viscous drag is higher; the correction is read from a per-float viscosity curve supplied with the flowmeter. For fluids above 200 cP, the rotor flowmeter is not recommended — switch to a Coriolis mass flowmeter or a positive-displacement flowmeter.

For highly corrosive or hot service, specify the PTFE float (compatible with virtually all chemicals except molten alkali metals and fluorine) or the Tantalum float (for hot concentrated acids and high-pressure water service above 100 bar). The Tantalum float has a density of 16.6 g/cm³, which means it sits lower in the tube at any given flow — the flowmeter's range scale is rescaled accordingly.

Typical Applications

Chemical-injection skid flow indication. The most common application is on a chemical dosing skid, where the rotor flowmeter is installed in the suction line of the metering pump (between the chemical reservoir and the pump suction) or in the discharge line of the Venturi injector. The flowmeter gives the operator a direct visual indication of the chemical flow rate — essential for verifying the pump or injector is operating correctly, and for trending the flow over time. A 5-25 L/h PVDF-body flowmeter on a sodium-hypochlorite injection line, for example, tells the operator at a glance that the chlorination is within the expected range.

Cooling-tower water treatment. Bleach, acid, and antiscalant are injected into cooling-tower recirculating water at 1-50 L/h. The PVDF rotor flowmeter with a 5-25 L/h range is the standard. The float is easily visible through the clear tube, and the chemical compatibility with hypochlorite, sulfuric acid, and most phosphonate / polymeric antiscalants is excellent.

Municipal drinking-water fluoridation. Fluoride (typically fluorosilicic acid at 23-25% concentration) is injected at 0.5-5 L/h into the treated-water clearwell. The PVDF rotor flowmeter is the standard for this application because of the acid compatibility and the small flow range. The flowmeter is typically installed on the suction line of a diaphragm metering pump, with a foot-valve strainer at the chemical reservoir outlet.

Laboratory and pilot-plant flow monitoring. In R&D and pilot-plant settings, the rotor flowmeter is the simplest way to monitor low flow rates of experimental chemistries. The PSU-body variant (clear amber tube) is the standard for lab use, because the operator can see the chemical color and confirm the absence of air bubbles in the flow. The flowmeter is installed on a portable cart, with quick-disconnect fittings for easy chemistry changes.

Food and beverage ingredient dosing. In a CIP or ingredient-skid, the SS316L-body rotor flowmeter with the optional 4-20 mA transmitter gives both a local visual indication and a remote signal to the plant DCS. The flowmeter is sized for the ingredient flow rate (typically 5-100 L/h) and the 4-20 mA signal is tied into the recipe-management system, which records the actual dose for batch reporting.

Installation & Maintenance

The rotor flowmeter is installed vertically in the suction line (or in the discharge line if there is sufficient back-pressure to keep the tube full of liquid). The standard installation includes a 5D straight-pipe section upstream (no valves, elbows, or pumps within 5D) and a 3D straight-pipe section downstream. The inlet and outlet are 1/2" NPT by default; 3/4" and 1" NPT are available for higher-flow ranges, and tri-clamp is available for sanitary service.

For the float to operate correctly, the tube must be full of liquid at all times. If the flowmeter is installed in a suction line that may see air (e.g. when the chemical reservoir is empty), the float will jump and give erratic readings. To prevent this, install an air-elimination device (a small air-separation chamber) upstream of the flowmeter, or specify the inverted variant where the flow enters at the top — but note that the inverted variant has a different (and smaller) flow range, and is not the same as the guided-rod variant.

Maintenance is minimal. The float and tube should be inspected annually for fouling or chemical attack. For chemical compatibility issues, the tube and float can be replaced as a subassembly without replacing the body — the standard replacement interval is 3-5 years, depending on the chemical. The optional 4-20 mA transmitter is calibrated at the factory and is field-adjustable via a zero and span trim; the calibration drift is typically less than 1% per year.

For sanitary service, the flowmeter should be included in the plant's CIP loop. The CIP chemistry (1-3% NaOH at 80°C followed by 1% HNO₃ at 50°C) is compatible with SS316L, FKM, and borosilicate glass, and the flowmeter can be cleaned in place without disassembly. The tri-clamp end connections allow the flowmeter to be removed for inspection in under a minute.

Frequently Asked Questions

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