An Introduction to Flow Measurement with Rotameters

Rotameter flow meters are devices that can measure the flowrate of liquids and gases. Generally, the rotameter is a tube made of plastic, glass or metal combined with a float, which responds linearly to the flowrate of the fluid in that tube.

OMEGA^™ laboratory rotameters are far more versatile due to the use they make of correlation equations. Advantages of the rotameter include the long measurement range, low-pressure drop, simplicity of installation and maintenance, and linear scale of measurement.

Factors to consider when choosing a rotameter:

• Operating conditions: minimum and maximum flow rates and process temperature and pressure
• Pipe size for installation
• Rotameters that give direct readings, or require lookup tables, are both available.
• Required accuracy
• Flow regulation valves optional (and may be useful).
• Will there be back pressure?

Working principle of rotameters

For the advantages listed above, the rotameter is the most widely-used variable-area flowmeter. It consists of a tapered tube; as fluid passes through that tube, it raises the float. Greater volumetric flow exerts more pressure on the float, lifting it higher. In liquids, the velocity of the flowing liquid combines with buoyancy to raise the float; for gases, buoyancy can be neglected and the float height is set mostly by the velocity of the gas and consequent pressure.

Generally, the tube is mounted vertically; with no flow, the float rests at the bottom, but once fluid passes upwards from the bottom of the tube, the float will begin to rise. Ideally, the heights that the float move through are proportional to the fluid flow rate and the annular area between the float and the tube wall. As the float rises, the size of the annular opening increases which decreases the differential pressure across the float.

The system reaches an equilibrium, and the float reaches a fixed position, when the upward force exerted due to fluid flow balances the weight of the float – the float is levitated by the fluid flow. You can then read off the flow-rate for a particular fluid’s density and viscosity. Of course, the size of the rotameter and its makeup will depend on the application; if everything is calibrated and sized correctly, flow rate can be read directly off a scale based on the position of the float. Some rotameters will allow you to manually adjust the flow with a valve. Early designs had free floats that rotated in response to changing gas and fluid pressures; since they rotated, the devices were called rotameters.

Rotameters are typically provided with calibration data and direct reading scales for commonly-used fluids – air and water. Sizing a rotameter for use with other fluids requires conversion into one of those standard formats; for liquids, the water equivalent in gpm; for gases, the air flow equivalent in standard cubic feet per minute (scfm). These calibration tables for standard flow values, combined with slide rules, nomographs, or computer software for determining the size of the rotameter, will often be provided by the manufacturer.

Glass Tube Rotameters

The basic rotameter is the glass tube indicating-type. The tube is made of borosilicate glass, and the float can be manufactured by metal (usually stainless steel to prevent corrosion), glass or plastic. Floats typically have sharp, or metering, edges which will point to a specific reading on the scale. The rotameter is completed with an end-fitting or connection depending on use. Similar glass tube and stainless steel float combinations are generally available, regardless of the type of case or end fittings; since the tube-float combination actually performs the measurement, this is the most important part to standardize.

Scales can be set to provide direct readings for air or water – or they may indicate a calibrated scale, or flow in units of air/water for conversion with lookup tables into flows of the relevant fluid.

Correlation rotameter scales can be compared to correlation tables for gases like nitrogen, oxygen, hydrogen, helium, argon, and carbon dioxide. This will prove more accurate although less convenient from direct-reading from a scale; that scale will only be designed for one fluid, like air or water, under very specific temperatures and pressures. Correlation flow-meters, once the conversion is done, can give you flow values under various conditions for a wide variety of fluids. Different flow rates can be measured simultaneously using multiple floats. Typically, fitting a glass-tube rotameter at eye level allows for readings to be taken more easily.

Industrially, safety-shielded gas rotameters are the standard for measuring flows of water or air under ordinary conditions; they can measure flows up to about 60 GPM. Depending on the chemical characteristics of the measurement fluid, plastic or metal end-caps can be used.

There are some examples of fluids for which glass tubes cannot be used. Water over 90°C (194°F), with its high pH which softens glass; wet steam, which has the same effect. Caustic soda dissolves glass; and hydrofluoric acid etches glass: for these applications different tubing must be sought.

The glass metering tube has pressure and temperature limits that tend to be the limiting factor on extreme-performance of the glass-tube rotameter. Small, 6 mm (1/4") tubes can work in pressure up to 500 psig. Larger 51 mm (2") tube may only operate up to pressures of 100 psig. Glass rotameters cease to be practical at around 204 °C (400 °F), but since temperature and pressure generally scale with each other, this means that the rotameter may be no use at lower temperatures in practice; high temperatures reduce the maximum operating pressure for the glass tube.

Where several streams of gases or liquids are being metered simultaneously, or mixed together in a manifold, glass-tube rotameters can be used; they are also indicated for use in cases where a single fluid is flowing out through several different channels, where multiple-tube flowmeters can allow you to mount half a dozen rotameters in a single frame apparatus.

Metal Tube Flow meters

Metal tubes – typically made of aluminum, brass or stainless steel – can be used for higher temperatures and pressures. Since they are not transparent, mechanical or magnetic followers on the outside of the tube are used to determine the float position; here, the spring-and-piston combination determines the flowrate. End-fittings and other materials are chosen dependent on the application to avoid corrosion or damage. Typically they can be used for corrosive liquids that attack glass tubes, in cases where water hammer due to flows that suddenly start or stop is important, or under higher temperatures or pressures such as those associated with steam, which can destroy glass rotameters.

Examples of fluids for which metal-tube rotameters are ideal include hot and strong alkalis, fluorine, hydrofluoric acid, hot water, steam, slurries, sour gas, additives, and molten metals. They can operate at pressures up to 750 psig, temperatures to 540 °C (1,000 °F), and can measure flows up to 4,000 gpm of water or 1,300 scfm of air.

Metal-tube rotameters can be used as flow transmitters, with analog or digital controls; they can detect the float position through magnetic coupling. This then moves a pointer in a magnetic helix to give an external indication of the float position. Transmitters often use microprocessors to provide alarms and pulse outputs for measuring and transmitting the fluid flow.

Heavy Duty/Industrial Pressure Transducers

Heavy Duty/Industrial Pressure transducers are enclosed with a resilient coating and can operate in heavy industry conditions. A scalable 4-20 mA transmitter is often used: it provides greater immunity to electrical noise, which can be a problem in heavy-industry locations.

Design Variations

As mentioned, there are many possibilities in choosing material and design for floats, packing, O-rings, and end fittings. Glass tubes are the most common, but metal tubes can be used under conditions where glass would crack.

As well as glass, plastic, metal, or stainless steel, floats can be made of materials including carboloy, sapphire, and tantalum. Floats have a sharp edge at the point where the reading should be observed on the tube-mounted scale.

Rotameter can be used in vacuum. Valves placed at the outlet of the meter can allow for this to occur. If a wide range of flows is expected, a dual-ball rotameter can be used; typically there is a black ball that measures small flows and a larger white ball that measures much larger flows. The black ball is read until it goes off scale, and then the white ball reading is used. Example measurement ranges include a black ball that goes from 235-2,350 ml/min and a white ball that ranges up to 5,000 ml/min.

A low-cost alternative for hot water, steam, and corrosive liquids is the use of plastic-tube rotamters; they can be made from PFA, polysulfone, or polyamide. The parts that get wet – to avoid corrosion – can be made of stainless steel, PVDF, or PFA, PTFE, PCTFE, with FKM or Kalrez^® O-rings.

Accuracy of a rotameter

It’s possible to calibrate laboratory rotameters to an accuracy of 0.50% AR over a 4:1 range. Industrial rotameters are slightly less accurate; typically 1-2% FS over a 10:1 range. For purge and bypass applications, errors are around 5%.

One can manually set flow rates, adjusting the valve opening while observing the scale to calibrate to the process flow rate; rotameters can then provide repeatable measurements to within 0.25% of the actual flow rate when calibrated to a specific process under the same operating conditions.

Rotameters tend not to vary too much with small viscosity changes, although it depends on the design: very small rotameters that use ball-measurements are the most sensitive, while larger rotameters are less sensitive. If the rotameter exceeds its viscosity limit, you’ll need to correct the readings for viscosity; typically the viscosity limit is determined by material and float shape, and limitations will be provided by the rotameter makers.

Rotameters do depend on the density of the fluid; if this is liable to change, two floats can be used, one that depends on the volume and one to correct for the density. Typically if the float density matches the fluid density, the density changes due to buoyancy will be more important, changing the float position more; mass-flow rotameters work best with low viscosity fluids such as raw sugar juice, gasoline, jet fuel, and light hydrocarbons.

The upstream piping configuration shouldn’t have an impact on flow accuracy; nor should installing the flow-meter after an elbow in the pipe. A further advantage is that – due to the constant flow of fluid through the rotameter – it should remain clean and free of debris; however clean fluids should be used for this, without particulate matter or the possibility to coat the walls of the tube, which will cause the rotameter to become inaccurate and eventually unusable.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.