Rotameters (Variable Area Flow Meters) Explained

A rotameter is used to measure the rate of liquid or gas flowing through a system of pipes at any given time. They provide a constant flow rate measurement and can also offer optional switching capabilities to control the flow in a system or process.

Rotameter flow technology is such a long standing, familiar, and popular flow meter technology type that for some it is used as a substitute word for all flow meters in general. This, however, is in error as rotameter flow meter technology is just one of many flow meter technology types. So, in essence, all rotameters are flow meters, but not all flow meters are rotameters.

Rotameter type flow meters are referred to by a number of names including rotameter, variable area flow meters, and float-type flow meters. Variable area rotameter technology has been around for more than 100 years. This type of flow meter is among the most commonly sold and installed flow meters in the world. Rotameters can be used to measure the flow rate of either gases or liquids. Different rotameters for liquids can handle media of different viscosities like water or oil. They are not suitable for steam measurement.

They are comprised of a few simple parts: a tube (usually made of glass or plastic and sometimes protected by a surrounding metal housing), a float (usually metal), a scale (printed on the flow tube), fittings for inline installation, and optional outputs or switches for flow control.

The rotameter principle of operation is a mechanical means of flow measurement. They measure flow using a float that is able to move freely up and down within the flow tube. The buoyancy force acting on the float, the flow resistance, and the weight of the float itself are all in equilibrium. This position of equilibrium provides the flow rate by referencing the top of the float against the scale printed on the flow tube. Guide rods are sometimes used to help stabilize the float and are more common in gas rotameters or large flow body liquid flow meters.

The float buoyancy is dependent on the density and mass of the float as well as the materials of construction and volume. The float must have a higher density than the media or it will move up in the flow tube even when there is no flow present, resulting in a measurement error. Essentially, the float must not actually float on the top of the media.

If you look closely at a rotameter tube, it usually will taper from a small diameter to a slightly larger diameter at the top. The shape of the tube and the increase in the volume capacity based on the location within the tube all contribute to create the environment for accurate measurement. Some models offer an obvious taper to the tube and others are more subtle in nature.

Types of rotameters can be defined by the material used for the flow meter. Most rotameter flow bodies are made of glass or plastic, some are metal. Acrylic and polyamide are common plastic types used in rotameter construction. Metal rotameters are generally made in stainless steel or brass bodies.

Rotameters can also be categorized by the media type. Most rotameters can handle gases like air or liquids like water. Because the scales on the meter are calibrated for the media type, you cannot use an air flow meter to measure water and vice versa, even though a rotameter for air and a rotameter for water maybe near indistinguishable from each other. In their actual build they may be, but the difference is that the measurement scale between the air and water model will have different spacing. Some special rotameters offered by KOBOLD instruments are built to compensate for higher viscosity media.

Rotameters can also be classified by their common applications areas. Rotameters are commonly found in laboratories, chemical injection systems, boiler control, purge applications, filtration loading, process analyzers, oil lubrication circuits, coolant flow, and as early warning tools for clogged systems. These are not the only application areas that rotameters are found. The applications they are used in across the globe is almost endless as they are both economical and versatile.

Rotameters are not classified by installation type as all rotameters are “inline” installations. Because of the very nature of the operating principle, there are no insertion or “clamp-on’ model types. This means that the flow meter is placed in between two sections of process piping by fittings on either side of the flow meter.

Variable area rotameters offer the following benefits:

• Visual flow indication through transparent flow bodies or magnetically coupled indicators
• A linear scale
• Typically, lower in cost
• Simple, reliable design
• Easy to install
• Minimal to no straight run requirements
• Low pressure drop that is usually constant
• Multiple meters can be installed on a single line without significant pressure loss
• Easy start up
• Minimal maintenance
• Wide material choice to accommodate many different types of media
• Can be used with corrosive liquids if compatible build material is selected
• No external power required for operation
• Good accuracy for low to medium viscosity media types
• Repeatable measurement

Yes, most rotameters offer options that can control flow. Most rotameters are available with optional switches that will shut down flow if a certain flow rate is reached. KOBOLD offers many models with one or more switches to enable multiple switch points for the process if needed.

Rotameters are also available with needle control valves. These look like little dials on the bottom or top of the flow meter body and can be turned to increase or decrease the flow rate and to hold it at a relatively steady rate once it is set.

Rotameters can also commonly be mounted in manifold blocks where the flow from one main line can be custom set per line feeding from the main line to a particular flow rate. Flow meter manifold blocks are used in both industrial and in residential applications. To learn more about rotameter use with manifold valves visit the BVB Manifold Valves product page.

Whether or not a rotameter is accurate enough depends on the exact application that it will be used in. Rotameters are known for being highly repeatable. There are other flow meter technology types that will offer higher accuracy, like magnetic and ultrasonic flow meter types, but those technologies also come with a much higher price tag. It is always important to weigh the accuracy needs of the application against the budget. Many applications are served quite well by the accuracy provided by a rotameter flow meter.

Rotameters generally offer an accuracy of 1.6% to 5%. This is average accuracy for most mechanical type flow meters. In comparison, flow meters with no moving parts offer the following accuracies: 0.1% to 0.5% for Coriolis flow meters, 0.2% to 2% for magnetic flow meters, and 0.7% to 1% for ultrasonic flow meters.

It is important to choose a meter that will operate in 60% to 80% of the meter’s flow range most of the time to avoid inaccuracies associated with flow rates that are at the upper and lower end of the meter’s capability. Rotameters are usually limited to a 10:1 turndown ratio, which means they cannot accommodate a wide variation in flow rates. To learn more about flow meter turndown, visit our turndown ratio article.

Rotameters cannot tolerate liquids that coat or crystalize as that would impede the proper movement of the float. They also do not handle large particulate matter well as it would not be able to easily slip by the float in the flow tube and would cause buildup under the float which would create measurement errors. Changes in the specific gravity, pressure, and media viscosity can also cause significant measurement errors. If your process is defined by changing media specifications throughout the process, then another flow meter technology may be a better option. Rotameters must be calibrated for density, viscosity, and compressed gases. Ensure that whatever meter you choose can handle the maximum operating pressure and temperature within your process and that the media is compatible with the flow body and float materials.

Rotameters are volumetric meters (as opposed to meters that measure mass) by nature so for liquids, they need no accommodation for changes in temperature and pressure as liquid volume is not greatly affected by changes in these 2 operating conditions. Gas volume however is greatly affected by operating temperature and pressure so referencing scales for gas measurement need to reference a standard such as SCFM (STANDARD cubic feet per minute) or NM³H (NORMAL meters cubed per hour) and any variant from the calibrated operating conditions will result in a change in gas volume and very inaccurate readings.

Rotameters are read at the top of the float against the scale on the flow body. Because of optical distortions, to get an accurate reading it is best to position yourself where you are at eye level with the top of the float and positioned in front of the measurement scale and not the side.

Most rotameters must be installed in a vertical position, with the flow moving from bottom to the top. Media that is not of a transparent nature will make reading the float very difficult so consider whether a rotameter is suitable also based on the media. They are also not good for pulsating flow. Without protection, the glass and plastic tubes can be broken if installed in an area where things are moving around and could impact the meter.

Most any rotameter suitable for gases will be suitable for the measurement of air as well. When choosing an air rotameter, consider the minimum and maximum flow range that the meter will accommodate, the maximum pressure and temperature that it can handle, and the area of installation. If a glass rotameter is chosen and there is a possibility of the meter being broken by moving people or parts, you may want to choose a model that offers a protective metal housing around the glass tube or consider a plastic model.

Most rotameters for liquids can handle measuring the flow of water. The exception would be rotameter designed specifically for oil measurement. The defining factor in considering a rotameter for water (in addition to flow range, pressure, and temperature capabilities) is whether the water has particulate matter. It if does, a rotameter may not be the best choice as floats can be hindered and errors can occur in the reading. If the media has particulates, a technology that does not have any moving parts, like magnetic, ultrasonic, vortex, or differential pressure, is generally a better choice.

Oil rotameters are usually their own specific media-type models as most water rotameters and air rotameters cannot handle oil due to the increased viscosity. Oil rotameters are typically constructed of metal as most processes using oil are also likely to be running at a higher pressure. KOBOLD offers a patented special oil rotameter design that compensates for changes in media viscosity and density that can occur because of changes in temperature or other factors.

Low flow rotameters offer the ability to read a much lower flow rate than standard models. They are available in both liquid and gas models. KOBOLD offers a wide range of low flow rotameters that can meet the needs of many different applications. Our low flow rotameters offer some of the lowest flow values in the industry, like our KFR Rotameter which offers a low-end flow rate of 0.2 GPH or our KSK Rotameter which offers a low-end flow rate of 0.006 GPM.

Metal tube variable area flow meters offer the distinct advantage of durability as they are not as prone to breakage like glass or plastic rotameters. This also means that they are capable of handling higher operating pressures. The disadvantage to metal tube variable area flow meters is that they do not offer any visual access to the flow like transparent models do. KOBOLD offers metal tube variable area flow meters like our SM High Pressure All-Metal Flow Meter and Switch. We also offer many models that have a glass tube flow body that is almost completely surrounded by metal protection, like our SV Rotameters.

There are two typical versions of an acrylic rotameter. The first is an acrylic tube model where the flow body is made of plastic and the meter is the shape of the circular flow body. The second kind is a “pillar” of acrylic where the meter body is essentially rectangular in shape, even though the flow tube within the meter is circular.

Acrylic rotameters often offer the advantage of being compatible with media that may be more aggressive or chemical in nature. Because plastic is less expensive than metal, acrylic flow meters are often cheaper than their metal constructed counterparts.

Variable area flow switches are built and operate in the exact same manner as variable area rotameters. In fact, many variable area flow meters offer optional switches. If all that is required is a switch and not a reading of the continuous flow rate that is provided by a flow meter, then there are also models that do not have measuring scales and simply offer the ability to set a pre-determined switch point like our SMN Flow Switch for Liquids or our S-Series All-Metal Flow Switches for Gas or Liquids.

Most rotameters will come pre-calibrated from the manufacturer. Calibrating a rotameter at the factory is done by comparing the readings on the rotameter with the readings provided by another more accurate flow meter installed in line with the rotameter on what is called a “flow stand”. A flow stand is basically a pipeline with pumps that will pump media through the instrumentation. Flow meters that are used on flow stands to verify the accuracy of a rotameter are usually magnetic or Coriolis flow meters as they offer a much higher accuracy than rotameters.

As the media moves through the flow stand, the reading on the verification flow meter is compared with the reading on the rotameter. If the reading is within the stated accuracy tolerance, then the rotameter passes inspection. If the rotameter reading is not within the stated tolerance, then a new scale can be created that will account for any variations in the meter that may lead to it to not confirm with the initial scale.

Rotameters do not generally require recalibration like other flow meter technologies because the measurement principle is simple and mechanical, and the float and tube dimensions do not change unless damaged. Some companies are ruled over by governing bodies that require them to periodically recalibrate their flow meters. Without a flow stand or the ability to reprint a scale, it is not possible or advisable to attempt modifications to recalibrate a rotameter by yourself. There are companies that specialize in flow meter accuracy verification and the meters are typically removed from the line and sent in to be tested.

Standard rotameters are among the cheapest flow technologies out there. Because of the simple mechanical design, they can offer cost savings over more complex technologies. The price of a rotameter will depend on the materials of the flow body, additions like optional switches and needle valves, and the size of the flow body. Heavy-duty all-metal industrial rotameters, like our BGF Variable Area Flow Meters for horizontal installations (a unique option, as most rotameters are for vertical installations only) and our BGN Variable Area Flow Meters for vertical installations, will be more costly due to the amount and type of materials used.