A rotameter is a commonly used type of industrial flowmeter that is used to measure the flowrate of a liquid or gas. It consists of a tapered tube with a float inside. Rotameters work in a simplistic way: fluid raises the float when it passes through the tapered tube. When there is no flow, the float stays resting at the bottom. Rotameters are widely used because they are easy to install and maintain and they have a fairly wide measurement range, a low pressure drop, and linear scales.
The Way Rotameters Work
The operation of a rotameter is based on the variable area principle. That is, the flow of a liquid raises the float inside a tapered tube, increasing the area through which the liquid can pass. The larger the flow, the higher the float will be raised. The level of the float is directly proportional to the flowrate of the liquid, and it moves up or down in proportion to the liquid’s flowrate and the annular area around the float. The tapered tube allows the annular area between the ball and the tube to be proportionate to the flow, in a balanced condition.
Determining Accuracy
The accuracy of a rotameter is established by the accurateness of the temperature, pressure, and flow control when it is initially calibrated. If there is any change to the weight and density of the float, there will be an impact on the reading of the flowrate. Also, if there are any changes that affect the liquid, like a change in temperature or pressure, the accuracy of the rotameter will be affected. Therefore, it’s important that rotameters are calibrated at least once a year to allow for any changes that may have happened.
Design / Style Variances
All rotameters consist of a tapered tube and a float, but what those components are made of and where they are best used can vary. The tubes may be made of glass, metal, or plastic, and float materials can vary. The following will provide more information about different rotameter designs:
• Glass tubes – Glass tubes are the most common, as well as the oldest, design of rotameters. The glass for these tubes is safety-shielded borosilicate glass, which is safe for water and air applications up to 60gpm flowrate for water and 200scfh for air. There are some materials that aren’t safe for use in glass tube rotameters, including wet steam or high-pH water that is over 194°F (which may cause the glass to soften), caustic soda (which can dissolve glass), and hydrofluoric acid (which can etch glass).
• Metal tubes – Metal tubes are used for rotameters when conditions include higher temperatures and pressures than glass can withstand. These tubes are typically made of stainless steel. To detect the position of the float, magnetic followers are used inside the tube along with readings on the outside of the tube. Metal tubes can be utilized for hot and strong alkalis, water, steam, fluorine, slurries, hydrofluoric acid, molten metals, and more.
• Plastic tubes – Plastic-tube rotameters are relatively low-cost devices that are designed for applications that involve corrosive fluids or deionized water. The tube itself can be made from polysulfone, perfluoroalkoxy (PFA), or polyamide.
• Float materials – Floats are usually made of glass, metal, plastic, or stainless steel for corrosion resistance. Other float materials include sapphire, carboloy, and tantalum.
Advantages of Rotameters
There are a number of advantages to using a rotameter rather than a more complex flowmeter (to read the differences between mechanical and digital flowmeters, go here). Rotameters advantages include:
• Rotameters are uncomplicated devices made with inexpensive materials, which makes them cost-effective.
• Rotameters can be installed into areas that don’t have power because they operate only on the properties of the liquid and gravity itself.
• Rotameters can easily be installed to existing piping systems with standard fittings or via a panel.
• A rotameter’s scale for measuring is linear because the flowrate is based on area variation. That means that you can count on accurate readings throughout the complete range.
• With a properly maintained rotameter you can expect sustained high repeatability.
• Smaller rotameters typically only have a few inches of water column pressure drop. That means that rotameters can be installed nearly anywhere in the process. Smaller pressure drops mean smaller pumps, which can mean lower pumping costs.
Disadvantages of Rotameters
You’ll also find a few disadvantages to using rotameters. These include:
• Rotameters have to be installed vertically because they use gravity to measure flowrate.
• When opaque fluid is used, the float may not be visible through the rotameters transparent tube.
• The scale on the rotameter is only valid for a specific fluid and the conditions where it was calibrated, which means the device will have to be recalibrated if the fluid or the conditions are changed.
• It can be challenging to adapt rotameters for machine reading. A magnetic float is sometimes used for that purpose.
Final Thoughts
Rotameters are very cost-effective and practical for measuring the flowrate of fluids in piping systems. Applications in which rotameters are used extend across many industries because the devices are still the most economical way of measuring very low to high flowrates.
