Variable-area flowmeters (or rotameters) have been one of the most versatile flow measurement instruments, providing proven performance for more than 100 years. In addition to their low cost and reliability, they can be used in a variety of applications with water (and liquids with water-like characteristics) as well as many different gases. Depending on the model, significant features include:
•Wide flow ranges of 0.002 ml/min to 500 LPM for liquids
and 0.1 ml/min to 2200 LPM for gases
•High pressure ratings typically up to 200 psi
•No power required / easy maintenance
Applications
Variable-area flowmeters are well suited for a variety of liquid and gas applications, including the following:
•Measuring water and gas flow in plants or labs
•Monitoring chemical lines
•Purging instrument air lines (i.e., lines that use a valved meter)
•Monitoring filtration loading
•Monitoring flow in material-blending applications (i.e., lines that use a valved meter)
•Monitoring hydraulic oils (although this may require special calibration)
•Monitor makeup water for food and beverage plants
The way in which the variable-area flowmeters work is based on the variances in the tapered bore size of the tube and the given density of the float material. These physical properties allow the factory to calculate the flow of the instrument.
The fluid moving through the meter is important as different fluids have different densities. For example, you would not get accurate results if you passed water through a meter that was meant for CO2 gas or air.
Variable-area flowmeter types
The two types are direct reading and correlated. Direct-reading flowmeters have a numbered scale and read out the actual flow rate of the fluid or gas moving through it. They are generally more convenient for water and air applications.
Correlated meters are capable of reading several different fluids or gases without having to switch out the flowmeter or flow tube, making them very versatile. They have a standard reference scale (usually 0 to 150 or 0 to 65 mm) which the operator can use to compare to a specialized correlation chart to find the flow rate for the fluid or gas that is moving through the meter. Most correlated variable-area flowmeters will have a wide variety of charts readily available for various gases and pressures, and charts for custom mixtures and pressures are usually available upon request.
Pressure and temperature effects
Pressure and the effects of temperature are also important factors to consider when using variable-area flowmeters. The typical STP (Standard Temperature and Pressure) with which meters are calibrated is 70ºF (21.1C)/14.7 psi.
When the back pressure increases, the flow rate decreases. This pressure change affects the viscosity and density of the gas and will cause the actual flow rate to deviate from the original calibrated flow rate. This relationship is extremely important and underscores the difficulty in measuring gas flow. Changes in gas temperature will also affect the reading, but to a lesser extent than with pressure. Correction factors are available to compensate for these variations, or the meters can be special-ordered from the factory precalibrated for existing conditions. However, conditions tend to vary. A mass flowmeter might be used in applications which are subject to pressure fluctuations.
Available options
•Dual floats for expanded ranges
•Multitube frames for multiple-channel configurations
•Gas proportioner multitube configurations for gas concentration mixing
•Alarm option on some units
Mounting
Mounting should only be done in a vertical position unless the design allows it, such as a spring-loaded or bidirectional design. Variable-area flowmeters can be panel mounted, placed in-line, or set on a base for benchtop use.
Maintenance
Particulates will affect the performance and can be mitigated by using a 50 µm filter before the inlet. Cleaning is easily done by disassembling the flowmeter and blowing out any particulates with compressed air. Any deposits on the float or tube can be cleaned off with isopropyl alcohol.
Reassemble it in the exact same order and use tweezers to replace the float so that oil from your fingers does not get on it. Following is a link to a demonstration video: http://www.coleparmer.com/TechLibraryArticle/1281
Advantages
•Economical; low cost/maintenance
•Versatile; works with many types of fluids and gases
•Wide ranges and good pressure capabilities
•Rugged; resistant materials available
Disadvantages
•Moderate accuracy; subject to fluctuations and reader error
•Variances in pressure and temperature can affect calibration
Other important factors
•Standard conditions are 14.7 psi (1 atmosphere) at 70°F (21.1°C
•Readings should be taken at the middle of the float and read at eye-level to prevent parallax errors
•Meter should be mounted in an upright position unless design allows
•Custom-calibrated tubes available for gas mixtures; specify percentages of each gas
When the application requires versatility, reliability, low cost, and ease of use, it is easy to see why variable-area flowmeters are popular for so many different applications.
For those requiring a digital display or a higher degree of accuracy, Cole-Parmer has the expertise to recommend a mass flowmeter or one of several other types from our extensive selection.
