Tips for Choosing Variable Area Flow Meters

Flow meters and flow measurement have been around for thousands of years, with early versions of the instruments found as far back as the Roman Era. The rudimentary meter was enhanced in the 17th century after the mathematical foundations of flow theory evolved. Today, the variety of flow meter styles has greatly expanded from those early beginnings. Some examples include variable area, positive displacement, ultrasonic and mass flow meters. Each type of flow meter has distinct features that complement use in the corresponding application and industry. End users need to know how to choose the right flow meter for their specific application. The variable area flow meter can be a cost-effective solution for many applications. The meter measures fluid flow by allowing the device’s cross-sectional area to vary in response to the flow, causing a measurable effect on the rate. The variable area flow meter offers many advantages over some more technologically advanced types, making it the meter of choice for many flow-measurement applications.

The flow meter body is a vertically tapered tube, allowing fluid to pass from the smaller end to the larger end of the taper. As the fluid flows through the tube, it forces an indicator (float) upward. The clearance space (the annular area between the float and tube wall) increases as the float moves toward the top of the meter. The larger clearance area toward the top requires a larger volume of fluid to force the float higher. The greater the flow, the higher the float is raised. Different flow ranges can be calibrated by varying the taper of the tube, the mass of the float and the length of the tube. A scale with flow increments or measurements is typically printed next to the tube or on to the tube itself. The flow rate can be read by matching the increments on the calibrated scale with the edge of the float. Proper installation of the variable area flow meters is critical to performance. The meter must be plumbed into the piping system with the narrow part of the taper at the bottom (inlet).

Economic Considerations

Variable area flow meters have become competitively priced and more cost-effective in recent years. As more manufacturers enter the market, prices remain steady or in some cases have decreased. They can be attractively priced compared to more advanced technology. A few of the industries where the use of variable area flow meters is common include the following: Food and beverage, agriculture, irrigation, car wash, and industrial water treatment: These industries require the installer and user to monitor flow accurately and economically, enabling critical process adjustments to be made. Semiconductors: Variable area flow meters are used in the manufacture and management of deionized water. Ultrapure water is used in the manufacture of printed circuitry, particularly the cleansing of computer chips. Commercial pool and spa: Flow meters measure flow though pool filtration systems. This enables operators and health inspectors to verify if an adequate amount of water is passing though the filtration system to ensure clean swim water.

Other Factors

Users of variable area flow meters must check chemical compatibility and not rely on compatibility charts. Users should perform their own testing. Many flow meter manufacturers provide kits so that the user can perform a compatibility test. Specific gravity (weight) of the liquid through the flow meter affects the instrument’s reliability and accuracy. Viscosity—the degree to which fluid resists flow under applied force—and elevated temperatures also affect accuracy. Most flow meters are manufactured and tested using water, so adjustments must be made when measuring other fluids. Accuracy and repeatability will determine the correct flow meter to purchase for each application. It is important to determine what various terminologies mean. For example, a flow meter that claims 2 percent accuracy does not tell the whole story. This could mean 2 percent of the full scale or 2 percent of the indicated flow—different accuracies that could caused confusion. Be certain to check with the manufacturer regarding accuracy claims. Repeatability is the flow meters’ ability to reproduce flow rates consistently under the same conditions. Users need to have a clear understanding of their flow-measurement requirements before purchasing a flow meter. They should consult an expert if there are any questions about which is the best for a specific application.