Flow meters and flow measurement have been around for thousands of years, evolving as farming, irrigation and water conservation incorporated new technology and became an even more integral part of civilization.
Rudimentary flow meters were evident as far back as the early Roman era. The flow meter was enhanced in the 17th century when the mathematical foundations of flow theory evolved, and today the variety of available flow meter styles has grown enormously from those early beginnings.
Examples of flow meter types include variable area, positive displacement, ultrasonic and mass flow meters. Each type of flow meter has unique features that fit with the corresponding application and industry, but how can a user know which flow meter is right for them?
Tapered Tube
The flow meter body is a vertically tapered tube. Fluid passes from the smaller end to the larger end of the taper. As fluid flows through the tube, it forces an indicator (float) upward. The clearance space (annular area between the float and the tube wall) between the float and the tube increases as the float moves toward the top of the meter. The larger clearance area toward the top requires a larger amount of fluid to force the float higher. The greater the flow, the higher the float is raised.
By varying the taper of the tube, as well as the mass of the float and the length of the tube, different flow ranges can be calibrated. A scale with flow increments or measurements is typically printed next to or on the tube. The flow rate can be read by matching the increments on the calibrated scale with the edge of the float.
Proper installation of a variable area flow meter is critical to performance. The meter must be plumbed into the piping system with the narrow part of the taper at the bottom (inlet).
Industrial Uses
A few of the industries where the use of variable area flow meters is common include food and beverage, agriculture, irrigation, car wash and industrial and municipal water treatment industries. These industries require the installer and operator to monitor flow accurately and economically, thus enabling critical process adjustments to be made.
Variable area flow meters, which are manufactured so that there is no metal in the fluid path, are a good option in applications where the manufacturing and management of deionized water is done. Ultrapure water is used extensively in the manufacturing of printed circuitry.
Compatibility
The variable flow meter user must be certain to check chemical compatibility. Rather than relying solely on compatibility charts, the user should perform their own testing. Many flow meter manufacturers are willing to provide material sample kits so the user can perform a compatibility test.
The specific gravity (weight) of the liquid through the flow meter affects flow meter 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 & Repeatability
Accuracy and repeatability will determine the correct flow meter for each application. It is important to determine what various terminologies mean. For example, a flow meter that claims 2% accuracy does not tell the whole story. This could mean 2% of the full scale or 2% of the indicated flow, which are different measures of accuracy and could cause confusion. Be certain to check with the manufacturer regarding accuracy claims.
Repeatability is the flow meter’s ability to reproduce flow rates consistently under the same conditions. Repeatability is crucial in the processing industry, where tracking flow changes is essential. It is best to have a clear understanding of flow measurement requirements before deciding on a flow meter. Consult an expert if there are any questions about which is the best flow meter for an application.
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