Flow Meters

A flow meter or flow sensor is an instrument used in almost all mechanical, electrical and instrumentation processes to measure the flow rate of a liquid or gas. In general, there are three primary types of flow meters (flowmeters):

• Mass flow meters measure flow rate in terms of the mass of the fluid substance and have units such as lbs/min.
• Volumetric flow meters measure flow rate in terms of the quantity of material that is flowing and use units like mL/min.
• Velocity flow sensors and flow meters measure flow rate in terms of how quickly the gas or liquid is moving. Velocity meters use units like ft/sec.

Methods of measuring the flow

Methods of measuring flow have advanced through several stages.

• The basic mechanism is the use of rotating anemometers, which are submerged in the flowing material.
• A Venturi Tube is another apparatus that narrows the flow of fluid and measures the differential pressure that occurs due to the contraction. This mechanism is used in gas pipelines to measure the conduction.
• Yet other simple technique to measure the flow rate is to place a plate with a hole in the path of the flowing fluid so that it narrows the stream. This plate is called an Orifice Plate and works on principles similar to a Venturi Tube.

Types of flow meters

• Variable area flow meters, more commonly known as Rotameters, are used to measure the linear flow of gases and liquids. It is simple to use, has high measuring range and is inexpensive.
• Mass gas flow meters are less affected by density, pressure, and fluid viscosity. They use differential pressure transducers and temperature sensors.
• Ultrasonic Doppler flow meters are used in slurries and wastewater applications. They use the Doppler effect principle to measure the flow rate.
• Positive displacement flow meters are used to measure the flow rate of fluids in areas where straight piping is not possible.
• Vortex meters are probably the most widely used flow meters. These are particularly insensitive to changes in process conditions. Additionally, they have long shelf life and easier to maintain.
• Pitot tubes are used in liquid and gas applications. The have a low installation and maintenance cost and don't wear out easily.
• Magnetic flow meters are ideal for use in conductive liquids, especially in wastewater applications. Remote monitoring is possible in magnetic flow meters.

Considerations for selecting a flow meter

An unambiguous understanding of your requirements will greatly help you choose the best flow meter for your application. Time should be invested in thoroughly understanding the use, installation and maintenance of a flow meter. Evaluate which features you need, as well as which features you want. Consider the following factors when choosing a flow meter:

• Flowing material: The most important aspect to contemplate is the flowing material, whether it is air, water or any other gas or liquid. Take care that the selected flow meter material will not react with or affect the flowing material.
• Accuracy, Sensitivity & Precision: Before the installation check the accuracy, sensitivity, and precision of the flow meter. Ensure that the flow meter is properly calibrated.
• Output type: The type of output desired is also a very critical factor when selecting a flow meter. The output can be digital or analog to be used locally or transmitted to a remote location.
• Limits of flow meter: Inspect the minimum and maximum permissible values of flow rate, temperature and pressure for a particular flow meter. Confirm that these values fit into the scope of your application.
• Area of installation: The area of installation also affects the selection process. Consider the layout and piping details prior to the choice of a flow meter. It helps you decide on the type and size of flow meter that satisfies your conditions. Vibrations and magnetic fields in the application area could also affect flow meter readings.