Water Flow Meters – How They Work

A water flow meter is an instrument capable of measuring the amount of water passing through a pipe. Several water flow meter technologies are available for selection depending on the water measurement applications, budgetary terms, and maintenance requirements. Each of these water flow meter types has a unique principle of operation, overall cost-of-ownership, and specific application benefits.

The question is what water flow meter should users select for their water flow measurement application? There are four major types of water flow meters: mechanical water flow meters,ultrasonic flow meters, vortex volumetric flow meters, or magnetic flow meters.

Four Types of Water Flow Meters

Mechanical Flow Meters

They are the most common and economical type of water flow meters which perform flow measurement through turbine rotation with a shunt, propeller, or paddle wheel design. The mechanical types of water flow meters work by measuring the speed of water flowing through the pipe that causes a piston or turbine to rotate. The volumetric flow rate of the water is proportional to the rotational speed of the blades. The disadvantage of mechanical water flow meters for water measurement is that they may clog up when the water is dirty or contain larger particles, which leads to increased maintenance costs. Mechanical water meters also do not work well when the water flow is low.

Vortex Flow Meters

Vortex water flow meters use vortices shed from a sensor immersed in the flow. Vortices are forces of nature, “swirls” produced when a fluid moves past an obstruction, like the wind past a flagpole or water flowing around a rock in a stream. In a vortex meter, a sensor tab flexes from side to side when each vortex flows past, creating a frequency output that is directly proportional to the volumetric flow rate. Multivariable vortex flow meters can measure up to five process variables with one process connection: temperature, pressure, density, mass flow, volumetric flow rate. Insertion vortex meters work well on very large pipes as they can be inserted into the flow by hot tapping with a retractor.

Ultrasonic Flow Meter

Ultrasonic water flow meters measure the speed of fluid passing through the pipe using ultrasound to measure the volumetric flow. In a transit-time ultrasonic liquid flow meter, an ultrasonic signal is transmitted in the direction of the flowing fluid downstream, and then another signal is transmitted against the flowing fluid upstream. In its most basic form, the time for the sonic pulse to travel downstream is compared to the time for the pulse to travel upstream. Using this differential time, the velocity of the following fluid is calculated. Then the meter calculates the volumetric flow rate in the pipe using this fluid velocity. In addition, BTU energy measurement can be derived from the volumetric flow rate and the temperature difference between the hot and cold legs. Clamp-on ultrasonic meters can measure water from outside of the pipe by shooting pulses of sound through the pipe walls, thus having application flexibility and making them suitable for water flow measurements in large pipes.

Magnetic Flow Meter

Magnetic flow meters measure the speed of a fluid passing through a pipe using a magnetic field to measure the volumetric flow. They are based on the principle of Faraday’s Law of Electromagnetic Induction, according to which liquid generates voltage when it flows through a magnetic field. The more rapid the fluid flow, the more the voltage generated. The voltage produced is directly proportional to the water movement; the voltage signal is processed into the volumetric flow rate by the electronics. Since the magnetic flow meters show an intermediate accuracy, they are not suitable for custody transfer applications and cannot be used to measure pure water as there are no ions to measure.

Conclusion

The best type of water flow meter to use depends upon the specific application. For instance, some flow meters work well when measuring different things. To learn more, customers are advised to read white papers, watch videos and use selection tools to identify the right Sierra flow measurement product for their liquid and water flow application.

This information has been sourced, reviewed and adapted from materials provided by Sierra Instruments.

For more information on this source, please visit Sierra Instruments.