Flow meters are devices that are used to measure the flow rate of a certain liquid or gas. They can measure these substances in terms of the volumetric flow rate or the mass flow rate. The volumetric flow rate is generally given in m3/second. The mass flow rate is given in kg/second.

Flow meters have many applications. For individuals, a peak flow meter can measure the lung capacity. This is also used by people with asthma and can be used to determine if an asthma attack is imminent.

Other than that, flow meters are used in many industries, like the food processing industry, water management, semi conductor fabrication and many others. This article will give a brief overview of how flow meters work.

There are a wide variety of types of flow meters. They are generally classified by what they do or how they operate. For instance, a mass flow meter is a general classification of flow meter that determines the flow rate of a medium in terms of mass even though the operating principles for each meter may be different.

The Coriolis flow meter is a point in case of a principle giving it’s name to the type of flow meter. The Coriolis effect is the force exerted on a liquid moving in a circular motion. In the flow meter this is done by vibrating the pipes that the liquid passes through. The vibrations (or waves) that are created in the pipes are changed when the liquid passes through them. By knowing the changes in the wave vibrations you can work out the flow rate of the liquid.

Two other examples of flow meters that analyze and calculate changes in waves to reach a measure for a flow rate are the ultrasonic flow meter and the Doppler shift flow meter.

For an ultrasonic flow meter, flow rate is measured by contrasting the measurements of two ultrasonic waves. These two waves are set off at the same time as each other but from different directions.

The ultrasonic waves are recorded by receivers at opposite ends of the pipe. One wave is sent in the direction of the flow of the liquid to be measured. The other wave is sent in the opposite direction to the flow.

The flow rate can be determined by the difference in the timings of each ultrasonic wave provided information on the material of the pipe and some details on the medium being measured are known.

Doppler shift meters use a sound wave that bounces off matter in the medium passing through the pipe. Due to the Doppler effect, the frequency of the waves changes when it strikes a moving object. This change in the frequency can be used to determine the flow rate of the medium passing through the pipe.

A variable area flow meter is a more straightforward meter. It consists of a calibrated tube in which a liquid or gas passes through. In the tube is a float that is displaced by the substance.

By taking the reading from the calibrated tube an indication of the flow rate can be determined. This is the principle of the peak flow meter used for lung capacity and asthma.