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.
Flow meters are generally classified by the principal that governs their operation or possibly by their applications. For instance, mass flow meters measure the flow rate of a medium in terms of it's mass.
However, there are types of mass flowmeters that use different methods to take this measurement. One example of this is the Coriolis flow meter that uses the Coriolis effect to determine the mass of a fluid passing through two vibrating tubes or pipes. The idea being that by understanding the change in the characteristics of the wave patterns in the vibrating tubes, you can correlate the flow rate of the substance passing through.
Ultrasonic and Doppler shift flow meters also use the changes caused to sound waves to correlate a rate of flow for the medium passing through the meter. In the case of the ultrasonic flow meter, two transmitters of ultrasonic sound waves are placed at the ends of a pipe where the measure is to take place. Each transmitter sends a pulse of sound.
The transmitters also act as a receiver of the wave sent by the other transmitter. One pulse of sound is sent in the direction of the flow of the liquid and the other is sent against the flow. If various constants are known about the metal of the pipe and the liquid or gas passing through the pipe, a rate of flow can be derived from the time it takes each sound wave to reach the receiver.
Doppler shift flow meters use the changes in the frequency and amplitude of a sound wave when it bounces off particulate matter in the liquid that is being measured to determine the flow rate.
A more traditional type of flow meter is the variable area flow meter that uses a float in a calibrated tube. When a substance passes through the tube, the float is displaced by the flow. 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.