​Open-channel flow meters measure water level, flow rate, and total volume in weirs, flumes, channels, and partially filled pipes.

Ultrasonic level meters operate by emitting ultrasonic waves and measuring the time for the waves to bounce back from the liquid surface.

Radar flow meters, by contrast, use microwave radar technology to determine the flow characteristics of a liquid or even a gas. These meters operate by emitting microwave pulses toward the fluid interface

The biggest difference between ultrasonic and electromagnetic flow meters is that ultrasonic flow meters do not require a conductive liquid to work. They also measure gases but fare less well with liquids that contain many solids or bubbles. Ultrasonic meters are able to withstand high temperatures and high pressure.

It cannot be used to detect vapor, steam and vapor containing a large amount of vapor. It can not be used to detect vapor materials with very low electrical conductivity, electromagnetic flowmeter can not detect petroleum products or organic solvents, etc.

They have limitations in the fluids they can accurately measure. Heavily contaminated fluids and slurry flows can result in the ultrasonic waves being unable to pass though the fluid, rendering the meter incapable of accurately measuring the flow.

Electromagnetic flowmeter is a commonly used measuring tool in water purification systems and dosing systems to detect the volumetric flow rate of conductive liquids. It works based on Faraday's law of electromagnetic induction: when a conductive fluid flows perpendicular to the direction of the magnetic field, an electromotive force is generated i