Ultrasonic flow meters are available in both single and dual-sensor versions though models using two sensors are more common and may be more accurate. Ultrasonic flow meters are ideal for wastewater applications or any dirty liquids that are conductive or water-based.
Ultrasonic flow meters are ideal in situations where low-pressure drop, chemical compatibility, and low maintenance are required. However, ultrasonic flow meters will generally not work with distilled water or drinking water because they are too clean and pure in which case water flow meters are the best option. These flow meters are made out of metal such as steel when used inline.
Important considerations to keep in mind while choosing an ultrasonic flow meter include the size of the pipe, the minimum/maximum flow rate required, and the minimum/maximum process temperature and pressure needed. Also, determining whether a handheld or continuous process monitor is necessary is also important. Many meters of this kind do not need to be part of the pipe system but can be installed outside of the piping. Ultrasonic flow meters are widely used by many industries that work with free flowing liquids.
The basic principle of operation employs the Doppler Effect which is the term used to refer to a shift in frequencies when an ultrasonic signal is reflected by discontinuities in the form of suspended particles or bubbles in motion. Ultrasonic sound is transmitted through a transducer into a pipe with flowing liquids. The solid particles or air bubbles reflect the ultrasonic wave with a slightly different frequency that is directly proportional to the rate of flow of the liquid. A second transducer picks up the reflecting waves and reports the difference. This process allows the meter to get an accurate measure of the liquid’s flow rate.
Clamp-on type flow meters are another option. They measure the flow through the pipe without requiring any parts to be exposed to the stream, ensuring that corrosion and other effects from the fluid will not damage the sensors. Plastic or metal-housed transducers are placed on opposite sides of a pipe and are connected to a handheld electronic device that can provide a reading as well as trend patterns and other details.
Many designs besides clamp-on types are portable, a feature particularly useful for acting as a backup for a preexisting flow meter or for double-checking existing meters in a number of locations.
Ultrasonic flow meters may be specially adapted to measure the flow rate of gasses while others are meant for use in open channels. Flow switches may be added if a report or alarm is desired.