For fluids with higher viscosity than water, magnetic and ultrasonic meters are popular choices because they are immune to viscosity changes. Since these meters do not have internal parts which can disrupt the flow, they also produce very small pressure drops. As a result, they can handle fluids with debris and particulate such as wastewater and cement or pulp slurries.
Transit Time ultrasonic flow meters are used to measure clean fluids. The meters require two sensors, one upstream and one downstream. An ultrasonic wave is sent through the fluid from one transducer to the other and the travel time of the wave is electronically measured. Another wave is then sent from the second transducer to the first and its travel time is also measured.
The difference in travel time between the opposing waves is proportional to the fluid velocity. An electronic converter uses the difference to calculate a flow rate.