There are two distinct kinds of flow that are measured by fluid flow meters. The first is open channel flow, which occurs when a stream driven by gravity is flowing down a path that's exposed to the atmosphere. Examples of open channel flow would include canals, sewers, and drains that are not full or overflowing. Closed conduit flow occurs when the fluid is being pushed through an enclosure (i.e. - a pipe) via pressure differences. Examples of closed conduit flow would include water supply systems and pipes that heat water.
Now that you have a basic understanding of the two flow types, here's a brief overview of the different kinds of meters that can be used to measure fluid flow:
List of flow meter types
Although describing the precise operation and applications of each meter would go beyond the scope of this basic resource, we've listed all of the different flow types below as a starting point to your research and comparison:
• Orifice Plates – One of the simplest and cheapest options that can be applied to almost any situation.
• Venturi Tubes – Best for use in applications with higher turndown ratios.
• Flow Nozzles – Commonly used to measure gas and air flow in industrial settings.
• Rotameters - Also called a variable flowmeter, known for highly accurate full-scale ratings.
• Pilot Tubes – Cheap and common method for measuring air flow in HVAC and ventilation systems.
• Calorimetric - Achieves high accuracy even at low flow rates
• Turbine - Incredibly accurate with a turndown ratio of 100:1. More expensive, typically used industrially.
• Vortex - For use in specialised scenarios.
• Electromagnetic - Can only be used to measure the flow of electrically conductive fluids (i.e. water).
• Ultrasonic Doppler - Uses sound waves to detect fluid flow.
• Positive Displacement - Good for non-abrasive materials like oils, fats, and inks.
• Mass - Directly measures the flow of fluid mass. Coriolis is a related type of flowmeter.
• Open Channel - Measures the height of the fluid as it flows over an obstruction.
Fortunately, you can compare all the above meter types within the catalogs of major flowmeter providers like Universal Flow Monitors. Contacting the manufacturer and describing the desired application may also help you obtain a useful recommendation for your project.
What to consider when choosing a flow meter
The most important factors to consider when selecting a flowmeter are accuracy, operating requirements, price, and maintenance needs. Those looking to incorporate the meter into a newly invented commercial product may also face certain legalities and technicalities to gain approval for such an application.
Relevant topics to study
In closing, now that you're aware of the different types, it would help to become familiar with the main metering principles that are used by each of the above flowmeters. Process control is another interesting topic that will improve your ability to utilize, invent, setup, and implement a wide range of conventional and hybridized meters. Ultimately, studying fluid flow meters is an inevitable step for any engineer who is researching for a project that involves the measurement of fluid flow.