The 5 Main Types of Flow Sensors – and How They Work

There’s no doubt about it: flow sensors are everywhere. Across the vast industry spectrum, businesses may call them a flow indicator, a flow gauge, or a water flow meter, but what they need them for is simple – a highly reliable and accurate way to monitor, measure, record, and analyze the flow of liquids or gases. Oxygen therapy and die casting machines need them for critical operations; semiconductor manufacturers need them for production; your car uses one to monitor your brake fluid – and the list is almost never-ending.

But are all of those flow sensors the same?  Definitely not!  In fact, while there are dozens of specific meter designs out there (in fact – at any trusty supplies distributor like RS Components) there are 5 main types of flow meters that you should know about:

1. Differential pressure

A water flow meter is a perfect example of this common sensor type, but they’re also widely used by the oil and gas industry, beverage manufacturers, and chemical and mining companies. It got its name because what is being measured is a pipe’s differential pressure.  The sensor itself monitors a restriction that is engineered into a pipe.  When the restriction comes into effect, the flow meter calculates the flow rate based on a calculation called “Bernoulli’s equation” – which is static pressure + dynamic pressure = total pressure.

2. Positive displacement

A positive displacement flow meter, on the other hand, is the type that measures the amount of fluid that is passing through the meter.  It does this with the help of a chamber which is used to trap pockets of fluid between components that rotate – with the rotation basically mimicking the ordinary flow of the fluid.  To understand how this works, imagine repeatedly filling a cup with water and counting the number of times you do this as you empty the contents downstream.

3. Velocity

There are various types of velocity flow sensors, including turbine, magnetic, vortex shedding, electromagnetic, and ultrasonic meters.  They all work vastly differently – for instance, the ultrasonic flow meter injects sound signals into the water and takes readings when the signal is bounced back from particles or bubbles in the liquid, while an electromagnetic sensor uses a coil to create a magnetic field.  But what they all have in common is that their various techniques are measuring the flow’s velocity – or speed.

4. Mass flow

The mass flow meter, on the other hand, is measuring the force exerted from the acceleration of mass over a given time-span.  They’re typically used for controlling mass-related processes, like the sort of chemical reactions involved in the pharmaceutical, power, and mining industries.  Engines, however, are typically fitted with mass airflow sensors, and in this particular application, the meter simply measures the amount of air entering the engine to calculate how much fuel is needed for the air-fuel ratio.

5. Open channel

Finally, when what is being measured is an open channel of water – like a weir, for instance – the open channel meter uses radar, microwave, or ultrasonic technology to point a signal at the surface of the liquid, using complex calculations to measure the level or speed of the liquid/flow based on the time it took for the signal to return.

As you can see, the type of flow meter you require for your organization’s operations depends not only on the type of liquid/gas that is being measured, but the property of the liquid/gas that is being measured – not to mention the range, the required accuracy, the technology type, and the operating conditions.

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Interesting Related Article: “5 Factors to Consider When Learning to Measure Liquid Flow Accurately“