How Flow Meters Work

July 08, 2015

A flow meter is a device that can be used in order to measure the ‘flow’ of gas or liquid through a specific vessel (such as a pipe for instance). ‘Flow’ is a term which in this context can mean the amount, speed or force of that substance as it passes through.

Flow meters can be used for a variety of different applications and are found in numerous industries. These include everything from plumbing, to maritime instrumentation to power plant facilities.

Due to their versatile nature, there are a number of different types of flow meters each of which works slightly differently and has various strengths and weaknesses. Here we will look at the different kinds and at how they each operate.

Positive Displacement Flow Meters

A positive displacement flow meter is a flow meter that is used in order to measure the volume of a substance passing through a pipe or something else as opposed to the speed or the force. As mentioned, the precise type of ‘flow’ being measured does vary from case to case.

The way in which positive displacement flow meters work is quite clever and inventive. Here, there will be an empty space attached to the pipe which has a set volume that is known. Volume here is the amount of ‘space’ that something takes up in total. It is different from ‘size’ in that it doesn’t include hollows and it is different from ‘mass’ in that weight and density don’t measure a factor. Volume then is the amount of space that something like a liquid or gas that can ‘spread out’ will fill.

Thus, by having a hollow space, it is possible to see how quickly this gets filled by the substance passing through and thus to make an accurate measurement of the volume. As the liquid passes through, a positive displacement flow meter will measure how long it takes for the space to be filled and this will provide a unit that corresponds to the volume of the flow.


With force flow meters, you are measuring the amount of force that is being generated by the liquid or gas as it travels. This is normally achieved by using something to slightly block the process of the liquid or gas which can then act as a switch. While the gas or liquid can still pass through, a certain amount of force is required to flip that switch and this can then be used to measure the force generated in increments. This can be considered somewhat similar to ‘pressure’ as it related to biology or plumbing.


There are many other types of flow meters and ways of measuring flow. Velocity flow meters measure the speed for instance, while clamp-on ultrasonic flow meters use ultrasonic sensors in order to ascertain dimensions of flow – by bouncing sound waves and seeing how they respond.

Flow is just one example of a crucial variable in all kinds of machine maintenance and diagnostics and flow meters are but one useful tool for measuring those factors.

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