Flow Meters – How Do They Work?

A flow meter works by measuring the amount of a liquid, gas, or steam flowing through or around the flow meter sensors. Flow meter sensors work in different ways, but with the same end goal: Provide the most accurate and repeatable flow measurements for a specific application, whether for process control, general research activities, or semiconductor processing.

Flow meters measure either volume or mass. The flow (Q) is equal to the cross-sectional area of the pipe (A) in a volumetric flow meter, and the velocity of the flowing fluid (v): Q = A * v. The mass flow can be expressed in a mass flow meter as follows: ṁ = Q ∗ρ (where Q is the volumetric flow rate and ρ is the fluid density).

Mass flow rate is the key consideration in many cases, especially in combustion, chemical reactions, or buying and selling gases.

How Different Types of Meters Work

  • Capillary Thermal Mass – Uses the thermal heat transferred between gas passing through a very small tube (the capillary) and a set of resistance temperature detectors (RTDs) to measure the mass flow of the gas.

Capillary Thermal Mass

  • Immersible Thermal Mass – Uses the thermal heat transferred from a heated sensor immersed in the flow to measure the mass flow of the gas.

Immersible Thermal Mass

  • Vortex Shedding – Measures mass and/or volumetric flow of liquids, gases, and steam using vortices shed from a sensor immersed in the flow and a computed fluid density.

Vortex Shedding

  • Transit-Time Ultrasonic – Measures the speed of a fluid passing through a pipe using ultrasound and can be fitted onto the outside of the pipe, thus eliminating the need for pipe cutting.

Transit-Time Ultrasonic

How Will the Flow Meter Be Used?

The next step is to find out what the flow meter is being used to measure:

  • Costly products that need to be measured very precisely?
  • Liquid, gas or steam?
  • Very large flows or only small flows?
  • Low or high pressure and temperature of the liquid, gas, or steam in the pipe?
  • Liquids, gases, or steam flowing in complicated or big piping arrangements?

The selection of the best flow meter working technology is based on the answer to the above questions. Certain flow meters work well when measuring different things.

  • Vortex shedding flow meters are ideally suited for measuring the flow of liquids and steam at high temperatures and pressures
  • Mass flow of gases from low to high flows can be precisely measured using thermal mass flow meters
  • Transit-Time Ultrasonic flow meters are ideally suited for measuring the flow of water or other liquids when no process shutdown or pipe cutting is ideal
  • Older technologies such as turbine meters, positive displacement, or differential pressure also have their place


This information has been sourced, reviewed and adapted from materials provided by Sierra Instruments.

For more information on this source, please visit Sierra Instruments.


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