Which flow measurement principle should you choose?

Flow measurement starts with the medium

When choosing a flow meter, knowing the pipe dimensions or the desired measuring range is not enough. The choice depends largely on what needs to be measured. Water, wastewater, chemicals, oil, solvents, gas, steam and aggressive media place different demands on the measurement principle.

Some flow meters require the medium to be electrically conductive. Others are better suited to gas, steam, large pipes, extreme temperatures or media containing particles and air bubbles. Flow measurement should therefore always be assessed based on both the properties of the medium and the specific application.

The most widely used flow measurement principles

Electromagnetic flow meters are often an obvious choice for conductive liquids such as water, wastewater and chemicals. They are easy to install, have no moving parts and are virtually maintenance-free. The measurement principle is based on Faraday’s law of induction, with the medium acting as the electrical conductor moving through a magnetic field. Electromagnetic flow meters can measure with an accuracy of as low as 0.25% of the measured value.

Variable area flow meters, also known as VA flow meters, are mechanical meters that do not require a power supply. They typically consist of a tapered measuring tube with a scale and a float whose position varies according to the flow of the medium. VA meters can be used for both gases and liquids and are often a cost-effective solution. Accuracy is normally around 1.5-3% of full scale.

Vortex flow meters are a versatile alternative because they do not require the medium to be electrically conductive. They can therefore be used for applications involving oil, solvents, liquids at extreme temperatures, gas and steam. The measurement principle is based on vortices formed behind a body in the process stream. The frequency of the vortices is proportional to the flow velocity.

How to choose the right measurement principle

The right flow measurement principle depends on the conditions in which the meter will operate. If the medium is a conductive liquid, electromagnetic flow measurement can be a strong choice. If a simple mechanical solution without a power supply is needed, a VA meter may be appropriate. If the medium is not electrically conductive, or if the application involves gas, steam or high temperatures, vortex may be more suitable.

Ultrasonic flow meters can be particularly useful for large pipes and aggressive media. There are two main principles: the Doppler principle, which is used for liquids containing particles or air bubbles, and transit time, which is used for clean, homogeneous liquids. Doppler typically measures with an accuracy of 3-5%, while transit time is typically around 0.5-3%.

Mass flow meters are used when direct measurement of mass flow is required, for example in kg/h. This can be an advantage for media whose density depends on temperature. Thermal mass flow meters are typically used for gas measurement and can be used for applications such as process control, consumption metering, supply monitoring, leak detection and distribution network monitoring.

When choosing a flow meter, you should clarify the following in particular:

• Whether the medium is a liquid, gas or steam
• Whether the medium is electrically conductive
• Whether the medium contains particles or air bubbles
• Whether the liquid is clean and homogeneous
• Whether extreme temperatures are involved
• Whether measurements are taken in large pipes
• Whether mechanical measurement without a power supply is required
• Whether the measurement should be volumetric flow or mass flow
• What level of accuracy is required
• How much maintenance the meter can be allowed to require

More accurate measurement requires the right match

Flow measurement is not just about choosing a meter that can technically detect flow. It is about choosing the principle best suited to the medium, operating conditions and intended measurement purpose. Choosing the wrong flow meter can lead to inaccurate measurements, unnecessary maintenance or a solution that does not perform optimally in the application.

Coriolis meters are an example of a highly accurate mass flow solution, particularly useful in sectors such as life sciences, food processing and chemical manufacturing. They can be used for a wide range of liquids and media, including cleaning agents, solvents, crude oil, vegetable and animal fats, alcohol, fruit solutions, ketchup, mayonnaise, gases and liquefied gases.

The best solution is therefore found by combining technical knowledge of measurement principles with specific insight into the process. Choosing the right flow meter from the outset makes it easier to achieve stable operation, valid measurements and a solution that meets the application’s requirements.