Flow measurement - What you should know

Along with temperature and pressure measurement, flow measurement is one of the most important tasks in industrial measurement technology. In principle, the physical measurand flow can probably be determined with most measurement methods. In practice, the selection of the measurement method depends on the respective application. Find out more about the individual flow measurement methods and make the selection of suitable flow sensors easier!

How do you define the measurand flow?

Flow measurement determines the amount per time unit that flows through a pipe system. The medium can be a liquid, a gas, or steam. In flow measurement, a distinction is made between volumetric flow measurement and mass flow measurement.

Volumetric flow

The volumetric flow is a volume that flows through a pipe per time unit (in l/h, cfm, etc.). Most of the systems presented here measure volumetric flow.

Mass flow

Mass flow is defined as the mass that flows through a system per time unit (in kg/h, t/h, etc.). If the density of a medium is constant, the mass flow can be determined by multiplying the volume flow by the density. If the density is not constant – as is often the case with steam and gases – then it must also be measured with measurement technology.

In which applications does the flow need to be measured?

In many applications, the only requirement is to detect whether a minimum flow of a medium is occurring. The use of flow monitors is then required to provide such things as dry-running protection for pumps.

However, the sensor technology presented here is used for continuous flow measurement and is used in such applications as:

• Filling end products into bottles, containers, or tanks
• Monitoring or displaying the totalized flow rate in different processes
• In legal-for-trade systems such as fuel pumps or hot and cold water meters, etc.

What are the requirements for flow measurement?

The requirements for the flowmeter vary greatly depending on the measurement task. Good repeatability is required for filling. In legal-for-trade systems a type examination certificate is necessary (at least in the EU). Similarly, for monitoring and displaying the totalized flow rate in the process, any number of requirements exist that are related to a specific application or industry. These result, among other things, from the type of medium to be measured and the accuracy requirements, but also from legal obligations such as the Pressure Equipment Directive in the EU or explosion protection according to the ATEX directive.

Which media does a flow sensor measure?

The media is divided into 4 different categories, for which only selected sensor technology can then be used:

• Conductive liquids (>20 μS/cm): acids, bases, beer, milk, drinking water, etc.
• Non-conductive liquids: alcohols, glycols, mineral oil-based liquids, deionized water, etc.
• Gases: oxygen, nitrogen, compressed air, natural gas, etc.
• Steam: saturated steam, overheated steam

What are the measurement methods for flow measurement?

Flowmeters work according to different measuring principles, but each of them cannot measure all media of the previously indicated categories. The following table lists the common measurement methods and shows which liquid categories can be measured with them.

Green = standard     Orange = use caution        Red = not possible or recommended

The table only allows the exclusion of a measuring principle for the respective liquid category. In the video and the following descriptions you will find more detailed information on the individual measurement methods.

How do you choose the right sensor for the flow measurement?

The measuring principle must be suitable for the medium – as described above. Other requirements that a flow device must meet include:

• Operating conditions (flow range, pressure, temperature, material compatibility)
• Installation conditions (inlet/outlet sections, pipe routing, installation)
• Environmental influences (temperature, moisture, vibrations)
• Legal requirements (explosion protection, calibration obligation)

The choice is further complicated by broader practical constraints:

• Solid or gas content
• Non-optimal installation situation
• Cost versus accuracy

Due to these different challenges, expert consulting on the choice of flowmeter is essential.