A Guide to Flow Meters for Water, Gas and Mass Flow
Choosing the right flow meter involves more than just knowing the pipe size and the required measuring range. The medium to be measured, temperature, pressure, accuracy requirements, installation and the type of flow data required all influence which measuring principle is most suitable.
Flow meters are used to measure, monitor and control flow rates in, for example, the process industry, water treatment, cooling systems, dosing and energy applications. At Swelex, we help you select flow transmitters based on factors such as the medium, flow range, pipe size, pressure, temperature, accuracy and installation conditions.
In this guide, we look at the differences between flow measurement for water and gas, and mass flow, as well as the factors that are important when selecting a flow meter
What is a flow meter?
A flow meter is used to measure how much liquid or gas passes through a pipe or a process over a given period of time.
Depending on the application, the measurement can be used for continuous monitoring, process control, dosing, consumption measurement or to detect anomalies in a system. We offer solutions for both continuous and intermittent flow measurement of liquids, gases and neutral media.
It is common to distinguish between two main ways of specifying flow:
Volume flow describes the volume that passes through over a given period of time, for example, litres per minute or cubic metres per hour.
Mass flow, on the other hand, describes the amount of mass that passes through over a given period of time, for example, kilograms per hour.
The type of measurement that is most relevant depends on the process and how the measured value is to be used.
Flow meters for water and other liquids
When measuring water and other liquids, it is important to take into account both the properties of the medium and the installation. Factors such as pipe size, flow rate, temperature, pressure and accuracy requirements influence the choice of flow meter.
Electromagnetic flow measurement
Electromagnetic flowmeters are used for electrically conductive liquids and have no mechanical parts in the measuring mechanism itself.
They are therefore suitable for use in many water and process applications where continuous flow measurement is required.
Ultrasound measurement
Ultrasonic flow meters use sound waves to determine the flow rate.
One advantage of so-called clamp-on systems is that the sensors can be fitted to the outside of the pipe, which means that the pipe does not need to be cut open to install the sensors themselves. Siemens describes clamp-on technology as a non-contact solution for both liquids and gases and highlights that it can be installed externally on existing pipes.
This can be useful, for example, during retrofitting or in situations where it is important to avoid interrupting the process.
Differential pressure-based flow measurement
Flow can also be calculated by measuring the pressure difference between two points in a system. We supply differential pressure transmitters and instruments used, amongst other things, for flow measurement in ventilation and process applications.
Gas flow meter
Gas measurement involves requirements that differ in some respects from those of liquid flow measurement. The density of a gas is affected by factors such as pressure and temperature. The measurement method must therefore be selected on the basis of both the properties of the gas and the specific information required by the process.
When selecting a gas flow meter, it is therefore important to specify:
- Which gas is to be measured
- Expected flow range
- Operating pressure
- Temperature
- Pipe size
- Requirements for accuracy
- Whether to measure volume flow or mass flow
- What signal or communication does the control system require?
For simpler air and ventilation systems, differential pressure may be a suitable measurement principle, whilst other industrial gas applications may require direct mass flow measurement or another measurement technique.
What is a mass flow meter?
A mass flow meter measures the mass of a medium passing through the system per unit of time.
This may be particularly relevant in processes where density varies or where the quantity of material is more important than the volume.
A common example is Coriolis measurement. A Coriolis flowmeter uses vibrating measuring tubes to measure mass flow directly and can be used for both liquids and gases. Some systems can simultaneously measure, for example, density and temperature.
Mass flow measurement may be relevant in areas such as:
- Dosage
- Chemical processes
- Gas distribution
- Process control
- Mixture
- Testing and laboratory equipment
Volume flow or mass flow – which should you choose?
The choice should be based on the medium and the process to be measured. The most important thing is to consider what the measured value will be used for.
Volume flow is often suitable when:
- The volume itself is relevant
- The density of the medium is relatively stable
- The measurement is used, for example, for water flow, cooling or circulation
Mass flow may be preferable when:
- The actual quantity of material is crucial
- The density may vary
- A high degree of accuracy is required in areas such as dosing or process control
- Gas or liquid needs to be measured independently of changes in volume
How to choose the right flow meter
Our industrial transmitters can be integrated with control systems via, for example, 4–20 mA, 0–10 V, pulse or digital communication. To specify the correct flow transmitter, you should start with the application rather than the product itself.
1. Which medium is to be measured?
Is it water, another liquid, air or a specific gas?
The conductivity, density, viscosity and chemical properties of the medium can influence which measurement principles and materials are suitable.
2. What flow range is required?
A meter should be selected so that the normal flow rate falls within its specified measurement range.
Please also take into account both the minimum and maximum expected flow rates.
3. What level of accuracy is required?
Not all applications require the same level of precision.
Simple flow monitoring may have different requirements to dosing, laboratory measurement or process control.
4. What pressures and temperatures are involved?
The sensor must be designed to withstand the maximum pressure and temperature of the process.
5. What does the installation involve?
Pipe dimensions, straight runs, installation position and available space can all affect both the choice of product and the measurement result.
In some existing systems, for example, an external ultrasonic transducer may be of interest, as clamp-on sensors can be fitted to the outside of the pipe.
6. What output signal is required?
The flow meter must be able to communicate with the system that will use the measured value.
Quick guide – which type of flow measurement is right for you?
| Application | Important points to bear in mind | Possible measurement principle |
|---|---|---|
| Water flow | Pipe size, conductivity, flow range | Electromagnetic or ultrasonic |
| Cooling water | Temperature, pipe size, installation | Electromagnetic, ultrasonic or other volume flow measurement |
| Air and gas | Pressure, temperature, density | Differential pressure or mass flow |
| Dosage | High repeatability and accuracy | Mass flow measurement |
| Existing pipes | Minimise disruption to the installation | Clamp-on ultrasound |
| Process fluids | Material compatibility and process pressure | Adapted to the medium |
| Direct mass flow | Mass rather than volume | Coriolis |
Frequently asked questions about flow meters
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Which flow meter should I choose for water?
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Can the same flow meter be used for both water and gas?
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What is the difference between a flow sensor and a flow meter?
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What is the difference between volume flow and mass flow?
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When is a mass flow meter required?
Do you need help choosing a flow meter?
The right flow meter depends on both the process and the technical requirements. We can help you select flow sensors and flow meters based on factors such as the medium, measurement range, pipe size, pressure, temperature, accuracy, connection type and installation conditions. The company also offers guidance on integration with existing systems and more customised solutions.