Flow Meters

Do you know what aspects you should take into consideration when selecting a flow meter for the current project you are working on?

Before you can start conducting experiments or developing skids for your customers, it is essential that your research setup or your system is working properly. A key component herein is often a flow meter. This can be a gas, liquid or vapour instrument. To select the right flow meter, you must follow some steps to make your choice. We will guide you along your way of making the best selection. Of course, there is a big difference between lab and industrial applications but most of the considerations are required for both application fields.

But first, let’s explain a bit more about what flow meters are, how they work, what they are used for and the criteria to select the best flow meter for the application.

Flow Meter with ultrasonic and Coriolis measuring principle

Flow meters

What is a flow meter?
How does a flow meter work?
How to select the best flow meter?
- Phase of the fluid: gas/liquid/vapour
- For which fluid do you use the flow meter?
- What is the flow rate?
- What is the inlet and outlet pressure?
- What is the ambient temperature and the temperature of the fluid?
- What is the location of the flow meter?
What do you want to achieve with your flow meter?
- Performance versus price
- Flow meter accuracy versus repeatability
- Flexible use
Which process conditions can be relevant?
Examples of applications in which flow meters are used

1. What is a flow meter?

A flow meter is an instrument that measures a mass or volumetric flow rate of a gas or liquid. You might have come across a variety of terms when referring to a flow meter, such as flow sensor, mass flow meter, mass flow controller, flow regulator etc. The purpose of a flow meter basically is measuring the flow of gas or liquid between two points in a process. Sometimes controlling or regulating the flow is necessary. This is done by combining a flow meter with a valve, creating a flow controller. In this case, besides measuring a flow, you can also control it to change the flow rate. The output can help you understand your process better to make decisions regarding product quality, speed of process and cost reduction.

2. How does a flow meter work?

There are two basic types of fluid measurement – volume and mass flow measurement. The volumetric flow measurement is temperature and pressure dependent and will be shown in units of volume such as ml/min or m³/h. When measuring mass flow, you see units of mass such as kg/h or g/min. Alternatively, mass flow can be expressed as standardized volumes e.g. ml_s/min or m³n/h. Therefore, you can either choose for a volumetric flow meter or a mass flow meter for your application. Besides these two types of measurement, there are different measuring principles that all have their specific advantages and disadvantages.

The commonly used mass flow measuring principles are:

Thermal measuring principle; within this category we distinguish three sensor principles:
Coriolis measuring principle

Coriolis mass flow measuring principle

The commonly used volume flow measuring principles are:

Ultrasonic flow measurement
Vortex
Magnetic inductive
Differential Pressure
Positive displacement

Some flow meters are developed for gas, some especially for liquid. There are also flow meters available on the market that are independent of the fluid properties and therefore can handle both gas and liquids.

You can find a glossary page on our website, in which you will find a lot of terms and abbreviations that are common in the field of flow measurement.

Thermal Mass Flow Meter/Controller for Gas (by-pass design).

3. How to select the best flow meter?

In this paragraph we will discuss some of the essential elements that go into the decision-making process of selecting a flow meter. Thereby, we consider the differences between various measurement principles. Read below what to think of when selecting a flow meter.

Phase of the fluid: gas/liquid/vapour

Some flow meters can be easily eliminated because they simply will not work with the application. For instance, electromagnetic flow meters will not work with hydrocarbons and require a conductive liquid to function. Many flow meters cannot measure vapours or slurries.

Listed below are some of the main flow meter categories paired with the fluid type the meters can handle:

Gas – Coriolis Mass, Thermal Mass, Ultrasonic, Variable Area, Variable Differential Pressure, Positive Displacement, Turbine
Liquid – Coriolis Mass, Thermal Mass, Ultrasonic, Variable Differential Pressure, Positive Displacement, Turbine, Electromagnetic
Slurry – Coriolis Mass, some subsets of Variable Differential Pressure, Electromagnetic, Ultrasonic
Vapour – Vortex, Ultrasonic, Diaphragm, Floating Element

For which fluid do you use the flow meter?

Chemical and physical properties of the medium can influence the material of the flow meter and therefore the working of the instrument. Commonly, the following wetted parts (parts that are exposed to or in direct contact with the medium) may be offered:

aluminium
stainless steel
hastelloy and
monel in combination with Viton (FKM), Kalrez (FFKM) or EPDM elastomer seals

Please note that MEMS or CMOS (chip) sensors which are applied in some gas flow meters, are only suitable for a restricted number of non-aggressive gas types.

Another aspect you must consider is the viscosity of the fluid, the density and dispersion (solid content). Not all measurement technologies can be used for all fluids, for example electromagnetic flow meters can only be applied for conductive liquids.

What is the flow rate?

The flow rate is usually the most important specification to consider when selecting a flow meter. Fluid quantity can be displayed in volume, standardized volume and true mass units. The flow rate is the quantity of fluid per unit time flowing through a measuring device.

Check out the blog to find out why it is important to know what reference conditions you are working with. A supplier usually indicates the minimum and maximum full scale range of a product series. This should meet your process requirements.

BLOG: Reference conditions

What is the inlet and outlet pressure?

When selecting a flow meter, it is important to know if you need a low pressure drop or not. The pressure drop is defined as the difference between the inlet and the outlet pressure. Next to this, flow meters have a maximum operating pressure. If you have a high-pressure application, you need to take this pressure rating into consideration.

In case of mass flow control, the inlet pressure (P1) and outlet pressure (P2) are required for selection and dimensioning of the most appropriate control valve.

What is the ambient temperature and the temperature of the fluid?

The temperature of your fluid and of the instrument’s environment are the next topics on the list to check.

Variations in fluid temperature may affect the accuracy of your measurement. In case of temperature fluctuations, it could be interesting to select a flow meter with temperature compensation (e.g. the EL-FLOW Prestige flow meters).

Too high or too low environment temperatures may also harm the electronic components of your flow meter during operation or storage. When you use a flow meter in a furnace of burner application, or in areas with very low temperatures, it is important to check whether the instrument can withstand these extreme temperatures. Therefore, check the temperature specifications as provided by the supplier before selecting your flow meter.

What is the location of the flow meter?

When selecting your flow meter, you must consider where you install it. Whether it is indoors, outdoors, in a laboratory or for a particular industry. For laboratories other specifications are applicable than for the oil and gas industry.

IP-rating
NEMA
Whether you need specific certificates or approvals for the area you install the flow meter in. For example: ATEX or IECex certificated (use hazardous area) or FDA approval, etc. Check the list of certificates available for our Bronkhorst flow meters.

4. What do you want to achieve with your flow meter?

When selecting your flow meter, you need to think of what is important in your process. What do you want to achieve?

Performance versus price

The most common criteria to select a flow meter are price and performance. If you place price at the top of your criteria, you are likely to get a basic instrument, with less than average performance.

Next to the price of the component, installation, maintenance, and repairs over time should be included in calculating the total cost of ownership. How much the meter costs to operate, like its electrical consumption, can also increase the overall cost of the flow meter.

Flow Meter Accuracy versus Repeatability

The specifications of the flow meter must be taken into consideration when selecting a flow meter. Accuracy and repeatability are important specs to look at.

Flow Meter Accuracy

Accuracy is how close the measurement is to the true value. For flow meters, the measured deviations are often visualised on a calibration certificate. This is expressed as a percentage, e.g., ±1%. Not all flow meters offer the same accuracy; however, not all applications require the highest possible accuracy. Nevertheless, absolute accuracy is important in quantitative research and development or catalytic applications.

Flow Meter Repeatability

Repeatability is producing the same outcome given the same conditions. In other words, a flow meter should produce the same readings when operated under the same variables and conditions. This, too, is expressed as a ± percentage. This is, for example, especially important for burner applications.

BLOG: Accuracy vs Repeatability

Flexible use

Sometimes it makes sense to select a flow meter that can be used in multiple applications. For instance, when you need an instrument in a research project and you know that other projects will follow in the future, but you have no idea what fluids are used then. In cases like this, it can be beneficial to select a flow meter that is fluid independent and has a wide flow range as well.

In case you have an application with high fluctuations in flow rate, you probably prefer a flow meter with a high turndown ratio. Turndown ratio is also commonly referred to as rangeability. It indicates the range in which a flow meter or controller can accurately measure the fluid. In other words, it's simply the high end of a measurement range compared to the low end, expressed in a ratio and is calculated using a simple formula: Turndown Ratio = maximum flow / minimum flow. Read more about turndown ration in our FAQs.

5. Which process conditions can be relevant?

Cleaning
In the food and beverage and pharmaceutical industry, cleaning your instrumentation is important to avoid cross contamination. Clean-in-place (CIP) is a method of cleaning the interior surfaces of pipes, vessels, equipment, filters, and fittings. A typical CIP cycle consists of various steps including washing with a hot cleaning agent and hot acid with temperatures up to 95°C. Steam-in-place, also referred to as sterilization-in-place (SIP), consists of a phase in which the instrument is sterilized with saturated steam with a temperature up to 140 °C. Not all flow meters are suitable for these cleaning methods, so it’s an important factor to consider when applicable. Please also note that these markets often require the application of FDA approved seals as well.

Available space
Is space limited in your process? Then select a flow meter that is compact and does not require a straight run of pipe at the inlet or outlet. There are ultra compact flow meters on the market based on MEMS technology (e.g. the IQ+FLOW gas flow meter).

Flow Meters for gas and liquid flow measurement

Mounting of the flow meter
Before selecting a flow meter it is essential to check where to locate and how to position the instrument in your installation. The accuracy of some instruments is more affected by its mounting position than others. Other relevant aspects with respect to the mounting of flow meters may be disturbances caused by vibrations, crosstalk, pressure shocks and the effects of bends, valves and reductions of pipe diameters up- and downstream of the instruments. These effects may also vary per operating principle.

Type of communication
Check whether you need a digital or analog flow meter. Besides that, you need to know what type of communication is used in your process. Popular types of fieldbus communication are Profinet, EtherCAT, CANopen, Ethernet/IP and POWERLINK but also the more established versions such as Modbus, Profibus and DeviceNet can be integrated. There is also the possibility of using a manufacturer's own fieldbus communication, such as Bronkhorst's FLOW-BUS. This has the advantage of a simple and cost-effective network setup that can be transferred to common interfaces such as RS232, Profinet and Profibus.

Moisture
Some flow meters are more sensitive to moisture or particles than others. Appropriate filtering to protect your instruments may be a good investment, saving costs for cleaning, repair, interruption of your process and possibly also the waste of raw material or finished product.

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