What is a flow meter? A complete guide

How to start selecting a flow meter

First you need to know what aspects you should consider when selecting a flow meter. Before you start to select the right flow meter, it is necessary to know what the purpose is of your application. Are you measuring gas, liquid or vapour?

But, let’s start by explaining 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.

What is a flow meter?
How does a flow meter work?
How to select the best flow meter?
What do you want to achieve with your flow meter?
Which process conditions can be relevant?

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 is basically to measure the flow of gas or liquid between two points in a process. Sometimes it is necessary to control or regulate the flow. 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 better understand your process to make decisions regarding product quality, speed of process and cost reduction.

How does a flow meter work?

There are two basic types of fluid measurement – mass and volume flow measurement. The volumetric flow measurement is temperature and pressure dependent especially for gas 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, because gas is compressible, it is more convenient to express mass flow as standardized volumes, e.g. mls/min or m3n/h. Therefore, you can either choose a mass flow meter or a volumetric flow meter for your application.

Besides these two types of measurement, there are different measuring principles that all have their specific advantages and disadvantages:

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

How to select the best flow meter for your application?

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.

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

In the case of mass flow control, the inlet pressure (P1) and outlet pressure (P2) are required for the 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 the instrument’s environment are the next factors to check.

Variations in fluid temperature may affect the accuracy of your measurement. In the 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 or burner application, or in areas with very low temperatures, it is important to check whether the instrument can withstand these extreme temperatures. So, 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 will install it, whether indoors, outdoors, in a laboratory or for a particular industry. For laboratories, other specifications apply than for the oil and gas industry.

IP-rating
NEMA
Whether you need specific certificates or approvals for the area in which you are installing the flow meter. For example: ATEX or IECex certificated (use hazardous area) or FDA approval, etc.

What do you want to achieve with your flow meter?

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

Performance versus price
The most common criteria for selecting a flow meter are price and performance. If you place priority on price, you are likely to get a basic instrument, with less than average performance.

Besides the price of the component, installation, maintenance, and repairs over time should also be included in calculating the total cost of ownership. How much the meter costs to operate, such as 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 considered when selecting a flow meter. Accuracy and repeatability are important specs to look at.

Flexible use
Sometimes it makes sense to select a flow meter that can be used in multiple applications. For example, 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 such case, it can be beneficial to select a flow meter that is fluid independent and has a wide flow range as well.

If you have an application with high fluctuations in flow rate, you will probably prefer a flow meter with a high turndown ratio. Turndown ratio is also commonly referred to as rangeability. This 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.

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 sterilisation-in-place (SIP), consists of a phase in which the instrument is sterilised with saturated steam with a temperature up to 140 °C. Not all flow meters are suitable for these cleaning methods, so this is 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).

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 may be 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.

Communication options
Check whether you need a digital or analog flow meter. You also need to know what type of communication is used in your system. 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. Using FLOW-BUS has the advantage of a simple and cost-effective network setup that can be used with Bronkhorst FlowSuite and other Bronkhorst software.

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.

So finally, understanding how a flow meter works, selecting the best instrument for your specific application, and understanding the purpose of your application are the key factors in optimizing your flow measurement process.

Selecting the best flow meter involves crucial factors, such as the specific fluid properties, flow rate requirements, fluid temperature, inlet and outlet pressure as well as environmental conditions. Besides that, process conditions like cleaning requirements, available space, assembly considerations and communication options should also be taken into account.