FLOW METERS; GUIDE

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 this instrument 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 – 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. ml_s/min or m³n/h. Therefore, you can either choose for 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:  

Gas & liquid flow measurement

Some flow meters are developed for gas, some especially for liquid. There are also instruments available 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. 

3. Examples of applications

Flow meters are used in a wide variety of applications; here are some examples: 

• Used in gas chromatography
• Used in medical applications
• Used in the worldwide automotive industry
• Used in the food industry for processing sugar beets
• Used for water treatment applications

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4. How to select the best flow meter for your application?

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

There is a big difference between lab and industrial applications. but most of the considerations are required for both application fields.

Phase of the fluid: gas/liquid/vapour 

Some 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 instruments cannot measure vapors 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 
• Vapour – Vortex, Ultrasonic, Diaphragm, Floating Element 

What is the flow rate?

The flow rate is usually the most important specification to consider when selecting an instrument. 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.  

For which fluid do you use the flow meter?

Chemical and physical properties of the medium can influence the material of the instrument and therefore whether it works as expected. 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 inlet and outlet pressure?

When selecting a flow instrument, 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, 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.

Calculate delta P & valve orifice

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 instrument 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 device 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.

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5. What do you want to achieve with your instrument?

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 an instrument 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 device. 

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 sensor 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.

6. 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 sensors 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 device 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).

Communication options

Check whether you need a digital or analog device. Besides that, you 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 FlowSuite and other Bronkhorst software. 
 

Moisture

Some 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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Do you need help selecting a flow meter? We can help you.