Chris King
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You would think that measurements of mass flow would be expressed in units of mass, such as grams/hour, milligrams/second etc. Most users, however, think and work in units of volume. That’s OK, at least when we are talking about the same reference conditions. Let me start with an example:

Mass versus Volume

Imagine you have a cylinder of 1 liter, which is closed by means of a moveable piston of negligible weight. This cylinder contains 1 liter of air at ambient pressure, approximately 1 bar. The weight of this volume of air at 0°C is 1.293 g, this is the mass.

When we move the piston half way to the bottom of the cylinder, then the contained volume of air is only ½ liter, the pressure is approximately 2 bar, but the mass hasn’t been changed, 1.293 g; nothing has been added, or left out.

According to this example, mass flow should actually be expressed in units of weight such as g/h and mg/s. Many users, however, think and work in units of volume. This not a problem, provided conditions under which the mass is converted to volume are agreed upon.

Using density in converting mass flow to volume flow

In order to use density in converting mass flow to volumetric flow, we must pick a set of specific pressure and temperature conditions at which we use the density value for the gas.

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The conditions that are agreed upon contain various references, normal reference and standard reference, available in European or American style. What is de difference?

Normal reference, European style

Following the ‘European’ definition, a temperature of 0°C and a pressure of 1,013 bar are selected as ‘normal’ reference conditions, indicated by the underlying letter “n” in the unit of volume used (mln/min or m3n/h). The direct thermal mass flow measurement method is always based on these reference conditions unless otherwise requested.

An example conversion to volumetric units using Normal reference conditions: The mass flow meter indicates 100 g/h of Air flow. • Density Air (@ 0°C) = 1.293 kg/m3 • X ln/m Air = 100 g/h / (60 minutes x 1.293 kg/m3) • Flow = 1.29 ln/m Air

Standard reference, European style

Alternatively, a temperature of 20°C and a pressure of 1,013 bar are used to refer to ‘standard’ reference conditions, indicated by the underlying letter ‘s’ in the unit of volume used (mls/min or m3s/h).

An example conversion to volumetric units using Standard conditions: The mass flow meter indicates 100 g/h Air flow. • Density Air (@ 20°C): 1.205 kg/m3 • X ls/m Air = 100 g/h / (60 minutes x 1.205 kg/m3) • Flow = 1.38 ls/m Air

If the prefix ‘s’ has been used, it refers to the American style.

Standard reference, American style

According to the ‘American’ definition the prefix ‘s’ in sccm, slm or scfh refers to ‘standard’ conditions, 101.325 kPa absolute (14.6959 psia) and temperature of 0°C (32°F).

Please be aware of the reference conditions when ordering an instrument. ‘Normal’ and ‘Standard’ can be relative to each customer.

Why is this important? Because mixing up these reference conditions causes an offset in what the customer expects to see by greater than 7%!

Learn more about the technologies of Bronkhorst and read more about the Mass Flow Theory

Table with the normal- and standard references, divided into European- and American style

Normal vs standard conditions

Hans-Georg Frenzel
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We all love cake, there’s no denying that, and especially with whipped cream. A celebration isn’t a celebration without a cake, whether you’re throwing a birthday party or attending a wedding. Or just when you’re having coffee with friends or family, simply because it’s delicious. Baking and decorating a cake takes a lot of time and patience. And that’s exactly why most choose the easy way of picking out an already finished piece , either from their local pastry chef or out of the freezer section at the supermarket. And today I would like to tell you something about how such a fancy cake is made.

Manufacturing the cake layers

It all starts with the base, which consists of one or more layers of cake that provide support to the whipped cream. These layers are factory produced, but they aren’t made in individual round spring forms. The dough is applied on a closed metal conveyor belt by using nozzles. This belt goes through an oven and at the end the individual shapes with the desired diameter are cut out of the dough.

Controlling air by using mass flow controllers

To make sure that these cake layers all have the same weight and consistency, foam technology is used in addition to the baking agent in the dough. In this case, a foam mixer generates a dispersion of dough and air which is then applied onto the baking steel belt. In this process, it is highly important that this dough always has the same consistency, density and quality. Thus it is not only necessary to control the delivery rate of the dough, but just as important is the amount of air. By using the Bronkhorst EL-FLOW Select mass flow controllers, precise control of the required air volume is ensured at all times throughout the whole process.

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Whipped cream

In cake decoration, the cake layers are covered with whipped cream and other sweet fillings. To produce whipped cream out of liquid cream, another foam mixer is used in combination with Bronkhorst mass flow controllers, proving their worth yet again by achieving continuous high accuracy and precise control. The whipped cream production is similar to dough production; however the requirements for this system are different.

Hygiene requirements: Cleaning in Place - CIP

In food production, high hygiene requirements apply. In the dough production process, the mixer is cleaned of residues by CIP (Cleaning In Place) using cleaning additives, guaranteeing a hygienic product. However, in whipped cream production, it’s highly important that all product-contacting surfaces in the foam mixer are clean and absolutely germ-free, since it’s a sweet dairy product and the cake needs to be preserved for a long period of time. This asks for even higher hygiene requirements, so these machines need a different cleaning approach. Using only CIP with cleaning additives can’t guarantee this, so they have to be sterilized in place (SIP) as well. Using a saturated steam at a temperature of 130° Celsius, the product area of the machine is thoroughly cleaned. This maintenance takes around 300 seconds to make sure all germs are killed. This gives the cake a longer shelf life when stored in the refrigerator or freezer.

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Hansa Mixer

Hansa Industrie-Mixer is a worldwide, medium-sized company that operates in the field of mixing machines and foam generators for the food and non-food industry. Technical equipment before and after the foam mixer is also included in the scope of delivery to the customers. These are not mass-produced products, but every system is customized and tailored to the needs of the customer. If you want to differentiate yourself from the competition, you need a custom-made machine and system. The heart of the foam mixer is a mixing head that uses the rotor/stator principle. Rotor and stator are fitted with rings of pins which are able to pass the pins on the opposite side when the rotor rotates in the stator. The generated turbulence and shear forces produce a fine dispersion from a pumpable medium and a foam gas, which in this case creates the used foam.

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Read more about aeration in Bronkhorst’s success story on how mass flow meters are important for adding the correct proportion and composition of air bubbles to ice cream.

Allard Overmeen
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Thermal flow instruments behave best using a laminar flow, at least if we look at the thermal mass flow meters and controllers with a bypass sensor. To conduct a precise measurement with this flow instrument, laminar flow is preferred.

However, in practice you will encounter a turbulent flow quite often. A turbulent flow can be caused by restrictions in an installation, such as valves or adapters, in combination with a high velocity of the used fluid. This effect is known as ‘turbulence effect’. A turbulent flow can affect the accuracy of your measurement, something you would like to prevent.

“Turbulence is a dangerous topic which is often at the origin of serious fights in the scientific meetings devoted to it since it represents extremely different points of view, all of which have in common their complexity, as well as an inability to solve the problem”. Marcel Lesieur, 1987

How can you prevent this turbulence effect? Let’s start with explaining what turbulent flow is:

Turbulent flow versus laminar flow

In general it can be said that there are two types of flows: a laminar flow and a turbulent flow. In picture 1 laminar flow has been visualized by an experiment using ink in a cylindrical tube. The ink has been injected into the middle of a glass tube through which water flows. When the speed of the water is still low, the ink does not appear to mix with water, the stream lines are parallel; this is called laminar flow.

laminar flow profile

If the speed of the water increases, a sudden change will occur at a certain speed. The flow completely disrupts and the water turns homogeneous through the ink. The stream lines are chaotic, not linear anymore, which is called turbulent flow.

In theory the flow pattern depends on four variables:

  • Diameter of the tube
  • Speed of the fluid
  • Density of the fluid
  • Dynamic viscosity of the fluid

The factors combined provide the so called Reynolds number (Re), an important parameter that describes whether flow conditions lead to laminar flow or turbulent flow. In general it can be said that a laminar flow occurs at a low Reynolds number (≤ ca. 2300) and a turbulent flow occurs at a high Reynolds number (≥ ca. 3000). In between these two numbers (Re 2300-3000) you have a ‘transitional flow’, meaning the flow can be laminar or turbulent (numbers mentioned are for a cylindrical tube).

When can turbulence effect occur?

As mentioned before, turbulence effect is a common effect which can occur in installations using (too many) restrictions, such as valves or adapters, in combination with a high velocity of the used fluid. In every restriction, the flow has been disrupted and the speed of the gas will change (as visualized in picture 2). Besides the usage of restrictions, the pipe length is something to take into account. As it takes some time for a turbulent flow to get laminar again, it is important to use the right pipe length.

A turbulent flow is something you would like to prevent at the inlet of your flow measurement instrument, as it can affect the accuracy of your measurement. It is preferable to have a laminar flow just before your flow instrument. However, the instrument itself used as flow controller, with a valve behind the meter, can cause a turbulent flow again. Not all kinds of flow meters experience this as disadvantageous. Mainly thermal flow meters using the bypass principle are sensitive for this effect. Flow meters based on the Coriolis, CTA (Constant Temperature Anemometry) or Ultrasonic principle are independent of turbulence.

Why are thermal flow meters with bypass sensor more sensitive?

Instruments with a bypass sensor work based on a main flow going through a restriction and a small part of the flow going through the actual sensor. The ratio between these two flows is determined by the pressure drop over the sensor and the restriction in laminar flow. The turbulence effect will disturb this ratio. As the instruments with bypass sensor are often used for very precise measurements, the turbulence effect can have a huge effect on the measurement results.

What can you do to minimize the disadvantageous effects of turbulent flow?

When using thermal mass flow meters with the bypass sensor, we advise it is advised to do the following:

1) Try to prevent restrictions in your process, such as valves, adapters and elbow couplings:

  • Do not mount the flow meter directly behind a restriction, such as a valve. However, if this cannot be arranged differently, than you could use a turbulence filter between the valve and flow meter or use a flow meter with integrated turbulence filter.
  • Using an elbow coupling close to a flow meter should be limited as much as possible.

2) Limit the speed of your flow by using the right pipe length. Generally it is advised to use a minimal pipe length of:

  • 10x the pipe diameter, at the inlet of the instrument
  • 4x the pipe diameter, at the outlet of the instrument (flow meters only)
  • For gas flow rates > 100 l/min it is common to use as a minimum a 12mm or ½” pipe.

laminar flow element lfe

Laminar flow element (LFE)

3) Use a ‘turbulence’ filter in your flow process. The turbulence filter will filter the flow before it reaches the sensor and makes it laminar again. Nowadays, flow meters often have such a filter integrated in the flow meter (for example Bronkhorst EL-FLOW series) or have an extended flow path inside the flow meter (for example the Bronkhorst Low delta P flow meters).

Extended flow path inside a flow meter

Extended flow path inside a flow meter


It depends very much on the application what the consequences are of turbulences. As an example in semicon processes, particularly in coating processes such as layer deposition, turbulent flow is out of the question. A stable process is essential here. However, in other coating processes, such as flame spray techniques, the impact of turbulences will be less due the high pressure in the flow.

It all depends on the process and application.

If you need any assistance for installation of your flow meter, contact our Customer Service Department by submitting the contact form.

For more information about the working principle of the Bronkhorst flow devices, have a look at the various working principles of flow instruments as applied by Bronkhorst.

Have a look at the related blog articles:

Why is the choice of piping important for thermal mass flow meters?

Why use thermal mass flow meters and mass flow controllers?

Thermal mass flow sensor: Bypass versus CTA

Allard Overmeen
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As working as a field service engineer for many years now at the company Bronkhorst, I have seen a lot installations in the field and I often get questions regarding the influence of the pipe length on the performance of a mass flow meter.

In today’s blog I will try to explain why the correct choice of piping is essential for an optimal performance of your installation using thermal mass flow meters or controllers and why this has an influence on:

• Deviation in measurement of the thermal mass flow meter

• Frozen pipes

Deviation in measurement of the thermal mass flow meter

Deviation in the measurement data can be caused by using a too short pipe length, because the pipe length is a parameter for the gas temperature. For an optimal performance we advise to avoid excessive temperature fluctuations during commissioning and process operation as much as possible, especially in a process with thermal mass flow meters and controllers. If you use mass flow meters based on the Coriolis principle, temperature fluctuations have no influence on the measurement data, as this measurement principle has been based on measurement of real mass.

In case of a high velocity of the gas flow, the temperature of the gas can change really quickly. In general it can be said, that the higher the flow rate, the more the gas temperature will change. This can interfere with the temperature of your instruments, as the temperature of a gas will lower much faster than the temperature of the instrument itself. This can cause a deviation in your measurement data.

Therefore, for optimal performance of a thermal mass flow meter the gas temperature should be equal to the instrument temperature. Choosing the appropriate length of piping can help you here. If the piping is long enough, the gas has the ability to cool down gradually, more at the same pace as the instrument. This will help you minimize the temperature deviation.

Frozen pipes

Another effect which I encounter in the field is frozen pipes. How do frozen pipes occur? When a cooled gas flows with a high velocity through the piping, the temperature of the piping will lower, especially when restrictions in the piping are used, such as narrowing of pipe diameter or the use of (shut-off) valves in the piping. As a result the piping will attract moisture. If the ambient temperature lowers beneath zero degrees the moisture will freeze. This can also happen within the pipe when the medium (gas) contains moisture.

In this case, using a refrigeration dryer can offer you a solution to make sure the gas which is used in the process is dry, to avoid freezing as well.

The pipe length in practice

I talk about “too short” and “long enough”. But what is long enough? Generally we advise to use a minimum pipe length of:

  • 10x the pipe diameter, at the inlet of the instrument

  • 4x the pipe diameter, at the outlet of the instrument

For gas flow rates between 100-1500 l/min it is common to use a 12mm or ½” pipe, and we advise a larger pipe diameter for gas flows > 1500 l/min.


The two effects discussed here are very common in all kinds of processes with high gas flow rates (>500 l/min), such as:

  • Plasma vapour deposition technique; used to provide rotor blades with a coating to make them suitable for high temperatures
  • Blast furnaces; to make stainless steel out of conventional steel

If you need any advice in this matter, contact your local UK Customer Service Department, we will gladly assist you and offer help and guidance 24/7!

Website Bronkhorst UK

gas flow meters and controllers

Rob ten Haaft
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For many years, Mass Flow Controllers (MFCs) and Mass Flow Meters (MFMs) have been used in Analytical instrumentation. There are some distinctive applications like carrier gas control or mobile phase control in Gas Chromatography (GC) and Liquid Chromatography (LC). I discovered that there are a lot more applications of Mass Flow Controllers in analyzers then I could imagine when entering the world of Mass Flow Controllers after many years working in Analytical Chemistry.

One application I would like to focus on in this blog is Mass Spectrometry or shortly, as chemists like to use abbreviations, MS. Mass Spectrometry comes in many forms and is often coupled to Gas Chromatography and Liquid Chromatography. A Mass Spectrometer coupled to a Gas Chromatography (GC) is called a GC-MS and a Mass Spectrometer coupled to a Liquid Chromatography (LC) is called a LC-MS.

Where are Mass Spectrometers applied?

The market for Mass Spectrometers is huge and expanding. The instruments are used for Analytical Research in general but increasingly important in Food Research. Research concerning aging of whiskey and fingerprinting of red wine to determine the origin of the grapes are some examples. Another emerging market is Biopharmaceutical Research where Mass Spectrometers are used to study proteins and how these proteins are digested in living organisms. There are even Mass Spectrometers on Mars (!), where the martian soil is studied.

schematic mass spectrometer

Figure 1: Mass Spectrometer(schematic)

What is a Mass Spectrometer?

The Mass Spectrometer is often compared with a weighing scale for molecules. Every molecule is built up from atoms and every atom has its own atomic mass and this is “weighted” by a Mass Spectrometer. Before it can weigh the different atoms that are present in one sample, the atoms have to be separated from each other. This is done by charging the atoms (to form ions) and using a magnet to deflect the path that the ion is following. The lighter the ion, the more influence the magnet has and the bigger the deflection. The detector detects where the ion hits and this is a measurement of the weight.

The place where the ionization takes place is called the ion source and there are a lot of different types of ion sources, depending on the matrix of the sample and on the ions that you want to form. The ionizing part is the most interesting part from a Mass Flow point of view because in this part different gases are used, depending on the technique of ionization.

There are two main techniques: hard ionization and soft ionization. With hard ionization techniques, molecules in the sample are heated and fragmented down to atomic levels giving information about the atomic structure of the molecule. With soft ionization techniques the molecule stays more intact giving mass information of the molecule. This is used in Food and Pharma research and has become very popular in the last decade.

Let’s look into detail to one of the most popular soft ionization techniques, the Electrospray Ion Source. The EIS vaporizes the liquid (coming from a Liquid Chromatograph, for example) by leading gas alongside a charged needle to form an aerosol spray. Leading a counter gas flow through the formed spray will evaporate most of the liquid that you do not want to measure, leaving the charged droplets going into the Mass Spectrometer.

atmospheric pressure chemical ionization electrospray ionization electroscopy

Figure 2+3: Electrospray ionization (ESI)

Mass Flow Controllers and Evaporation used in Electrospray Ion source

The interesting part is that the flow needs to be very constant as you want the process of forming droplets and evaporating solvent to be the same, day after day and at different locations with different circumstances. An important parameter in this reproducibility is the gas flow. By using Mass Flow Controllers for Nebulizer gas and Evaporation or Drying gas, the ion source will always have reproducible gas flows.

Our solutions department can design compact gas modules for analytical applications to supply gases for ion-source combined with other gas flows with high accuracy and good reproducibility. Combining components like pressure switches and/or shut-off valves with the flow channels can give a compact gas handling module to fit in the small footprint demanding designs of the Mass Spectrometers. Furthermore, the changes on leaks are decreased significantly as the whole manifold can be leak and pressure tested before it is shipped to the customer.

If you would like to learn more about Bronkhorst customized flow solutions, you can watch this Video or visit our website.

James Walton
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Thermal mass flow instruments that make use of a bypass (capillary bypass or bypass sensor) are what most people have in mind when they think of thermal mass flow instruments. What are the differences?

In instruments based on the thermal principle, power is applied to heat the sensor tube. Accordingly the temperature of the tube is measured at two points. With no flow measured, the temperature differential between the two points will be zero.

When the flow increases, the temperature at the first measuring point will decrease, as fluid carries away the heat. At the same time the temperature at the second measuring point will increase as the fluid carries heat to it. More flow will result in a greater temperature differential and this temperature differential is proportional to the mass flow.

Another technology used to measure mass flow is CTA (Constant Temperature Anemometry). In a CTA (through flow, straight tube) instrument there are two measurement “probes” inserted into a straight tube flow path. The first “probe” both heats and measures the temperature of the fluid, as the second “probe” measures the temperature of the fluid.

Again, as the gas flow increases the gas will carry heat from the first measuring point to the second one. In a CTA, however, the power is varied to keep the temperature between the two measuring points constant, and it is this power level that is proportional to the mass flow.

Each technology has its advantages and disadvantages which generally are application specific.

A clean, dry gas application where higher accuracy is as important as repeatability, may be a better application for a bypass instrument like the Bronkhorst EL-FLOW series.

An application with a dirty or slightly moist gas, or where lower accuracy but high repeatability and robustness is required, may be a better application for a CTA instrument like the Bronkhorst MASS-STREAM™ series.

Curious about using a thermal Mass Flow Meter or Controller? Or the top 5 reasons why we use Mass Flow Meters with CTA measurement?.

For more information, please visit our website

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