Ron Tietge
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Part 1 - What are low liquid flows?

What do micro reactors, catalyst research and odorant dosing have in common? Well, they all require the handling of low liquid flows. In the world of flow control & measurement, we distinguish between ‘low flows’ and ‘high flows’. But what does this really mean? Bronkhorst High-Tech is a renowned supplier of flow meters and flow controllers in the ‘low flow’ range. So, time to explain what we mean when we refer to ‘low liquid flow’.

We have therefore prepared a blog series with recommendations for a low liquid flow setup. Besides low flow definition and tips for flow meter selection, these blogs also give advice on system lay-outs, connection material and liquid supply systems. Because flow setups and process conditions are rarely the same for different customers; there is no one fix for all available. Providing the best advice requires insight into the customer application.

What are (ultra) low liquid flows?

The definition of 'low' is arbitrary and depends on the area of business. In bulk industry, flows of much less than 500 kg/h are considered low flows, whereas in research this term is attributed to flows that are smaller than 100 g/h. The current blogs focus on handling - measuring as well as controlling - liquid flow rates up to 100 g/h. We also focus on ultra low flows - which we define being in the range < 5 g/h.

To give you an idea, consider a water droplet. With a typical diameter of half a centimetre, 100 grams per hour is equivalent to about 2000 water droplets per hour - quite low. And 100 drops are an equivalent of 5 grams - to be dosed in our hour.

Accurate instruments for measuring and controlling low liquid flows have proven their use in a wide array of applications. For example:

• The supply of 100 g/h of drilling oil as a lubricating agent is monitored during hole drilling in the manufacturing of aircraft fuselage part. Read application note: Lubricant dosing in airplane manufacturing.

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• An ultra low liquid ethanol flow of 2 g/h is evaporated to generate a stable ethanol vapour flow as a carbon source in R&D for high-quality graphene production. Read application note: Research high-quality graphene production.

• In the investigation of catalysis at high pressure, low liquid flows of hydrocarbon compounds need to be dosed as a stable flow without pulsation. Read application note: Catalysis at high pressure.

• Labs-on-a-chip and other microfluidic devices in pharmaceutics and biotechnology significantly reduce the number of chemicals and experimental time compared to traditional means. Read appliction note: Flow measurement in microfluidics.

• The typical smell of natural or biogas originates from a ‘warning agent’ that has been added artificially to the gas, injected in a small but continuous amount as a liquid additive. Read application note: Controlled supply of odorant to natural gas.

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In all these cases, the measuring or dosing of the correct amount of liquid - not too much and not too little - is critical for the good performance of the process concerned.

Mass flow versus Volume flow In the previous paragraph, the flow is expressed in units of mass, such as grams/hour or milligrams/second. However, many users think and work in units of volume. This is fine, at least when we are talking about the same reference conditions. Check our blog ‘Do you know why mass flow reference conditions matter?’ to learn more about reference conditions.

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What is so typical about low flows?

How is a low liquid flow of less than 100 g/h different from 'normal' or high flows? Well, (ultra) low flow applications involve some phenomena which are not observed in or are not relevant to larger flows. Due to the (very) small amount of liquid that is being moved, (ultra) low flows are so sensitive that even the tiniest disturbances in process or ambient conditions can have a massive effect on flow stability. The influence of external conditions on flow stability is therefore key here - as well as the means to control these external conditions. For example, even small leaks of gases or liquids into or out of the process have a considerable influence on the intended liquid flow. Furthermore, any obstruction caused by solid particles or contaminations in the small liquid flow lines will obviously reduce the flow in an undesired way. For low liquid flow dosing in particular, unstable pressures will lead to unstable flows. Variations in pre-pressure, pulsation due to excessive pump stroke volumes compared to the flow rate, and dissolution of gas (pressurised air) when pressurising the liquid to be dosed will all result in an unstable flow.

Knowledge of the application as well as the physical transport phenomena of the process are essential to deal with this complex matter of low flow handling. Optimising flow stability and performance of fluid systems requires in-depth knowledge of fluid characteristics and system components in a wide range of circumstances. Every component used in a fluid system can affect the behaviour of a fluid or interact with other components, especially when it comes to low flows.

Solutions for optimal performance

In the Bronkhorst range of products, thermal-based μ-FLOW and LIQUI-FLOW mass flow meters and controllers, as well as Coriolis-based mini CORI-FLOW ML120 and mini CORI-FLOW M12 device, are particularly suitable for (ultra) low liquid flow applications. Where a mass flow meter consists of a sensor that only measures the flow rate of the medium, a mass flow controller combines such a sensor with a control valve to control the medium flow rate. Check out the ‘mass flow controller theory’.

Flow controllers are typically used to generate a stable flow. However, optimal performance requires a good deal more than just an excellent flow controller. For example, make sure that there are no leaks in the setup and use small volume tubing. Moreover, in pressurised containers, avoid using gas that dissolves in liquid, or use means to remove this gas. In the next part of our blog series, we will discuss these and other matters in more detail by focusing on practical tips on how to select the right low flow device. Image descriptionImage descriptionImage description

Stay tuned for part 2!

Are you looking for tips and recommendations for dimensioning, choice of material and best practice procedures? Read our blog about this in!

Lynn Woerts
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2020 is a few days old and the plans and goals of this new year are starting to take shape. Let’s look back on last year. What did we achieve? Which blog post was the most fun, useful, gripping or interesting to you? Oh and by the way, we’ll assure you that we’re going to share all our knowledge about low flow, mass flow and flow meters even more often this year. From the overview of the 2019 statistics, we’ve come up with a top 5 of most popular blog posts.

  1. How to deal with vibrations using Coriolis mass flow meters
  2. Do you know why Mass Flow reference conditions matter?
  3. Real-time pressure and temperature compensation to optimize flow control
  4. Flow Meter Accuracy & Repeatability
  5. Flow control valves; the most used accessory in flow control

Top 5 most popular blog posts in 2019

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1. How to deal with vibrations using Coriolis mass flow meters A Coriolis mass flow meter is known as a very accurate instrument and it has many benefits. To be quite frank we were quite surprised that this blog post came in first place. In industrial applications, all kinds of vibrations with different amplitudes are very common. However, the question is whether these vibrations influence the measuring accuracy of a Coriolis mass flow meter. Ferdinand Luimes, Liquid Flow Technologies product manager, shares the advantages as well as the disadvantages of these flow meters and provides some handy hints in using these instruments.

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2. Do you know why Mass Flow reference conditions matter? The world of flow measurement applies reference conditions, which can be further divided into standard reference and normal reference. Another distinction is between European and American style. Chris King, Bronkhorst USA General Manager, sheds light on this apparently complicated construction in his blog post, detailing exactly what the differences are and explaining why these reference conditions matter.

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3. Real-time pressure and temperature compensation to optimize flow control This blog post topped the charts in 2018 and is still in the top 5 today, once again proving the relevance of this topic. As it turns out, various external factors can have an influence on the measurement accuracy and control stability of mass flow controllers. Vincent Hengeveld, Gas Flow product manager, explains the theory behind real-time pressure and temperature compensation.

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4. Flow Meter Accuracy & Repeatability Choosing which flow meter is right for your application is a pivotal element in its success. Generally speaking, the two important statistics are flow meter accuracy and repeatability. In his blog post, Chris King explains what these two parameters mean and why they are crucially important.

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5. Flow control valves; the most used accessory in flow control Finishing the list is a blog post about control valves, perhaps the most often used accessory in flow meters. A control valve is used to control a flow by varying the size of the flow passage. Do you know which valve is best for your flow meter? Stefan von Kann, senior engineer in applied physics, provides a number of tips and tricks for the most pressing areas of attention.

2019 guest bloggers

We wish to thank our guest bloggers very much for their fascinating studies and compelling stories. It fills us with pride that you contributed content to our website in 2019.

• Roland Snijder, medical physicist resident at Haaglanden Medisch Centrum (NL), worked as a researcher on the multi-infusion project at the department of Medical Technology & Clinical Physics of University Medical Center Utrecht. In his guest blog, he focuses on investigating physical causes of dosing errors in multi-infusion systems. • Jean-François Lamonier (University of Lille) is an expert in the catalytic treatment of volatile organic compounds. In this blog post, he explains how his team uses flow meters for this purpose. • Jornt Spit, researcher at the Radius research group at Thomas More University of Applied Sciences in Belgium, has a background in biochemistry and biotechnology. He is working on renewable biomass. Read his blog post on controlled CO2 supply for algae cultivation and its valuable contribution as an alternative source of carbon. • Prof. Michaela Aufderheide (Cultex Technology GmbH) has been working in the field of cell-based alternative methods with a focus on inhalation toxicology for more than 30 years. Increasing pollution of the environment and workplaces demands new testing methods. Read her blog post: ‘The e-cigarette – A blessing or a curse?’

Would you like to become even more inspired? All blog posts can be read on our website.

On behalf of the entire Bronkhorst team, I wish you a healthy, wonderful and innovative 2020!

Which topic would you like to be the focus of a blog post in 2020?

Vincent Hengeveld
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Efficiency and yield in a process require a stable gas flow. This gas flow can be measured and controlled by a thermal mass flow controller. As a Product Manager at Bronkhorst High-Tech, I experienced that various external factors can have influence on the measurement accuracy and control stability of mass flow controllers (MFCs). Some examples of external factors are:

  • Temperature fluctuations
  • line-pressure fluctuations

These fluctuations can occur due to reduced pressure in a gas cylinder or due to cross talk between multiple flow controllers. How does Bronkhorst solve these problems, and what solutions do we offer?

Cross talk with mass flow controllers

What is cross talk? Cross talk typically arises when multiple mass flow controllers are positioned in close proximity in the same pipe, or installed upon the same rail or frame. The line-pressure from a gas regulator is affected by the flow demand of the flow controllers. When the flow instrument is changing its setpoint, it will affect the line-pressure. Due to this pressure change, the flow measuring section in a conventional flow controller is affected, indicating an incorrect flow measurement that does not represent the actual flow through the MFC. The smaller the nominal flow of the flow controller, the bigger the effect will be to a setpoint change of a larger, parallel installed MFC.

Static and dynamic pressure compensation

Static pressure compensation is the compensation for slow pressure changes, for example the slowly reduced pressure from a gas cylinder. By integrating a pressure transmitter to the mass flow controller, together with an on-board conversion algorithm, real-time calculation of the actual fluid properties can be performed. For semi-caloric measurement the density, viscosity, thermal conductivity and heat capacity are used in the calculation. Under influence of pressure and temperature, these properties change. Thus, actual temperature and pressure are measured and processed, resulting in accurate flow measurement and control stability.

Dynamic pressure compensation is the compensation for rapid pressure changes. This can occur when a higher-flow mass flow controller on the same supply line changes setpoint, an undesired effect which is also known as ‘cross talk’. The moment that these rapid pressure changes are identified by the pressure sensor, the valve control will be adjusted accordingly so that the flow remains stable.

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Stable flow control with on-board conversion The on-board conversion algorithm makes it possible to convert the stored calibration fluid into one of the 100 on-board gases (multi-fluid multi-range functionality). The actual measured temperature and pressure is used in the on-board conversion model to compensate for changes in the process conditions. This leads to a more reliable and accurate conversion and control stability.

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Benefits for the user

  • Firstly, due to the improved and accurate flow measurement and control, optimized and more constant process conditions are possible, resulting in an improvement of your process yield.
  • Secondly, ease of installation since there is no need for exactly providing/meeting the process conditions the instrument was ordered for.
  • Thirdly, because the supplied line pressure becomes less important for the accuracy and control stability of the instrument, less accurate components or even reduction of components in the supply line are needed. This allows saving costs on, for example, a pressure regulator.

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Rob ten Haaft
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It might surprise you but already since the Middle Ages, people are interested in the smell of a person’s breath. In those days it must have been quite a challenge to investigate but as early as those days diseases like diabetes (associated with a sweet, acetone odor) and liver failure ( a fish-like smell) were diagnosed upon one’s breath. I will not discuss the treatments during those years; let’s say that much has improved since the Middle Ages. For example, nowadays, we use Ion Mobility Spectrometers (IMS) to measure Volatile Organic Compound (VOC).

At the beginning of the 21st century, research studies show that dogs are able to detect cancer by smell. The dogs are trained to detect different kinds of cancer in exhaled breath of human patients, as they can smell with a sensitivity of parts per trillion (ppt). To give an example: the scent of one cc of blood, diluted in 20 Olympic sized swimming pools can still be detected by the dog.

It was concluded that dogs are probably discriminating between breath samples based on a specific breath odor but it is still unknown what odor or mix of compounds dogs detect. The detection of cancer by trained dogs seems to be obvious when you think about it, but it requires a lot of training of the dogs and it is still unknown why not all cases of cancer are detected.

Volatile Organic Compound (VOC)

This is the reason that researchers have started to develop analyzers that can do the dog’s job. In the last few years it was discovered that Volatile Organic Compounds (VOC) can be distinctive biomarkers in the diagnosis of human diseases. Volatility is the tendency of a substance to evaporate, therefore Volatile Organic Compounds are organic compounds that will easily evaporate or sublimate at room temperature.

The exhaled human breath contains a few thousand volatile organic compounds and the composition of the VOCs is used in breath biopsy to serve as a biomarker to test for diseases such as lung cancer.

An increasingly popular analytical technique to measure VOC’s is Ion Mobility Spectrometry (IMS). This technique is ideal for analysis in medical applications since the analysis is fast, not affected by humidity, highly sensitive, and operates at ambient pressures. This makes the technique very suitable for portable or Point of Care application.

Ion Mobility Spectrometry (IMS)

The Ion Mobility Spectrometer operating principle is based on the drift, or time-of-flight, of ions that are formed in the reactant section. The ions travel, supported by an electric field, through the drift tube where they encounter a drift gas (N2 or Air). The shape and the charge number of the ion will make it easier or harder to flow through the drift gas which will cause a separation of the ions in the sample and after detection give an IMS-spectrum as shown in Figure 1.

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Mass Flow Controllers

To deliver the gases to the drift tube, Bronkhorst has the knowledge and experience to supply the right products. Our products address the specifications that are important for controlling the gases in Ion Mobility Spectrometry such as:

  • cleanliness,
  • small instrument size
  • fast response
  • good reliability
  • low power and
  • low cost of ownership. Our MEMS (Micro Electro Mechanical Systems) based instruments, such as the IQ+FLOW thermal flow meters, are very suitable for Ion Mobility Spectrometry.

Image description IQ+FLOW thermal flow meter

  • Are you interested to read more about mass spectrometry (MS) and how mass flow controllers and evaporation is used for Electrospray Ion source, check our blog ‘A closer Ion them’.
  • Check our success story using IQ+ gas flow meters and pressure controllers for a gas chromatography application.
  • If you have questions or ideas on Ion Mobility Spectrometry other analytical applications that involve controlling of gas or liquid, feel free to contact us.!
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

Stefan von Kann
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A control valve is used to control a flow by varying the size of the flow passage as directed by a signal from a controller, such as an on-board PID controller in a flow meter. It is one of the most used accessories in flow control.

An accessory for mass flow controllers

Control valves can be furnished as an intergral part of mass flow controllers and pressure controllers or as a separate component used in combination with a flow- or pressure meter. Together with a feedback loop from the mass flow controller or pressure controller, the valve controls the amount of flow passing through to go to an imposed flow- or pressure setpoint.

Depending on the application it is often clear whether your mass flow controller needs a shut-off (open-close) valve or a control valve, or whether one needs a normally opened or normally closed valve. Within the group of control valves, there are a number of different valves available, each having their own parameter ranges, advantages, and disadvantages.

In this blog I will highlight some valves and focus on how to cope with higher absolute and differential pressures, and how to get higher flowrates at low differential pressures.

The direct control valve

A direct control valve consists of an orifice for controlling the flow and a controlled surface that determines the size of the opening that flow can pass through, and thus determines the amount of flow passing through the valve.

• Advantage: such a valve is relatively fast, cheap, and uses only little power to control the flow. • The disadvantage here is that it can only handle limited pressures and flows.

Let’s take an electromagnetic valve as an example:

For a valve, the force (F) needed to overcome to open the valve is determined by the orifice diameter size (d) and the pressure difference (Δp) over the valve , (F ~ Δp * ¼ d2). When either the pressure differential or the orifice diameter gets higher, the direct control valve will not open adequately due to this pressure force, which can be > 15 N for a 200 bar differential pressure over a 1mm orifice, pushing the valve shut.

An electromagnetic valve can only exert a force of ca. 5N on its plunger. It could be a possibility to use a stronger coil, delivering a larger magnetic force. However, mass flow controllers often have a limited power supply and the amount of heat that is produced can become a problem as well. Resulting in a limited maximum flow, proportional to pressure and the diameter squared.

In summary, most direct control valves are not suitable for high flows, or to handle high differential pressures or absolute pressures due to these restrictions. The direct control valves could be used for low flows from 1mln/min up to approximately 50ln/min.

What alternatives do we have?

  • Redesign the direct valve for higher pressures
  • Using a 2-phase valve, an indirect control valve
  • Using a pressure compensated valve, to reach high flows at low pressures

  • High pressure direct control valve

The easiest solution to cope with higher pressures is a redesign of the direct control valve. As the orifice size is limited, it can be used for relatively small flows (up to 20ln/min) . To handle the larger pressure differences, up to 200 bar differential pressure (bard), the valve and mass flow controller body have to be more robust. Most valves can not handle a burst of 200 bard; either the sealing material can rupture, or mechanical parts can not handle the sudden force bursts that are possible at 200 bard.

The dimensions of the valve are only slighty larger than for a common valve, and thus the entire mass flow controller. On the other side, low flows are often limited due to leakage through the valve at high pressure differences.

  • Indirect control valve, a 2-phase valve

To go to even higher pressures and more flow, up to 200ln/min, we have to take a larger step in changing our mass flow controller. With a so called indirect control valve (figure 1) higher flows and higher absolute and differential pressures can be reached.
[Figure 1 – Indirect control valve]

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An indirect control valve (or 2-phase control valve) consists of:

  • a direct controlled pilot valve (A), with the behavior as described before, and without needing any extra power.
  • an additional valve in the body; a pressure compensation part (B) to maintain a constant pressure difference (P1 -P2) of only a few bars across the pilot valve (A). By doing so, both the inlet and outlet pressure may change without having any impact on the valve’s function. The pressure force over the pressure compensated part keeps the valve closed. Only when the top valve opens, the pressure force is brought back to a small enough value to open the valve and control the flow.

So, the indirect control valve consists of two valves in series (A+B), where both the pressure drop and the orifice size together determine the resulting flow.

The disadvantages of this valve are its size and the relative high costs. Besides that, a minimal pressure difference is needed to close the pressure compensation part of the valve. Also, the orifices are still limited in size, thus to get to 200 ln/min a minimal inlet pressure of > 150 bara is needed. To get such flows at lower pressures, a whole different kind of valve is needed, like a pressure compensated valve, a bellow valve.

  • Pressure compensated valve

It is possible to use larger orifices and reach higher flows with a direct control valve, but to do that, the pressure force in the valve has to be reduced. This can be done with a pressure compensated bellow valve, where the effective orifice for the pressure force has been reduced significantly (figure 2). With a bellow valve, flows of several hundreds of liters per minute can be reached with a minimum pressure difference. However, the absolute pressure is limited due to the design and the valve is much larger and more expensive than a common direct control valve.

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Conclusion: Depending on the pressure that you want to put over your mass flow controller and the outlet flow needed, you can either use:

  • a direct controlled high pressure valve (up to 200 bara and 20 ln/min), or
  • an indirect pressure compensated valve (up to 700 bara or 400 bard and 200 ln/min).

To reach high flows at low pressures, a pressure compensated valve will be the best solution.

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Have a look at the control valves we often use in combination with our flow meters or pressure meters.