Semiconductor chip technology is enhancing our lives in many ways. Emerged from semiconductor technology, MEMS chip technology is also present in devices around you in the form of sensors. Think of your smartphone that captures your voice and senses the smartphone position, orientation and movement by means of Micro Electro Mechanical Systems (MEMS). Adding those features is barely impacting the physical dimensions of a smartphone: it still fits in your hand and pocket.
This blog is about instrument miniaturization by MEMS chip technology and the benefits of miniaturized gas flow instruments for application in the field of gas chromatography. As a MEMS Product Manager at Bronkhorst High-Tech, I can see the benefits of miniaturization by MEMS technology in such applications.
IQ+ FLOW solution based on MEMS modules
Miniaturization by MEMS chip technology
Further miniaturization is achieved by combining MEMS modules in customer specific flow solutions.
In a laboratory environment, it is advantageous to work with desktop-sized equipment. Advantages of increasing functionalities in table top equipment are: reduced space requirements, enhanced ease of operation and often reduced cost of ownership.
Gas chromatography equipment is a good example of a concentration of functionalities on a small footprint. Many types of gas composition and vapour composition can be analysed with high accuracy and for very low concentration levels. Additionally, there is a certain degree of automation involved. This is all within arm’s reach of a laboratory analyst.
The goal of gas chromatography analysis is to identify and measure the concentration of gas components in an analytical gas sample. Within the gas chromatograph (see picture 3), there is often a need for gas flow or pressure control. The picture shows a gas flow controller for the carrier gas stream (red) and a pressure controller for the split flow stream (yellow).
The principle of gas chromatography involves a controlled carrier gas stream that passes an injector, column and detector. A sample gas is injected for a short period of time, creating a gas sample plug. The gas sample plug is separated into gas components across the column, which become visible as peaks during detection.
Picture 4 shows an example of a gas chromatography output.
Let’s zoom in on dynamic headspace sampling that is used in combination with gas chromatographys. Headspace sampling refers to the gas space in a chromatography vial containing a liquid sample. The liquid sample is a solvent, containing material to be analysed. E.g. volatile organic compounds in environmental samples, alcohols in blood, residual solvents in pharmaceutical products, plastics, flavor compounds in beverages and food, coffee, fragrances in perfumes and cosmetics.
This is explained in picture 5. Dynamic headspace sampling is performed by purging the gas space and the adsorbent. The adsorbent collects the sample gas. After transport, the adsorbent is purged again to release the sample gas into a gas chromatograph.
Where a gas flow controller comes into play is at purging the headspace with a constant Helium or Nitrogen flow for a pre-determined period of time at a specified temperature between 10 - 200 °C. The gas flow, containing the headspace sample gas, passes an adsorbent that collects the headspace sample gas.
The adsorbent is usually made of Tenax TA material. Now, the adsorbent is transported to the inlet of a gas chromatograph. While the adsorbent is heated between 20 - 350°C, a controlled Helium or Nitrogen gas flow passes the adsorbent to release the headspace sample gas into the inlet of the gas chromatograph. The gas chromatograph does its job to analyse the sample. Different signal peaks in time show the different components and their concentration.
IQ+FLOW gas flow meters and pressure controllers
For flow instruments, a number of specifications are important in headspace sampling and gas chromatography in general. The IQ+FLOW product line addresses these specifications with small instrument size, fast response, good repeatability, low power, low cost of ownership and the excellent support that you can expect from Bronkhorst.
Read more about the IQ+FLOW chip based product line
For more information about gas chromatography in combination with IQ+FLOW flow and pressure meters and controller have a look at our application note ‘Gas Chromatography'.
The future of MEMS technology
Bronkhorst is committed to look ahead and find applications that can be enhanced with MEMS chip technology. Feel free to contact us for questions. We will keep you informed!
Read more about MEMS technology in our blog 'Miniaturization to the extreme: micro-coriolis mass flow sensor'
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.
Let’s start with an example:
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.
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.
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%!
Mass Flow Theory
Since the day of their introduction Instrumentation devices have always been required to evolve. One of the main reasons for this is to accommodate new, better and more complex communication protocols.
What a lot of people still do, due to ease of use and consistency is specify instruments with analogue or RS-232 serial communication. Analogue communication (4-20mA, 0-5v or 0-10v) and RS-232 which was the original way in which computers communicated was the original, mainly due to cost and available technology.
It is still a very robust and solid way to send and receive information over a small group of instruments however it does have some very practical set-backs. As a point-point communication protocol it requires a port both on the instrument and the controller, this can be very limiting in size affecting both the amount and length of cables needed.
The development of Fieldbus communications meant it became possible to have multiple (100’s) of instruments connected through only one communication port at the controller level. This means that you had a huge reduction in both the number and length of cables needed. This development allowed the complexity to increase and size decrease of instruments containing multiple sensors.
As with VHS and Betamax there will always be competing technologies and ‘bus’ development was no different. Of course everyone hopes for a single unified solution because it makes things simpler, cheaper and more efficient. However the reality is that most ‘buses’ are utilised differently in different industries.
Different industries are described as either; a ‘process fieldbus‘ used in many process automation applications (flow meters, pressure transmitters and other measurement devices) or a ‘device network’ which is a large number of discrete sensors are used, motion, position etc., the best example of this is in automotive manufacturing.
There is an IEC standard that was developed for the European Common Market and interestingly I have learnt that the common goal was not focused on commonality but more the elimination of restraint of trade between nations. This standard is IEC 61158, it is almost 4000 pages long. Issues of commonality are now left to the consortium that supports each of the standard fieldbus types.
What next is always a good question, Ethernet based communication systems are one area that has seen large development over recent years and its definitions are being added into the International standards.
In all of this, what is our involvement in Fieldbus. As you may know, we are mainly involved in Process and control industries. We support almost all of the major bus systems out in the market and also have our own in-house ‘Flow-bus’ system that can be used link multiple instruments together. You run them through a single PC running our Flow-Plot or Flow-View software.
The latest addition to the communication range is the ‘Gateway’ solution. This allows multiple or manifold instruments on a Flow-Bus network to communicate with PROFIBUB or PROFINET DP through a specific fieldbus interface. This can be a very cost effective solution as multiple PROFIBUB or PROFINET DP instrument can become very expensive, very quickly.
Bronkhorst Field-Bus Technology
Chromatography has a long and interesting history. To discuss such a vast subject is a challenge, people are understandably passionate about such a subject. There is huge potential of not charting or discussing an area another person believes to be critical, for example we are discussing here Liquid Chromatography, we didn’t feel there would be room or time to discuss Gas Chromatography (GC) in an appropriate amount of detail.
So before we begin, please do let us know if you have any additions or corrections, we are always open to learning more from experts within any industry as that helps us to grow and learn.
Since the mid-19th century multiple types of chromatography have been developed. To start at the beginning, the word ‘Chromatography’ stands for ‘color writing’ and was initially used for the separation of plant pigments such as chlorophyll (which is green) and carotenoids (orange and yellow). However, it soon became apparent that it could be used for a wide range of separation processes as new forms of chromatography were developed starting in the 1940s.
In the modern analytical solutions discussed here, from HPLC to GC and SCFC we have an instrument that is currently in use, allowing the finished Analytical Instruments to achieve their full potential. We have provided solutions for liquid and gas applications, using thermal mass flow technology allowing manufacturers to achieve their end goals and meet the customer’s expectations.
As solutions providers we have relationships with all of the leading manufacturers around the world, these relationships are based on partnership where we provide the flow expertise. Our goal is to deliver the solutions of the future by listening to and understanding the trends in the market now.
One of those techniques is High Pressure Liquid Chromatography (HPLC).
Schematic drawing of a typically HPLC instrument
HPLC stands for High Performance Liquid Chromatography and is a technique used to separate and allow the user to quantify different compounds. A high pressure pump is used to push the solvent through a column, due to the interaction between the compounds and the column material a separation of different compounds is possible. From here you can analyse the time taken to elute the compund ot the amount of compound detected.
Schematic of our instruments installed in the HPLC system
UPLC stands for Ultra High Performance Liquid Chromatography and is a special version of HPLC. Compared to HPLC, UPLC columns contain smaller particles sizes (2 um for UPLC vs 5 um for HPLC), which results in a better separation of compounds. The pump pressure in UPLC can go up to 100MPa in comparison to HPLC where this is 40MPa. UPLC has, in some applications, improved chromatography significantly. The run times are much shorter; therefore very fast analysis is possible.
UPLC is the abbreviation mostly used in writing; however this is a trademark technology of one of the major corporations in this field and is officially called UHPLC (Ultra High Performance Liquid Chromatography).
SCFC, the last trend we will discuss today is the growth of research into the way that liquid CO2 can be used as a super-solvent. With the ever increasing cost of chemical solvents used in the mobile phase, both purchase and disposal is increasing yearly.
Developing a system that utilizes a solvent, such as CO2, that can elute different compounds and provide a gradient effect purely through adjusting and controlling the system pressure is an incredible potential cost saving development. However as with all things, the cost reduction has to be off-set by the cost of installation of such systems. That day is only getting closer, but we are not yet at the point of wide-spread liquid CO2 solvent usage.
- Working with liquid CO2 can be a real challenge and one that we take seriously, our Coriolis and EL-Press instruments are perfect for this application giving flow and pressure control without the need for a thermal based system that would affect system integrity.
The industry is changing, that’s obvious! We will be on top of this………
Quality Control has a very high value. The value comes from the security it provides. Air and Liquid tightness can be of critical importance, also on a small scale. It is our mission to support the development of tools that can help achieve this goal. Leaks can lead to unstable process conditions which in turn can create potentially dangerous results. An example is the Bhopal disaster : a gas leak incident in India which is considered as the world’s worst industrial disaster. Over 500,000 people were exposed to methyl isocyanate gas and other chemicals.
There are three major solutions that we have provided to demonstrate leak integrity:
- Valve seat leak testing
- Fabric permeability testing
- Ventilation and Air conditioning systems (HVAC)
Each of the challenges has different demands and requires an independent approach.
Two of these solutions required a flow pressure combination and one was a Coriolis solution. The valve seat leak test required a Coriolis sensor to meet the specific demands of that industry. Both the fabric and ventilation systems required a Thermal Mass Flow meter and Pressure instrument and that is what we are talking about this week.
Fabric permeability testing
The fabric test was completed by using a closed loop flow-pressure control solution. A chamber with an outlet, Digital Electronic Pressure Meter (DEPM) to control the internal pressure and an inlet, Mass Flow Controller (MFC) to control the inlet flow was built with a custom made fabric clamp on top of the chamber for the fabric.
This system allows the end user to clamp in a chosen fabric sample, always a consistent size, give set-point to the pressure meter, which via a closed loop control cable then controls the valve on the mass flow controller to allow the addition of air into the chamber until the pressure set-point is reached and then maintain it. Once reached the flow controller records the flow of AiR required to maintain the set-point giving a value to the permeability of the fabric. This process provides data acquisition for the test.
Ventilation and air conditioning system
The ventilation leak test system requires a more industrial approach. Also, with larger volumes to fill and potentially several devices under test (DUT) the system is controlled by a Digital Electronic Pressure Controller (DEPC). A Mass Flow Meter (MFM) is used in series to meter the flow of AiR required to maintain the set pressure.
With an MFM and DEPC in-series you can set a pressure on the DUT via the DEPC and fill with AiR, the DEPC then controls the pressure while the MFM records the amount of AiR required to maintain the pre-determined pressure. This will then give a reading on the leakage rate of the DUT.
There are a few key factors to making sure that you get repeatable and consistent test data:
Firstly, if the test volume would require a long fill time, or secondly, a small leak rate might be measured then a safe by-pass would be required to achieve a quick fill and stabilisation time.
The third and most important parameter that can affect the result of the test is temperature. The gas and components must be allowed to equalise before testing begins, for example a 2 Deg C shift in temperature can result in a 0.7% change in volume measured due to gas expansion. Any leaks smaller than this volume would not be measured and the integrity and repeatability of the test would be under question.
To help counter some of the issues discussed we have developed a new flow controller that will provide your process with cutting edge performance, click here for more information.
As with any inter-dependent parameters there are consequences for changing one parameter if the controller of the other is responsive.
For example; if you installed a mass flow controller and a digital pressure controller in-line, each with their own inputs and outputs, changing the set-point on the digital pressure controller would adjust the valve and therefore the flow. This reaction would be detected by the mass flow controller as either a decrease or increase in flow and the mass flow controller valve would adjust accordingly to achieve its given set-point. This second reaction would then be seen as a change in pressure by the digital pressure controller and you can see how quickly this would become a feedback loop of increasing instability.
One of the most frequent challenges that we come across, and one that people are always surprised we can solve, is the ability to control and meter flow and pressure in-line. Many of the applications we are involved in require control and measurement of the flow and pressure of the fluid. However, as the two parameters are different but inter-dependent it can make it difficult to determine the optimal process conditions in which both parameters are fully supporting each other. Our experience teaches us that each case has its own unique solution.
We learned by experience that it is very beneficial to draw together the process owners of the application under discussion. I/we use this ‘drawing’ skill to help identify which areas are critical and where value could be added through the use of digital instrumentation. This can make it easier to determine what the best course of action is.
Applications for use:
The need to control/meter both pressure and flow can be important in:
• Consumption reactions
• Fuel cell development
• Burner/Ignition lance applications
• AiR Permeability testing
If you need to measure the AiR permeability of medical packaging; you need to set a standard pressure while also metering and controlling the AiR required to maintain the standard pressure. This measure gives you the flow required to achieve stability and therefore the AiR permeability.
In consumption reactions you may want to hold your reaction vessel at a set pressure and control/meter the feed gas to maintain the set pressure. This set-up, with electronic instruments and PC/PLC communication software will give you total consumption, consumption rate and allow you to profile the life cycle of the reaction you are conducting.
Adding digital control can allow you to control maximum flow as you build up to desired pressure levels which can be important in certain applications.
As with all application solution requirements, talking the application aim through with your supplier can be crucial to getting the best result.
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