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.
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%!
In our daily life we use plastics or polymers in many different forms whether as a disposable product such as packaging film or as a long-lasting component in the automotive industry, in construction or in sports equipment and toys.
Nowadays, plastics are tailor-made for the respective application, depending on the properties desired. In this way, properties such as hardness, mold ability (or formability), elasticity, tensile strength, temperature, radiation and heat resistance can be adjusted as well as the chemical and physical resistance can be adapted to the desired function.
This large variety can be modified within wide limits by the choice of the basic building blocks (macromolecules), the production process and additives. The respective macromolecules are polymers of regularly repeating molecular units. The type of crosslinking and the used additives determine the final properties of the material. In 2016, the world-wide production of plastics for bulk materials and films was over 300 million tons (source: BMBF) of which almost one third was produced in China. Europe and North America follow with slightly less than 20 percent each.
Precise dosing for operational efficiency and minimization of unnecessary waste
Typical additives in the plastics industry are antistatic agents, dyes, flame retardants, fillers, lubricants, colorants, stabilizers and plasticizers. Many of these additives are liquid.
Precise dosing of the additives leads to operational efficiency and the minimization of unnecessary waste.
Additives are frequently added by use of needle valves, which is inexpensive, but always has a risk on malfunction because of fluctuation within the process (e.g., pressure and temperature). In particular the use of plasticizers is increasingly critical since some of these substances are directly absorbed by human beings or accumulate in the food chain.
With the proven CORI-FILL dosing technology, Bronkhorst offers an easy-to-use setup to ensure the required accuracy and reproducibility. By combining a mini CORI -FLOW with a pump or a suitable valve, fluids can be dosed continuously or as a batch into the reactor with high reproducibility. These systems can be integrated or used as an add-on in already existing processes and production lines.
mini CORI-FLOW flow meter combined with a Tuthill pump
5 Reasons why additive dosing with a Coriolis instrument supports process efficiency for plastic manufacturers
No need for (re)calibration in the field – fluid independent flow measurement and control
Gas and liquid can be measured with the same sensor
Ability to measure undefined or variable mixture
The CORI-FILL™ technology features an integrated batch counter function and enables direct control shut-off valves or pumps
Traditionally, and in most cases we see, dosing- or metering pumps are believed to be accurate because the theory is that a known pump head displacement will move a known volume over a known time giving a known delivered volume. In practice however it will never achieve a high level of accuracy with deviations of 10-15% being normal. Inaccuracies like this are caused by many changing process conditions, such as:
Wear of components
These factors can each be the cause of an inaccuracy in the expected volume of displacement from a pump head movement. If you then multiple each of those factors you can realise quite large measuring errors that create both inaccuracy and inconsistency.
Please refer to our earlier blog about ‘High Accuracy’.
What can be done to improve the accuracy?
Option 1) Add a flow meter between the pump and the process
By adding a flow meter between the pump and the process, you can take information from the flow meter to adjust the speed of the pump. Traditionally, this would be managed with an analogue output signal, 4….20 mA or similar, from the flow meter into a separate PID controller that compares the real flow signal to the desired flow. Subsequently, the electronic controller can then adjust the speed of the pump to achieve the desired dose or flow.
Using this solution will mitigate the issues in the original solution, however it introduces more:
Slow flow signal due to signal filtering in the PID controller
Slow pump response due to extra control relay
Increased complexity with extra components
Time to achieve stable flow can be long
Additional price of meter and PID controller
Option 2) Direct mass flow measurement with a flow meter with built in PID control
Now we need to discuss the next possible solution, using a direct Mass Flow measurement device with built in PID control
that can drive a pump to achieve the desired dose or flow.
With this solution you do not need to include the pump in the control system, just give a set point demand to the mass flow meter and it will drive the pump to achieve the desired dose or flow. This solution will give you several advantages, such as:
Direct mass flow control of the flow
Mass flow dosing is independent of temperature and pressure, in contrast to the volumetric dosing when only a pump is used;
Accurate delivery mitigating normal pump issues
Alarm functionality of low flow
Preventative maintenance based on pump performance over time
Consistent flow measurement based on actual not assumed numbers
Coriolis mass flow meter in modular dosing system
These advantages can be utilised in many different industries:
Anywhere that liquid is dispensed into a container that will require quality assurance, and commonly the quality control test is carried out on a small percentage of the vials to ensure general compliance. If you use a mass flow meter to control the dose you can achieve 100% QC checking of your product with reduced human input.
If you need to dose additives, performance chemicals or mix liquids then the ability to control the flow of the additive and know what that flow is can be a huge advantage to the outcome of the application.
Pump control can offer accurate dosing solutions for house hold chemicals like detergents and cleaning products.
Quality is becoming more and more important for customers, but what is quality? Some people refer to quality as the accuracy of an instrument, others as the reproducibility of an instrument. I am writing this blog to let you know how we organize quality at Bronkhorst.
How does Bronkhorst deal with quality?
As Supplier Engineer at Bronkhorst, I have co-responsibility for the quality of our purchased parts. Together with our suppliers, we set up the supply chain using the Lean Six Sigma philosophy. The ultimate aim is to create a repeatable and reproducible supply chain, which is becoming more and more important for our Copy Exactly customers. By properly setting up the supply chain, we adhere to the ‘first time right’ principle, which reduces the chance of subsequent changes.
Cooperation between Bronkhorst’s Research & Development department, the supplier that produces the article and the measuring chamber that ultimately carries out the inspection is essential for setting up this reliable supply chain. If you know how a product is used, you can set up a good process. If you know the critical specifications of the function and the critical parameters of the process, you know what to measure in order to say something about the repeatability and reproducibility of the product.
Pilot Series Process
Once a design has been approved through a ‘Proof of Principle’, the release for the series can start. For this purpose, Bronkhorst has set up the ‘Pilot Series Process’. All new products undergo the Pilot Series Process, including our newest product, the ES-FLOW. The ES-FLOW is an ultrasonic Liquid Flow Meter/Controller, which has recently come on the market and works on the basis of ultrasonic measurement techniques.
Let’s take this as an example.
Pilot Series Process for the ES-FLOW, Liquid Flow Meter/Controller
An important part of the ES-FLOW liquid flow meter/controller is the stainless steel body of the instrument, which contains the measuring sensor. The technical requirements that this component must meet have been specified by the Research & Development department at Bronkhorst. Some of the important requirements of the ES-FLOW body include pressure resistance, leakage density and installation length.
The supplier will then set to work and will carry out a number of tests during the production process to convince Bronkhorst and himself that the body of the instrument and the sensor meet all the requirements.
During the Pilot Series Process, extra data (including measurements) of the critical function specifications
and the critical process parameters of the component is requested. In this way, we can obtain a good overview of the reproducibility and repeatability of the process and, based on that information, a decision can be taken to release the component, the so-called ‘Article Supplier Approval’ (ASA). As soon as the article is released with an ASA, the Purchasing department can order the article in large series from the supplier concerned.
The results of the Pilot Series Process also form the basis of the component’s measuring plan, describing which specifications must be inspected in what manner when the product in the series is delivered.
Working with Pilot Series ensures that the knowledge of both the developer and the supplier comes together and that the measuring chamber knows what and how to measure in order to monitor the process functionally. The introduction of the Pilot Series enables Bronkhorst to reduce the risk of interruptions in the series and create a reliable supply chain together with its suppliers.
This entire process helps Bronkhorst to deliver high-quality products, which we consider a top priority.
In my previous blog post ‘A digital alternative to traditional VA-meters/purge meters’ I have written about the MASS-VIEW mass flow meter which is a digital alternative to traditional VA-meters/purge meters and the reasons why to use such a thermal mass flow meter instead of a traditional purge meter. This time I will answer the 5 most frequently asked questions about this type of thermal mass flow meter.
1) Can the MASS-VIEW be used for gas mixtures?
Since the introduction of the MASS-VIEW, I have regularly been asked whether the instrument is also suitable for gases or gas mixtures that are not stored in the instrument by default.
Despite the fact that the pre-installed gases might be the most common gases the MASS-VIEW is used for, the wetted parts of the instruments allow to use them for other gases and/or gas mixtures as well.
Traditionally it is possible to apply a fixed conversion factor, calculated with the free online calculation tool ‘FLUIDAT® on the Net’ , to the measurement of the MASS-VIEW. It’s a solution. However, Mass Flow ONLINE offers a more user-friendly solution, called the FluidAdd, which is actually an additional calculated fluid curve.
As a MASS-VIEW user, such an additional curve allows you to overwrite one of the default gases. The creation of a FluidAdd file requires the following information specified by the user:
Conversion factor. The calculation of the gas conversion factor can be made via FLUIDAT® on the Net using the CFMassView module.
Standard density. You may check Fluidat® on the Net for the correct data. The density is needed to show the flow not only in normalised volume flow units (like ln/min or ls/min) but also in mass flow units like g/h.
Serial number of the instrument. This information is required to make sure that the FluidAdd file is adopted to the instrument.
Custom gas name. The custom gas name will be shown in the list of selectable gases in the instrument.
The new calculated curve is delivered by e-mail. Using the FluidAdd software, the FluidAdd file can be easily uploaded into the instrument.
2) Can the MASS-VIEW be used at sub-atmospheric pressure conditions?
To answer this question properly, it is actually required to start with a closer look at the physical properties of gases. In physics and engineering, mass flow rate is the mass of a substance which passes per unit of time. Considering a sub-atmospheric flow rate, you can imagine that the gas flow rate need to be (significant) higher to transport a particular amount of gas (mass).
Since the MASS-VIEW series operate on the principle of direct thermal mass flow measurement, the combination of its structural design and the increased (volumetric) gas flow rate in sub-atmospheric conditions might have negative side-effects to ensure a proper measurement. Therefore the measurement output cannot be guaranteed in sub-atmospheric conditions.
3) Will the instrument (calibration) be affected by dust, humidity and/or oil mist in the process gas?
Humid air for instance can be seen as a mixture of water and air and additionally to this, dust and oil might be present in the process gas. The physical properties of those mixtures will differ from those of dry and clean air. As from a technical point of view, MASS-VIEW is able to deal with the humidity as long as there is no condensation inside the flow channel. However, the inaccuracy of a thermal mass flow meter calibrated for dry air could increase when humid air is applied instead of dry air.
As for the oil and dust, potential clogging of a thermal mass flow instrument in general is just around the corner.
However, the thru-flow nature of MASS-VIEW’s working principle from Bronkhorst is relatively insensitive to possible clogging in potentially polluted industrial gas applications. However, the insensitiveness does not mean that clogging is impossible. It is highly recommended to filter those process gases before it enters the mass flow meter.
4) What is the recommended calibration interval?
Mass flow instruments, in fact all process instruments, experience wear from the conditions of the process in which they are installed. Temperature, electronic component tolerance shift, contamination built up over time (even very slight), plus other factors will all contribute to affecting the accuracy of an instrument.
Your instruments should regularly undergo a calibration check if not a recalibration. But how often? Because the nature of each application is different (conditions, running time, etc.) a calibration interval can last three years or three months.
Bronkhorst instruments do not have specified due dates for calibration. We suggest that our instruments be calibrated every year. However based on the application conditions, and perhaps company quality procedures, each customer must determine when they need to send in an instrument for recalibration.
Properly calibrated instruments will be more accurate, more reliable, help ensure consistency, and help improve production yields.
5) How to hook up the alarm settings?
MASS-VIEW instruments have built-in programmable alarm functions available to make the flow meters as versatile as possible. These functions enable the user to get warned for or in cases of:
Master slave alarms
When alarm settings are activated, the flow alarm will automatically open or close Alternating Current (AC) electrical circuits, activating warning lights, bells, pumps or other process control equipment. In fact, today’s highly flexible and versatile alarm trips can be found working in a wide range of applications, under an impressive list of pseudonyms.
As a standard feature, the MASS-VIEW flow meter has two alarms connected to a relay/switch.
One side of the switch is connected to ground (pin 4, 0 Vdc)
and the other side is connected to Pin 3 (alarm high).
The other relay is also with one side connected to ground (pin4, 0 Vdc)
and the other side is connected to Pin 5 (alarm low).
For more information about the MASS-VIEW, please visit our website or watch the video
Within the medical arena there is increased pressure on budgets and financial accountability, with a significant trend for the sector to look again at how resources are used and where savings can be made.
One of the largest expenditures in most hospitals is the cost of purchasing or producing the various medical gases needed, such as Medical Air, Nitrogen, Oxygen and Nitrous Oxide. Often the usage and consumption of these gases is neither monitored nor measured or, whenever it is done, it is often a crude estimation, inaccurate and recorded only by pen and paper.
Most hospitals rely on the rate at which the cylinders (in which the gas is supplied) empty to determine the amount and rate of gas used. There are of course many issues associated with this method, such as:
The amount of gas in a particular sized cylinder can vary greatly, even when directly delivered by the gas supplier
Total gas consumption and peak times of consumption cannot be accurately determined
Leaks can go undetected
Specific point of use consumption is impossible to determine
This makes it very difficult to manage costs overall and to assign invoicing costs to individual departments and sections.
A company specialising in the design, installation and maintenance of gas systems was asked to install the medical gas network in a new hospital. An approach was made to Bronkhorst UK Ltd for the supply of gas meters which could then be communication-linked to the building maintenance system.
Thermal mass flow Instruments with integrated multi-functional displays were offered to fulfil both the accuracy and reliability requirements . As a result of their through-flow measurement (Constant Temperature Anemometry - CTA technology) the thermal mass flow instruments offered the additional benefits of no risk of clogging, no wear as there are no moving parts, minimal obstruction to the flow of the gas and hence ultra-low pressure drop, all based upon the fact that the instrument body is essentially a straight length of tube.
In addition to the local integrated displays both 4…20 mA and RS232 output signals were available ensuring integration with the Building Management System (BMS). This gave the end user real time continuous data logging and remote alarming should the gas supply enter low- or high-flow status for any given event. As a double failsafe the instrument offers both on-board flow totalization and further hi/lo alarms.
The installation of the mass flow instruments for this hospital application provided the following benefits to the client:
1. On primary networks:
Separated invoicing for hospital/clinic/laboratory departments sharing the same source of medical gas
Monitoring and acquisition of consumption data
Leak detection within gas line, safety vent and medical gas source
2. On secondary networks:
Independent gas consumption invoicing between the health institution departments
Monitoring and acquisition of consumption data
Leak detection within gas line
Subsequent installations across Europe have followed the trend of increased accountability by installing a Mass Flow Meter for the incoming bulk delivery, obtaining a totalized flow reading and cross matching this to the bulk invoice. This could be useful in the event of inadvertent errors or typos when a bulk delivery invoice is being raised.