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%!
The first variable area (VA) meter with rotating float was invented by Karl Kueppers in Aachen in 1908. The device was patented in Germany that same year. Felix Meyer was among the first to recognize the significance of Kueppers’ work and implemented the process for offering the meter for sale. In 1909, the firm of "Deutsche Rotawerke GmbH" was created in Aachen (Germany). They improved this invention with new shapes of the float and of the glass tube. It didn’t take long for the new device to capture attention in Europe, the United Kingdom, and other areas.
VA flow meters (or purge meters)
Over time, different types of VA flow meters (also called purge meters) have been developed, usually in response to some specific need. Nowadays a purge meter usually consists of a tapered tube, typically made of glass or plastic. Inside this tapered tube there is the ‘float’ which is made either from anodized aluminum or ceramic. The float is actually a shaped weight that is pushed up by the drag force of the flow and pulled down by gravity. The drag force for a given fluid and float cross section is a function of flow speed squared only.
While the meters are still relatively simplistic in design, relatively low cost, low maintenance and easy to install they are used in many kinds of application. Despite these facts, the traditional VA meter has a number of drawbacks. For instance, graduations on a given purge meter will only be accurate for a given substance at a given temperature and pressure. Either way, due to the direct flow indication, the resolution is relatively poor. Especially when they are built into a machine, reading might be hard. Moreover, the float must be read through the flowing medium, so you can imagine that some fluids may obscure the reading.
9 reasons why to use a thermal mass flow meter instead of a traditional purge meter
As for the current century, Bronkhorst has developed a thermal mass flow meter series (MASS-VIEW, as shown in picture 1) which is the digital high-tech alternative to the traditional VA flow meters. Thanks to today’s digital possibilities, many other advantages arise for many industrial processes and chemical plants.
MASS-VIEW flow meter in application
The MASS-VIEW flow meter series operate on the principle of direct thermal mass flow measurement (no by-pass); rather than measuring the volume flow it measures the actual mass flow, without the need of temperature and pressure correction.
The digital OLED display provides an easy direct or relative reading of the actual flow. Herewith parallax errors are excluded.
With this digital mass flow meter it is easily possible to obtain the accumulated flow. This availability of data gives insight in costs, leading to data driven decision making power.
In contrast to the traditional VA meter which need to be mounted in a vertical position, this digital alternative can be mounted in any position.
The flow path is made of sustainable aluminum rather than plastic or glass which is fragile.
The instruments are standard equipped with 0-5V, RS-232 and Modbus-RTU output signals. Note that the traditional VA meters usually do not have any output signal available at all.
As a standard feature, there are 2 built-in relays which indicate an alarm situation. Herewith, external devices can be controlled.
Multi Gas; as opposed to traditional VA meters, which are produced for one particular fluid only, the digital alternative has up to 10 pre-installed gases available as a standard feature.
Multi Range; traditional VA meters usually have a rangeability of 1:10 and one single full scale range only, the digital alternative has a rangeability of 1:100 as well as up to 4 pre-installed flow ranges.
Achieve a stable flow
A VA meter, whether it is a conventional or a digital one, can be equipped with a built-in needle valve. This needle valve enables the user to regulate the flow rate by means of a restriction inside the flow channel. As long as the inlet pressure is stable, the subsequent flow will be stable too. On the other hand, once pressure conditions are susceptible to change, the flow rate will become equally unstable. If this is not desirable, you’ll have to compensate these pressure fluctuations.
Manual control valve
This effect can be eliminated by using a manual control valve like the FLOW-CONTROL series which keeps the pressure drop across the needle valve (delta-P) constant. This is accomplished by a second (normally open) valve, though it is operated by a membrane this time. The operating principle is based on a balance that forms between the pre-pressure, back-pressure and the spring force on the mebrane. A change in the pressure conditions leads to a change of the equilibrium and thus a change in the valve position as well (as shown in the picture below).
Working principle of a pressure compensated control valve
Although Bronkhorsts’ pressure compensation technology is suitable for either gases and liquids, the nice thing about this is that both technologies, the digital VA meters and pressure compensation, lend themselves well to being built together. However, in that case it is applicable for gases only.
Learn more about the different models in the manual constant-flow control series
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.
Following the trend of big-data in today’s digitial world here at Bronkhorst, we see that it has also affected the world of flow measurement. The amount and speed of data transmission continues to increase and digital flow meters are employed in an even wider range of applications, such as at Universities and R&D institutes.
Thermal mass flow meters are a well-known instrument in this field. They measure the mass flow of gases, employing a combination of heated elements and temperature sensors, with thermodynamic principles used to derive actual flow. Mass flow meters need limited correction for changes in temperature, pressure or density and are extremely accurate, especially when measuring low and very low flow rates, and are no longer regarded as high cost.
Throughout the years, we’ve seen the technology improve, from a simple VA-meter to today’s ‘smart’ measuring device, like Bronkhorst’s EL-FLOW Prestige mass flow meter. Bronkhorst has used big-data to improve the technology within the mass flow meters and we still do, to keep the devices ‘intelligent’. The improved technology within the device makes it possible to select corresponding gas properties which are on-board, such as density and heat capacity, and to automatically adjust measurement values for environmental influences, for long-term sensor stability. Which, in turn, will directly relate to process yield.
There are many parameters, such as temperature and pressure, that have an influence on the accuracy, stability and reproducibility of a mass flow meter or controller. These are very important conditions within laboratory research.
Furthermore, to investigate the influence of as many gases as possible in a certain period of time, there is a need for short settling times as well as a quick change-over to a new gas with a very short downtime. Using a multi-gas/multi-range device can make a difference here, which offers the ability to use multiple gases at various process conditions with just one device.
Download white paper EL-FLOW Prestige mass flow controller
For those of you interested in how we used big-data to optimize our thermal mass flow meter, the EL-FLOW Prestige, we have written a white paper containing in-depth information about influential factors and the impact on the accuracy, stability, linearity and pressure correction.
Please fill out the form and you’ll receive the whitepaper.
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.
Did you know that natural gas is odorless? I didn’t… I always find it having a penetrating sulfur scent. Well, it appears that this penetrating scent is added to the natural gas on purpose. Let’s see why this is.
As natural gas is combustible and odorless by nature, the government requires some safety measures here. Many countries have established safety regulations how to handle natural gas and which gas needs odorisation. This is mostly done by the Health and Safety department (HSE) of the local government.
What about natural gas odorisation?
Today’s question is about this subject. Why does gas smell when it is odorless by nature? This is the point where gas odorisation comes in.
Odorisation of natural gas is done to act as a ‘warning agent’ in case of leakage. The idea is that people can smell the gas prematurely if it is present. Because, if there is too much gas present it can be explosive.
As shown in the picture, the LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit) are crucial here. If the concentration of the combustible substance present in the air is too low (< LEL), than no combustion will occur. It the mixture is too rich (> UEL), there is a huge amount of gas in the air and only partial combustion will occur. Gases become dangerous in between the LEL and UEL. Therefore, it is most important for people in the surroundings to smell the gas in time, before the concentration is too high and it exceeds the LEL.
As a result, it is stated in the safety regulations that natural gas has to be detectable at a concentration level of 20% of the LEL and this is done by odorisation. Needless to say that the odor used in the gas is not dangerous to people’s health.
When is an odor added to gas?
This depends on the type of gas line. We know ‘distribution lines’ and ‘transmission lines’.
Distribution lines are local natural gas utility systems that include gas mains and service lines, such as the commercial gas used at domestic environments. All these distribution lines need to be odorised. For the transmission lines it is stated in the regulations when to odorise it.
Picture LEL and UEL
For the odorisation there are many different odorants available, such as Tetrahydrothiophene (THT) and Mercaptan. Selecting the odorant depends on the properties of the gas to be odorised, pipeline layout, ambient conditions etc.
Tetrahydrothiophene or THT is a well-known odor. THT is under ambient conditions a colourless volatile liquid with an unpleasant smell.
Controlled supply of THT using mass flow controllers
Bronkhorst had the pleasure of developing a solution for a Dutch customer to add THT to their biogas. Biogas was generated from anaerobic decomposition of organic matter and upgraded to natural gas quality to inject into the Dutch natural gas main. As commercial natural gas in the Netherlands has to contain at least 18mg of THT per cubic meter gas, the process of adding this to the commercial gas had to be done really accurately.
ATEX Zone 1 Coriolis Mass Flow Meter
The traditional approach to add THT is using a pump with a fixed stroke volume. However, low gas flow rates using a pump for batch-wise injection may lead to liquid THT remaining in the gas lines. THT may not be mixed well with the gas and might have the wrong concentration. A homogeneous injection of THT is therefore much better. Besides this, THT is a relatively expensive odor which also makes an accurate injection very much desired.
A better solution here would be using a combination of a pump with a Coriolis mass flow controller, in our case the mini CORI-FLOW™ series mass flow controllers. The Coriolis instruments make it possible to dose both continuously as well as accurately.
Something to be taken into account is the classification of the area. As gases in principle are explosive, it is very common for the environment around gases to be classified as a hazardous area. Most common classifications (in Europe) are marked as ATEX zone 1 or zone 2. Just make sure to select the right material to use.
For solutions such as THT odorisation processes, Bronkhorst can offer both ATEX/IECEx zone 1 and zone 2 solutions. Our mini CORI-FLOW Exd mass flow meter, for zone 1 applications, is a collaboration with one of world’s leading manufacturers in explosion protection, Electromach member of the R.STAHL Technology Group.