A flow meter’s specifications are pivotal elements in choosing which one is right for your application. Two important statistics are its accuracy and repeatability. Let’s start with explaining what these two parameters mean:
Flow Meter Accuracy
Accuracy is how close the measurement is to the true value. In flow meters, that means how close the output of the meter is to its calibration curve. This is expressed as a percentage, e.g. ±1%. It means that any given reading can be in error 1% above or below the calibration curve. In general it can be said the lower the percentage, the more accurate the meter. However, this also depends on the specification of either FS (Full Scale) or Rd (Reading). The meaning of Full Scale and Reading will be explained later in this blog.
Flow meters are becoming more and more accurate, especially with the advent of mass flow meters.
Flow Meter Repeatability
Repeatability is producing the same outcome given the same conditions. In other words, a flow meter should produce the same readings when operated under the same variables and conditions. This, too, is expressed as a ± percentage.
While accuracy usually takes the spotlight in the measurement world, repeatability is the foundation on which accuracy rests. You can have high repeatability without high accuracy but you cannot have high accuracy without high repeatability. It is not helpful if the meter is highly accurate only once in a while. If your data is unreliable, if you get different numbers under the same circumstances and setup, there is no way those numbers can all be accurate.
Is accuracy always important?
No one wants an inaccurate meter, but not all applications require high amounts of accuracy. It may be acceptable to stray further from the calibration curve if you are only looking to get an idea of how much is flowing through a pipe. It isn’t acceptable if you are mixing pharmaceuticals for consumption or volatile elements. How accurate your meter needs to be is important when selecting a flow meter, because usually the more accurate a meter, the higher the price.
When you see an accuracy specification, it should be expressed as a percent of Full Scale (FS) or Reading (Rd or RD). The difference between those can be significant.
Read our blog “Is the high accuracy trend right?”
What is Full Scale (FS)?
The definition of Full Scale is “Closeness to the actual value expressed as percentage of the maximum scale value.”
With Full Scale, the error remains the same but the percentage changes as the flow goes up and down the flow range. If the accuracy is calibrated 1% of 200 ln/min then the error is 0.01 x 200 ln/min = 2 ln/min. If the flow is 100 ln/min, the error is still 2 ln/min or 2%, a much bigger percentage.
What is Reading (Rd or RD)?
The definition of Reading (Rd) is “Closeness to the actual value expressed as percentage of the actual value.
With Reading, the accuracy is the percentage of what is being read. The percentage stays the same, no matter where the flow is in the flow range. If it is 1% at 200 ln/min it would be 1% at 100 ln/min. So the error for a 200 ln/min flow would be 2 ln/min but for 100 ln/min it would be 1 ln/min rather than the 2 ln/min of Full Scale.
Depending on the application, the difference between Full Scale and Reading can quickly add up and have a significant impact on the end product.
Full Scale (FS) versus Reading (Rd)
Full scale is actually a carryover from mechanical gauges when readings were dependent on physical marks on a dial. Digital meters now can give much more precise readings, so high-end meters generally use Reading rather than Full Scale.
Although you don’t want an inaccurate flow meter, not all applications require high amounts of accuracy.
In terms of mass flow, accuracy requirements can change the type of sensor being discussed. If you need very high accuracy you can have a Coriolis flow meter, if high accuracy is less important, you may need a Constant Temperature Anemometry (CTA) or other sensor type.
See out Coriolis flow meters.
View our CTA flow meters.
Anglian Water Services cleans water to the highest standard, delivers it to millions of homes, and carefully manages it to ensure it never runs out in an area of the UK. They started a project to optimize and further control dosing of phosphates in the public water system.
The functionality of orthophosphoric acid in the public water system
Public water systems commonly add phosphates to the drinking water as a corrosion inhibitor to prevent the leaching of lead and copper from pipes and fixtures. Inorganic phosphates (e.g. phosphoric acid, zinc phosphate, and sodium phosphate) are added to the water to create orthophosphate, which forms a protective coating of insoluble mineral scale on the inside of service lines and household plumbing. The coating serves as a liner that keeps corrosion elements in water from dissolving some of the metal in the drinking water. As a result, lead and copper levels in the water will remain low and within the norms to protect the public health..
What was the original process ?
In the original process a down-steam analyser was in-place to measure the concentration of orthophosphoric acid in the main flow. The measurement results were checked against the required concentration and used to adjust the pump speed and therefore the level of orthophosphoric acid in the main flow. With this process Anglian Water Services can secure copper and lead concentration levels in the water acceptable to protect the public health. Nevertheless the process had room for improvement, which will be discussed in this blog.
The original process of record
What are the limitations in the original process?
The reactive feed-back loop mechanism for dosing phosphates was not a preferred working method. We could not react quickly enough to the changing main flow to reduce or increase the dose proportionally. We had to ensure that we dosed to a level meeting the legal requirements assuming the station was processing maximum flow.
Secondary costs were added to the system by needing double redundancy on the analyser to ensure there is no break in the measurement of orthophosphoric acid levels.
- Reducing phosphate levels.
- Reducing the cost of meeting legal environmental standards for the business.
- Remove the downstream analyser and redundant spare in the process of record.
Two sensor technologies were evaluated to enhance the process ; Differential Pressure and Coriolis technology.
The Differential Pressure instrument was the most cost effective and allowed us to meter the Orthophosphoric acid flow as a volume, it would take an analogue signal input and adjust the dose proportionally to the main flow.
The Coriolis Mass Flow Meter utilizes direct Mass Flow Measurement, which is preferable over volume flow for this application and is more accurate and repeatable, but is more expensive. It would also take an analogue signal input and adjust the dose proportionally to the main flow.
Combination of mini CORI-FLOW with Tuthill pump
Making a decision appeared to be based around return on investment. Essentially the time taken to generate sufficient savings. However, during the demonstration of the Coriolis Mass Flow Meter we learned something new that would change the direction of our final design. The Coriolis Mass Flow Meter gave the density of the fluid being metered as an output.
Why was this important?
Phosphoric acid it sold in diluted concentrations , usually 80% in solution. What we have found is that there is a variation in the actual concentration at the point of use.
At this point we already knew that either the Differential pressure or Coriolis technology could support us to enhance the process of record. Now we had the chance to go to the next level and take a previously unavailable but very important parameter and use it to really refine the dose ratio.
The extra density parameter available with the Coriolis Mass Flow Meter made the decision for us. Dosing would now be controlled proportionally to the main flow and the density/quality of the phosphoric acid being used.
The enhanced process
What are the projected benefits using Mass Flow Meters:
As we look to go live on the first five installations of this technology, we are projecting the following:
- Stable concentration of orthophosphoric acid in the public water system.
- Maintaining the public health commitments of the Water Industry.
- Decreasing the addition of phosphoric acid into the environment by significant levels.
- Two-fold cost reductions: by eliminating the down-stream analysers and the consumption of phosphoric acid.
At Anglian Water Services they live with a Love Every Drop approach. The Love Every Drop approach is a vision for how they believe a modern utilities company should be run. That vision means creating a country with a resilient environment that enables sustainable growth and can cope with the pressures of climate change. Creating infrastructure that is affordable and reliable, meeting the needs of customers, communities and the environment. We want our people and our communities to be resilient too. Phosphoric acid is connected with the concept of planetary boundaries according to Rockström et al. 2009. Anglian Water Services was able to reduce the consumption of phosphoric acid in their processes without sacrificing the quality of the water. This fits with the way they run their business.
Download the brochure for our [Liquid Dosing Module](https://www.bronkhorst.com/getmedia/f64e57bd-e31a-4a3a-a18b-cc08d22ef310/Water-Treatment-LDM.PDF ).
If we want to live healthy lives, we need to know the nature and content of undesirable chemical elements in our environment. If a municipal council wants to clean up a piece of land to develop a new suburb, it needs to know whether heavy metals or toxic substances such as arsenic remain in the soil from the previous use of the land. Likewise, the managers of drinking water sources, surface water bodies and fishing areas need to know about the quality of their water, to determine whether it contains excessive levels of undesirable substances that will have to be removed. And in order for air quality to be considered good, the trace element content in the solid particles floating in the air must not be too high.
Outside of the environmental field, there are other places where it is helpful to be able to identify and quantify the elements that are present – such as establishing the concentration of metal in lubricating oil to determine how quickly an engine will wear out, or the concentration of fertilisers in agricultural soil to determine whether additional fertiliser is required.
Flow meters and regulators also play a major role here. As an industry specialist in the analytical market, allow me to explain how it all works.
Inductively Coupled Plasma – Atomic Emission Spectrometry, ICP-AES
As you can see, there are many applications in which it is useful to know what chemical elements are present and in what quantities. ICP-AES is a good analytical technique for measuring the nature and concentration of elements in solids, liquids and gases. This acronym stands for Inductively Coupled Plasma – Atomic Emission Spectrometry. Due to its high accuracy – up to the ppb (parts per billion) range – ICP-AES is particularly well suited to analysing trace elements, i.e. very low concentrations. This technique is excellent for detecting metals (such as mercury) and metalloids (such as arsenic), and dozens of elements can be analysed simultaneously. But what is behind this technique – and how does the careful delivery of gases play a role?
Controlled supply of argon gas through a flow regulator
The short version: The ICP-AES method of elemental analysis uses an inductively coupled plasma to produce excited atoms and ions of the elements in the sample to be measured, whose characteristic spectrum is measured using atomic emission spectrometry (AES) as they return to their ground state. The intensity of the lines in the spectrum is directly proportional to the concentration of the elements in the sample.
The ICP-AES equipment can only analyse samples in liquid form. That’s not really a problem for water, but things get a bit tricky with soil samples and other solid substances. To unlock the chemical elements, you have to dissolve the sample in a strong acid: aqua regia, a mixture of hydrochloric acid and nitric acid. A peristaltic pump sucks the sample liquid out of a storage vessel and transports it to the nebuliser, which turns the liquid into an aerosol form or mist. To accurately regulate the concentration of the mist – and to dilute it if necessary – a flow of argon gas is supplied to the nebuliser, with the assistance of a flow regulator. The mist then enters the reactor chamber, where it collides with the plasma that is already in the chamber.
If you supply a gas with sufficient energy – by passing a high electrical voltage through the gas using a coil – then some of the gas atoms release electrons. In addition to the original gas particles, you now have a mixture of negative electrons and positively-charged ions. This ‘ionised gas mixture’ of charged particles is called a plasma; plasma is considered to be the fourth state in which matter can exist, in addition to the solid, liquid and gaseous states. With ICP, argon gas forms the basis for the plasma, and this gas must be supplied with great precision, using flow regulators. The plasma has a very high temperature of around 7000 degrees Celsius. Because the plasma must have the correct composition at all times, a precise and continuous flow of argon gas is important. And to protect the outside world from this high temperature, a cooling gas (often but not always argon) is channelled around the outside of the reactor.
Regulating the mist
When the mist with the chemical elements to be measured collides with the plasma, these elements are also converted into plasma. The elements absorb so much energy that they enter an excited state. Elements don’t like to be in an excited state, so they try to return to their ground state at a lower energy level. During this transition, the elements emit radiation that is characteristic of each element. This radiation is measured by a spectrometer, and the intensity of the measured radiation is directly proportional to the amount of the element in question in the sample.
Since each element has its own characteristic set of wavelengths of the emitted radiation, you can use this technique to identify multiple elements at the same time. And if you have a calibration curve for the elements concerned, or if you entered an internal standard into the nebuliser earlier in the process, then you can also quantify these amounts.
Spectrometer, ICP-AES or ICP-OES
The spectrometer within the AES part is a combination of mirrors, prisms, bars, monochromators/polychromators and detectors, which guide and ultimately measure the emitted radiation. To prevent any disruption to this process – such as the absorption of radiation by gases containing oxygen – the area where these optical objects are located is continuously flushed with nitrogen. This gas flow does not have to be very precise, but it does have to be highly reproducible. The use of flow regulators is essential to ensure this reproducibility. Incidentally, you may come across the term ICP-OES (optical emission spectrometry), which is an alternate name for ICP-AES (atomic emission spectrometry). These are two different names for the same technology.
ICP-MS is a similar technique for elemental analysis; the biggest difference is that the method of detection is not optical. The charged particles from the plasma enter a mass spectrometer (MS); here, they are separated on the basis of their mass-to-charge ratio, and the relative ratio of each of these charged particles is recorded. ICP-AES is performed at atmospheric pressure, but ICP-MS requires a vacuum. The detection limit for ICP-MS is lower than for ICP-AES.
In an environmental analysis, you can look not only at the total quantity of an element in a sample, but also at whether the element occurs in its free form or as a component of a chemical compound. By way of illustration: inorganic arsenic compounds are often more toxic than their counterparts in organic compounds. You can use ICP-AES and ICP-MS to distinguish between different forms of elements, a process known as ‘speciation’. However, this requires the different forms to be separated from each other before the ICP process, for example through ion exchange chromatography (IC). For this reason, the IC/ICP combination is very common.
Mass flow meters and flow regulators for ICP-AES
Digial Manifold Solution
When ICP was first invented, the gases were added manually. When ICP became automated gas regulation was automated too, and mass flow meters were introduced. Mass flow meters and flow regulators are devices used in ICP-AES to supply inert gases. If you have good gas regulation, the entire system is more accurate and more stable, enabling lower detection limits. Which is helpful, given the increasingly strict quality and environmental standards.
Bronkhorst supplies flow meters for the analytical market; our customers include a number of large suppliers of analytical equipment. These customers are often supplied with specific ‘manifold’ solutions. In these solutions multiple functionalities are integrated into a single body, custom built for the customer. Compact instruments with small footprints are becoming more and more important in laboratories where space is increasingly restricted.
Read the application story “Controlled supply of gases in Inductively Coupled Plasma (ICP-AES) for environmental analysis”.
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.
Miniaturization by MEMS chip technology
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 chromatography. 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, flavour 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 and adsorbant with a constant Helium or Nitrogen flow.. The gas flow, containing the headspace sample gas, passes an adsorbent that collects the headspace sample gas. Now, the adsorbent is transported to the inlet of a gas chromatograph. Again 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 and different peaks 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. The IQ+FLOW product line, which is based on MEMS chip technology, 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'
A Coriolis mass flow meter is known as a very accurate instrument and it has many benefits compared to other measuring devices. However, every measuring principle has its challenges, as also the Coriolis principle. It can be a real challenge using Coriolis instruments in low flow applications in the heavy industry where you may have to deal with all kinds of vibrations. In this blog I would like to share my experiences with you regarding this topic.
The Coriolis principle
Coriolis mass flow meters offer many benefits above other measuring devices. First of all Coriolis flow instruments measure direct mass flow. This is an important feature for the industry as it eliminates inaccuracies caused by the physical properties of the fluid. Besides this benefit, Coriolis instruments are very accurate, have a high repeatability, have no moving mechanical parts and have a high dynamic range, etc.
Read more about the importance of mass flow measurement and the relevance of Coriolis technology in a previous blog.
Do vibrations influence the measuring accuracy of a Coriolis mass flow meter?
In industrial applications, all kinds of vibrations with different amplitudes are very common. A Coriolis meter measures a mass flow using a vibrating sensor tube, which fluctuation gets intentionally out of phase when the fluid flows through. As explained in the video [link] at the end of this article.
This measurement technique is somewhat sensitive to unwanted vibrations with a frequency close to the resonance frequency of the sensor tube (this depends on the sensor tube design, e.g. 360 Hz) or a higher harmonic of this frequency (see picture below).
Coriolis flow meters are only sensitive for the resonance frequency or a higher harmonic of this frequency
The likelihood of the occurrence of these unwanted vibrations is higher in an industrial environment. Coriolis flow meter manufacturers do their utmost to reduce the influence of vibrations on the measured value by use of common technical solutions, such as using:
- higher driving frequencies
- dual sensor tubes
- different sensor shapes
- mass inertia (mass blocks)
- passive and active vibration compensation
So yes, vibrations can influence the measuring accuracy of your Coriolis flow meter, but only if the vibrations have a frequency close to the resonance frequency. What can you do about this? This depends on the kind of vibration.
What kinds of vibrations do exist?
In an industry zone frequencies can be generated by:
- environmentally related vibration sources (such as: truck, rail transportation, industry activities)
- building-based vibration sources (mechanical and electrical installations, like air conditioning)
- usage-based vibration sources (installed equipment and machines, e.g. pumps, conveyor belts).
These vibrations travel through a medium like the floor, in the air, through pipes or the fluid itself. If these vibrations disturb the Coriolis frequency, the measured flow could be incorrect in some extent.
To minimize the effects of vibration it is useful to identify these sources. Sometimes, it is possible to move the flow meter just a little bit, turn it (Coriolis flow meters are in most cases less sensitive to vibrations if the meter is rotated 90 degrees), make use of a big(ger) mass block, use flexible tubes or U-bend metal tubes, or use suspension alternatives.
How could you check the performance of a Coriolis flow meter?
A well performing flow meter and controller will give the best process result. Therefore, it is advisable to test a Coriolis flow meter in your application if you expect heavy industrial vibrations before you trust it to the full extent. Be careful when filtering the measuring signal. In some cases it makes sense (e.g. when a quick response isn’t required), but if you want to test the performance of a flow meter, filtering could blur your judgement.
Coriolis flow meter in action
If in specific circumstances the Coriolis flow meter isn’t performing the way it should, the operator will see a shift in the process output – for example in an application dosing color to a detergent it can result in differences in product color by incorrect dosing and/or unexpected measuring signal behavior. In these cases it makes sense to check the raw measuring signal (without filters!), because it will give you a good insight in the performance of the flow meter. Ask your flow meter manufacturer how to switch off all signal filtering.
Standards regarding vibrations
Remarkably, the influence of external vibrations is not clearly defined in a standard for Coriolis flow meters. Several standards are written about vibrations, but none in respect to measuring accuracy in relation to vibrations. However, two useful standards in relation to vibration are:
- IEC60068-2, Environmental testing for electronic equipment regarding safety
- MIL STD 810, Environmental engineering considerations regarding shock, transport and use
As a user of Coriolis flow meters it is important to understand your application, especially about potential external vibration sources. As low flow Coriolis specialist we work together with knowledge partners like the University of Twente and TNO (a Dutch organization for applied scientific research) to get a continually improved understanding of this topic.
With in-house test facilities we are able to do special vibration tests. Together with the experience we gained from customer applications and custom made solutions, we are always aiming for improving our Coriolis flow meters to give our customers the best performance they need.
Watch our video explaining the Coriolis principle:
Read more about the importance of mass flow measurement and the relevance of Coriolis technology in a previous blog.
Check out our success story using Coriolis mass flow controllers for odorization of our natural gas.
Industrial low flow applications have to cope with a wide variety of environmental and process conditions, but what does this mean when we talk about ‘industrial’? Knowledge about the specific application and low flow fluidics will help a lot to prevent slipping.
We often refer to ‘uncontrolled macro-environments’ for equipment, when we talk about ‘outdoors’. However, it can also be a room or factory without (local) climate control in which equipment is experiencing comparable temperature and humidity variations as outdoors.
What is important in low flow applications and what kind of challenges do you encounter? Let me share my ideas in this blog.
What is IP-rating?
I experienced that IP-rating is not always interpreted correctly. Having the highest possible IP-rating is often mistaken with having an ‘industrial-device’. But what does the IP-rating actually indicate? The first digit of an IP-rating only refers to dust ingress protection and the second digit refers to the liquid ingress protection.
Therefore, a higher IP-rating does not always mean that the instrument is better and more suitable for your application. Hence, it can even make things much worse in practice. A reason for this is that even the tightest IP-rated constructions may breathe in and out, caused by internal and external temperature variations. This can lead to internal condensation, especially in high humidity environments, if no further precautions are taken.
The importance of dedicated low flow equipment
Not surprisingly, things are often a lot smaller in low flow applications. The other side of this coin is that common process and environmental disturbances have a proportional larger impact on these low flow applications compared to traditional ‘normal’-flow applications.
In general, an industrial flow instrument, like a flow meter, needs to be suitable to a lot of external influences, such as resistance to corrosion and impact or pressure ratings. These requirements often lead to selecting more standard industrial flow meters instead of specialized low flow instruments. This is not always the best solution for the required low flow ranges and can lead to unsatisfactory results.
What we want to achieve is to have rigid flow measurement and control, suitable for the application during the economic lifetime of the installation. Therefore, it would be best to select the best flow instrument fit for purpose. In case of low flow applications I therefore recommend to use dedicated low flow equipment. These flow meters are designed and tested for these kinds of applications.
Our industrial low flow mass flow meters and controllers can be equipped with integrated control valves or dedicated pumps, especially designed for low flow purpose. Stable control characteristics are combined with signal-to-noise ratio plus being proportionally less sensitive for disturbances.
Bronkhorst industrial low flow instruments
We gladly support you in process and environmental equipment selections including system design aspects, starting with selecting the most suitable measuring and control principles. Our flow meter product portfolio contains laboratory-style and light-industrial flow meters to heavy-duty IEC-Ex/ATEX-rated industrial versions (…having a “high” IP-rating as well).
Drive your low flow control reliable and safely!
Our product manager for liquid technologies, Ferdinand Luimes, explains how to deal with vibrations using Coriolis mass flow meters.
Visit us at the Hannover Messe (April 1-5, Hall 11, booth A50)) and have a sneak preview at our new industrial Coriolis flow meter.