Litre Meter strengthens South Asia team

LM1 - Claus Weihermueller(1)As part of part of the TASI group of companies Litre Meter now has new representation in Singapore. A new KEM/TASI Flow office in Singapore has been established with Claus Weihermueller appointed as regional manager of KEM/TASI Flow Asia Pacific & Middle East Operations. 
Claus has spent the past 15 years working in the field of flow metering and has been living and working in Singapore for eight years. He will promote and further expand the KEM/TASI business in the region, including Litre Meter.

Slim line customisation

Litre Meter’s ability to highly customise its meters has led to the company winning a significant order to supply meters to a company that produces hydraulic systems to the oil and gas industry. The fluid to be measured was Castrol Transaqua HT 2, with a viscosity of 3,95 cSt at 20 degree Celsius. The meters were bespoke slimline ones with a wide turn-down to allow for additional meters on a skid.

VFF positive displacement meters had to be made to operate at different pressures and flow ranges with different connections ranging from a VFF8 meter with a design pressure of 44 Bar, a scale of 0.3-30L/min with 25mm Female BSPP or equivalent size connections up to HP20 Flowmeters designed to work at a pressure of 950 Bar with a scale of 0.1-10L/min and 3/8” MP Autoclave connections.

All the flowmeters were located and certified for in Zone 2, Gas group IIB and Temperature class T3 so had to be EEx D or E. The minimum accuracy had to be within ±1.0% of full scale and the meters were constructed from 316SS quality stainless steel for all wetted parts.

The solutions provided were:

VFF8/690bar/AGPVD/V/9/16″AE MP

VFF8/1035Bar/AGPVD/V/9/16AE MP

HF20/207Bar/AGPDV/V/1″NPT all with


Significantly we were able to offer measurement over the whole of the flow ranges required which competing flowmeter manufacturers were not able to deliver. In addition, the bespoke slimline VFF8 1035bar on drawing enabled the end user to get an additional meter on the skid without having to increase the available space.

Now we are 40

LM1 - Mug shotLitre Meter is 40 this year and to celebrate we are giving away commemorative mugs.

To get yours while stocks last email us at and we will get one in the post.

See you at ACHEMA

Staff from Litre Meter will be at the ACHEMA exhibition in Frankfurt in 2015. We will be joining over 3,000 other exhibitors at the show.

ACHEMA will this year be focusing on products, technologies and solutions for the bio-based economy, industrial water management and innovative process analytical technology. At Litre Meter we have solutions for flow control and measurement in all three areas.

To register for the show visit the ACHEMA website.

Next Generation Flowmeters for Fluid Measurement and Control Solutions

Charles Wemyss, Litre Meter Limited

Types of Flowmeters fall into many categories. One could use the involvement of moving parts and electronics to define this. Mechanical meters, used and invented before domestic electricity was prevalent must be Old Generation. These would include what you and I have outside our houses for the measurement of domestic water. They would also include meters in our gas supply for the measurement of our consumption of gas. The very first turbine meters credited to Woltmann in 1790 were considered for calculating the loss of energy in open canals. It would be true to say that these were used for counting or totalising flow rather than providing an instantaneous rate display or output.

From the Old to the New

Those using electricity or electronics with a moving part like a rotor are also Old Generation as turbine meters have been around for several decades, for example. The first of these were axial turbine types developed, in essence from Woltmann, in the Second World War for accurately determining the fuel consumption of military aircraft and torpedoes. The pick-up or sensor with a magnet and rotating conductor enabled the number of rotations to be counted, totalised and used for rate display.
If we define Next Generation meters as having no moving parts so that the definition encompassed Thermal, Coriolis, Ultrasonic and Electromagnetic, then there would be a modern outlook. Apart from the fact that Faraday tried making an electromagnetic meter to measure the River Thames almost 200 years ago! He only failed because his instrumentation wasn’t sophisticated enough.
The obvious question to ask is: What is Next Generation, What is Current Generation and What is Old Generation? We can be certain that Old Generation does not mean unusable. We can also be certain that Old Generation in some people’s eyes is more than adequate for various tasks. This article explores the provenance of some flowmeter technologies, what might be round the corner and how to select the best meter for each project.

Some new and not-so-new flow measurement techniques:

New Technology

Coriolis, inertial force was first formulated by Gustave Coriolis in 1835 but MicroMotion didn’t release a commercial unit until 1977.

Electromagnetic, proven by Faraday but commercially produced from 1952. *Ultrasonic, from 1963.

Vortex, using the van Karmann effect of the generation of alternate vortices past a bluff body commercially from 1969, famously spotted by Leonardo da Vinci in 1504.

Thermal, hot wire anemometers were used from the early 1900s, commercially from the 70s.

Sonar, unconventional and measures turbulence since 2003.

Optical, measuring the speed and direction of individual particles using a laser beam, in research labs in the 70s and 80s but only commercially used in flare gases.

Traditional Technology

Differential Pressure like an orifice plate or Dall tube with a separate differential pressure transmitter. Also nozzles, pitots, Venturis and wedges. Still the most popular non-domestic meter type.

Positive Displacement, commercially pre 1830s for diaphragm gas meters with sheepskin diaphragms and sheet steel enclosures.

Turbine, first drawn up in 1790, commercially available post Second World War.

Variable Area, available for most of the 20th century.

Low Flow technology and the next ten years

There are various technologies that present themselves for low flow shown below. Many of the others mentioned elsewhere do not scale effectively.

Coriolis: Most manufacturers concentrate on ½” (15mm) and above. The issues of balance and producing thin wall tube to the required dimensional tolerances are hard to overcome. Smaller sizes exacerbate this.

Thermal: Microelectromechanical systems (MEMS), generally 0.01 mm to 0.1 mm in size, consist of a CMOS circuit on a thin silicon substrate. For lower flows these will replace a larger heated element and sensor. Liquids have a massively different thermal conductivity so the same device can measure at grams per hour rather than grams per minute.

Positive Displacement: Generally their purpose is to positively measure a trapped volume of fluid ? either gas or liquid. Gas versions tend to be for higher flows with the most popular one being used for domestic gas measurement. At lower flow the leakage between successive volumes is too large for effective measurement. For liquids where there is more viscosity the PD meters work well. Developments focus on some novel types and constant improvements to existing designs. There is a law of diminishing returns as the smaller the mechanical parts are, the harder they are to manufacture accurately. Also, leak paths are proportionally larger. One of the new types is the pendulum. This has one moving part with low mass and minimal friction loss, enabling it to respond to extremely low flow volume rates from 0.3 litres/hour. Unusually, this unit only works with viscosities up to 5 centiStokes. The rotary piston meter also has one moving part. In terms of flow rate, like most PDs, these prosper on viscosity. At 10 cSt a typical meter will start measuring at 0.08 l/h and when water is measured this increases to 0.4 l/hr.

In line ultrasonic: What happens when the pipes reduce in size and the type where a sensor is clamped on the outside of the pipe is no longer applicable? The sensors are mounted inside the pipe usually contrived in the shape of a U so that the ultrasound is passed between sensors at the base of the U. By knowing the diameter of the tube and the velocity between sensors, the volumetric flow can be calculated. Liquid flow rates down to 2 ml/min can be measured.

So if it’s not the methods of measurement we use that define Next Generation what is it? Perhaps: intelligence? The rise of smart meters i.e. those with digital communications and with the ability to self-verify are undoubtedly modern but were defined decades ago and have been in use for many years.

What’s Next?

Wireless communication is similarly up-to the- minute. HART digital communication has been around since the mid-1980s when it was developed by Rosemount Inc. for a range of measuring instruments, not just flowmeters. The HART foundation was formed in 1993 and the wireless version came along in 2007. So quite modern but Next?
So, is it the flowmeters that inhabit university laboratories and the R&D departments of flowmeter manufacturers that constitute Next Generation? Can we speculate what a cutting edge meter might look like in ten years’ time?

No Moving Parts

It would be fair to say that this Future meter would have no moving parts. This improves the chances of long term use as it would not suffer from mechanical degradation either planned or unplanned. It would ideally be non-invasive i.e. it would fit on the outside of a pipe and nothing would actually breakthrough the pipe wall. Currently, just ultrasonic meters match this criterion so let’s say that’s less than likely and the meter will therefore be non-intrusive. The sensor will break through the pipe wall but won’t impede the flow or perhaps just negligibly. What will the sensor measure, what techniques will it employ? That’s the $64,000 question. A single sensor is less likely as there will have to be a reference point for comparison.
Probably two sensors set apart, then, monitoring a property of the fluid. The clever part will be the intelligence of the signal processing; looking for perturbations in the signal amplitude and comparing it to the next sensor. Dumping thousands of comparisons for the sake of a few, locking onto patterns and pumping out high strength signals. In fact, the real hurdles will be firstly customer acceptance and secondly, electronic component obsolescence. Will the customer accept this meter and will it continue to find the small perturbations in property? Can he see it in action? Does he get a sense of goodness in the signal, in the rejection rate? What if the pipe vibrates, if the temperature ramps up, if the ‘property’ disappears? Then we find out that metering and measuring is about confidence, experience rather than Next Generation.

Bringing the Oil and Gas Industry Up-to-Date

The Oil & Gas industry is relatively conservative, relatively slow moving. The prevalence of HART and 4-20 mA signals decades after their introduction speaks volumes. Wireless, Bluetooth and fancy bus protocols are only just now making significant in-roads offshore. The creep of domestic innovation exemplified by the rise of the smart phone encourages instrument designers to bring their act up-to-date. Most instrumentation can only be compared with the most basic mobile phone. There is an inherent expectation that the modern user will have something easy-to-use, colourful and dangerously (?) customisable. The smartphone has many different uses of course beyond that of making calls. Arguably, it’s an instrument display in its own right. The logical conclusion is that the meter ‘display’ will be with the operator the whole time, in his/her hand. The obsolescence of components that bugs the subsea side of the industry is irrelevant in the actual instrument as this is replaced by the mobile phone and it’s ‘app’.
That still leaves the problem of the fast-moving world of miniature components for the clever parts – that will always be a thorn in the side of designers. Just as with most technologies, we’re not trying to design something to last for 30 years; the likelihood is that it will be overtaken by a new Next Generation device in ten years and then again in twenty years. All we can hope for is that the unit is still working in ten and twenty years and only needs replacing in thirty.
To select the best flowmeter for each application it is not just a question of looking up the first flowmeter you thought of on Google. Nor is it asking the engineer on the next desk or even consulting the internal specification guides issued by your employers. And it certainly shouldn’t be by selecting the cutting-edge meter of the day. It should be by consulting a flowmeter specialist – a specialist that has a wide range of solutions, not just one that is shoehorned into every application. Ideally, an independent specialist who can give unbiased advice and who will, if necessary, recommend an external solution.

Looking to the Future

In conclusion, the Next Generation of flowmeters is already operating, they’re already proven and they’re probably on the specification lists. Most applications can be met, more than adequately, by existing techniques. But the manufacturers aren’t standing still. They’re continually leveraging current technology with creeping demands. It’s more evolutionary than revolutionary but we’re all getting there – safely, economically and technically.

Safety research highlighting key ‘drivers’

Results of our first survey into safety issue in the oil and gas industry have shown that concerns over risks to personnel and the environment are key drivers for the implementation of international safety standards for instrumentation.

It will be interesting to see how the results of that survey ? which concentrated on Safety Integrity Levels (SIL) ? compare with the results of the second (still current) survey which focuses on the Pressure Equipment Director (PED). You can take part in our PED survey until 5 September by going to One lucky participant will win a Kindle for taking part.

Our ‘SIL survey’ gathered the opinions of senior engineers worldwide with technical design and management roles within their organisations. It showed that when specifying flowmeters and other instruments, environmental safety (70 per cent) was cited as the main reason for safety standard compliance, followed by business-critical concerns including personnel safety (59 per cent) and maintaining process integrity (65 per cent).

Risks of injury to personnel (70 per cent), risk of explosion (65 per cent) and damage to the environment (50 per cent) were the chief concerns relating to the consequences of equipment failure. Business concerns including costs of shutdown (25 per cent) and damage to equipment (15 per cent) were of less significance.

While the majority of companies in the oil and gas sector comply with an international safety standard for instrument specification a significant number ? almost 40 per cent ? do not, the survey found. However, most of these stated that they will be seeking to comply with an international safety standard in the future where relevant.

One of the aims of the research was to find out to what extent engineers use SIL in specifying equipment and what reliance they place upon it.

Almost 40 per cent said that SIL level 1 was the minimum acceptable for instrumentation in their operations with 22 per cent citing level 2, 26 per cent stating level 3 and just 13 per cent saying that the highest level (level 4) is the minimum acceptable.

We wanted to make sure that our manufacturing focus is on safety in relation to both the environment and industry trends. These figures show that by complying with SIL we have a reliable benchmark for safety and reliability.

SIL was considered to be an effective measure of safety performance by 70 per cent of respondents but 54 per cent believed that a lack of consistency in applying SIL across all functional safety standards significantly affects trust in products designed to work in particular SIL level environments.

Now we are asking a similar range of questions about PED ?and with a similar purpose.

There has been increased focus on safety issues in the offshore sector over recent years. We want to make sure that our manufacturing focus is on safety in relation to both the environment and industry trends.

To take the PED survey visit and spend just a few minutes answering the questions.

SIL is the degree of likelihood that a safety instrumented function will operate effectively when it is required to. Four SILs are defined within the European Functional Safety standards based on the IEC 61508 standard, SIL 4 being the most dependable and SIL 1 being the least, taking into account such things as the development process and safety life cycle management.

Litre Meter, Company of the Year

Text from Industrial Process News article, issue 622

It gives us great pleasure to present Litre Meter Limited with the prestigious and highly sought after Industrial Process News Flow Measurement ?Company of the Year?.

Speaking with CEO Charles Wemyss and Industrial Process News Editor Thomas Gill, we unveil the reasons why Litre Meter Limited has been selected for this prestigious award, why the company has been so successful over the past twelve months and how they have weathered the economic crisis.

Thomas explained: ‘If I’m being honest, it didn?t take much deliberation when selecting our Company of the Year. From our research, Litre Meter were head and shoulders above the rest in the category and have proven their worth especially over the past twelve months.

‘As we all know, weathering the economic downturn is crucial for all businesses and very difficult at that. But Litre Meter have managed to not only sustain their position in such difficult times but have also seen some encouraging signs, continually moving forward, and this was the real clincher for the team at Industrial Process News.’

Charles explained: The last time we won an award was back in 2005 and that was a springboard to part of our success, and we are hoping that this will generate the same outcome. It feels great to know that our hard work and dedication has been noticed. A job well done I say.

The past twelve months has been fantastic for us, as we have seen significant growth, acquiring more orders than the previous two years combined. This has particularly been driven by our presence in the Oil & Gas sectors. We are now in a position where we are very busy exporting, have a huge level of stock and have a large back log of orders that we are currently working on. Having so much business has given us a great deal of confidence.

To be honest the current economy hasn’t affected our business at all, which has allowed us to take strides forward in the industry. As the majority of our business is executed abroad, we haven’t had to shelter ourselves from any hardship in the UK. Our sales in the UK have risen and we are expecting, once the UK economy is stabilising, home-grown business to get even better, opening more doorways.

Our forecasts were significantly exceeded with a 29% rise in sales over budget and a 91% rise in bookings compared to 2012.

Our most successful product has been the LF05, part of the VFF range, which has been available for its first full year in 2013. Particularly successful in the Oil & Gas industries, the LF05 has pulled along other flowmeter sizes with its success, plus accompanying  instruments and accessories.


A simple construction with only one moving part and reed switch pulse output, the LF05 is a positive displacement flowmeter with 316 stainless steel body. With a PVD coated titanium rotor only, the LF05 is hazardous area approved as standard. Available with any end connections to your individual applications, the product has a flow range of 0 to 30 I/hr and is accurate to ±1% linearity, ±0.5% linearization and ±0.25 repeatability.

Other Litre Meter 2013 innovations have included: LF03 estimated at 60% flow rates of the LF05; a new connection type ? ½ inch OD tube; and QR codes on each meter, with links to material certs, calibration certs and instructions via the internet.

Looking ahead, we are aiming for more of the same. We currently have four times more orders, and we are looking to utilise our additional resources and information to increase production for the forthcoming years. With more resources available our ability to plan ahead has dramatically increased and we continue to grow and innovate.?

Company Profile:

Founded in 1975, Litre Meter Limited is a world-leading designer and manufacturer of flowmeters, which are installed on a wide variety of applications across the UK, Europe, America, as well as the rest of the world. Litre Meter works in a number of high profile industries including Oil and Gas, Aerospace, Automotive, Marine, Mining, Power Generation, and many others.

Litre Meter is a member of the TASI Flow Division, as well as the Institute of Manufacturing at Cambridge University, FPAL, Achilles, Institute of Measurement and Control and Northampton Chamber of Commerce. Litre Meter hold ISO9001:2008 approval.

Charles Wemyss, Chief Executive of Litre Meter Limited, commented: ?When flow measurement is critical, consult the specialists. Litre Meter has been leading the field since its founding, with a well established reputation for the design, manufacture and supply of flowmeters of all kinds.?

Firstly, Litre Meter pioneered the development of the Pelton Wheel flowmeter: an accurate, effective and versatile design suitable for many different fluids including acids, chemicals, fuels, spirits and water, at both high and low flow rates.

Then since 1986, Litre Meter has developed the popular VFF meter (literally, Viscous Fluids Flowmeter) for low flow & high pressure applications for the offshore industry particularly for chemical injection. And in 2011 the company supplied the largest ever chemical injection flowmeter order for the Gulf of Mexico.

Litre Meter?s portfolio for the UK now includes flowmeters of all kinds, including rotary meters, gas flowmeters, helical screw flowmeters, electromagnetic and ultrasonic flowmeters to handle a wide range of viscosities, flow rates and pressures, aggressive liquids and hazardous or adverse environments. They have particular expertise in measuring low flow rates and flows at high pressure.

‘To fulfil our customer’s needs in the UK we also distribute other flowmeters from many flowmeter manufacturers. For gas flow we offer Sierra thermal mass gas flowmeters, vortex flowmeters, Ritter gas meters, Hoffer turbine meters and Ritter gas sampling bags. Sierra also manufacture thermal mass gas flow controllers,’ added Charles.

For more information please don’t hesitate to contact one of the team on 01296 670200 or email: Alternatively, to view a full product portfolio or download spec sheets visit



Safety first – new safety survey launched

There has been increased focus on safety issues in the UK offshore sector over recent years. With that focus in mind we at Litre Meter want to make sure that our manufacturing focus is on safety in relation to both the environment and industry trends.

According to the most recent raw data available from the Health and Safety Executive in Q3 of 2013 there had been 27 offshore hyrdrocarbon releases in the UK. This represented a significant reduction in HCR releases in response to the HCR reduction plan initiated by Step Change in Safety ( in 2010.

The HSE has defined asset integrity as the ability of an asset to perform its required function effectively and efficiently while protecting health, safety and the environment. Asset integrity management was defined as the means for ensuring that the people, systems, processes and resources that deliver integrity are in place, in use and will perform on demand over the asset?s lifecycle.

Issues surrounding the environment and hydrocarbon releases, asset aging and life extension drive the focus on safety. We want to be able to help in the process of recognising hazards and reducing risk as well as help engineers take ownership of risk and asset integrity through proving assertions about the functionality and construction of instruments.

Litre Meter has therefore launched the first in a series of oil and gas industry safety surveys that will be introduced throughout 2014. The first survey concentrates on safety integrity levels (SIL) and one lucky respondent will win a Kindle for taking part.

SIL is the degree of likelihood that a safety instrumented function will operate effectively when it is required to. Four SILs are defined within the European Functional Safety standards based on the IEC 61508 standard, SIL 4 being the most dependable and SIL 1 being the least, taking into account such things as the development process and safety life cycle management.

The Litre Meter survey aims to find out to what extent engineers use SIL in specifying equipment and what reliance they place upon it.

To take the SIL survey visit and spend just a few minutes answering the questions.

Chemical injection – from Africa to Arctic

Two articles in the current issue of Offshore magazine cover very different areas of the world ? West Africa and the Arctic.

They also cover very different scenarios. One is talking about ‘brownfield’ development – extending the life of existing fields and brining new reserves online through existing infrastructure. The other is focused on new technologies and challenges in safely exploiting hitherto unreachable reserves.

The common element is that both articles mention chemical injection as a key technology. This is, of course, an area in which Litre Meter has considerable experience.

Over the past year we have shipped a large number of meters to be used subsea, on a variety of chemicals, over a wide range of flows at high pressure and calibrated at specific viscosities. For example, Litre Meter rotary piston flow meters are part of systems used to control the amount of ‘antifreeze’ injected into pipelines at high pressure (430 bar) in subsea gas exploration on fields in the Caspian Sea and the North Sea.

‘Antifreeze’ fluids like methanol are used as thermodynamic inhibitors, which lower the freezing point of gas hydrate. They are injected into pipelines where there is a risk of hydrates (dew) forming then freezing at low temperature.

The antifreeze prevents gas hydrates solidifying as crystals and blocking pipelines – which can result in a costly shutdown and the risk of explosion or unintended release of hydrocarbons into the environment.

Litre Meter has also recently shipped flowmeters for use in chemical injection skids on a number of fields in the Gulf of Mexico and the Persian Gulf off Dubai. These meters are used in the flow measurement of wax dispersants and pour point depressants (PPD) to control their use to a very high tolerance.

Wax dispersants break apart and prevent the reformation of hydrocarbon sludge deposits and improve flow by reducing the viscosity of the fluid. PPDs are used to reduce the viscosity of oil and to maintain flow rate by preventing the build-up of wax crystals at low temperatures.

Sludge deposits are typically composed of varying concentrations of hydrocarbon, asphaltene, paraffin, water and inorganic materials. They are commonly found in storage tanks and vessels, production and transportation pipelines, process systems and hydrocarbon-producing formations where they have an adverse effect on the flow of crude oil from the well head.

Our expertise is nicely summarised in our new chemical injection brochure which highlights the enormous success of the VFF meter over the past few years in solving chemical injection measurement problems around the world.

With 3,000 VFFs in active use on chemical injection Litre Meter has demonstrated a depth of experience and knowledge that is unmatched in the low flow arena. The brochure provides further detail of the VFF range in one eight-page document.

Positive displacement meters: pros, cons and selection

Positive displacement flowmeters, sometimes known as PD meters, have been around for more than 100 years. They are commonly used in a wide range of applications from domestic water measurement to measuring ultra flow rates of chemical at high pressures subsea.

First off – what is a “positive displacement” meter? Well, as the name suggests it involves the positive displacement of a volume of fluid – this is usually a liquid but there are some units suitable for gas. There is a chamber and inside the chamber, obstructing the flow, is a rotor.
The shape of the rotor and chamber vary greatly with each meter type but they all provide an output for each rotation. Most meter designs therefore lend themselves to being totalisers. Most can have the flow rate calculated from this primary data.
An accurate PD meter will have minimal ‘leakage’ across the rotor seal. This is generally minimised with the use of more viscous liquids and accuracies of ±0.1per cent are sometimes quoted. On the other hand rotary piston flowmeters are used by the water industry in the UK for measurement of water over a normal flow range to accuracies of ±2 per cent.
Because they measure a volume precisely it does not matter if the flow is pulsing. They will follow the increase and decrease of flow found in reciprocating pumps of all types. With higher viscosities the turndown ratio can be high. Even with water 100:1 is not uncommon and 3000:1 is possible at 250cSt. Few applications require this but it does enable measurement of ultra low flow rates without miniature parts or normal flow measurement at minimal pressure drop.
Most meters are simple to maintain as they have only one or two moving parts and are coupled with simple readouts that are easily understood in the field. There is no requirement for straight pipe lengths like that might be needed for electromagnetic or turbine devices. They can be connected directly to elbows or valves and in most cases in a variety of orientations.
Designs are relatively easy to adapt for high pressure applications eg over 100 bar.
All PD meters require clean fluid so a filtration level of 100 micron is usual. Some meters can actually block the flow if a larger particle is trapped in the wrong place. Many meters are not made in high specification materials and therefore corrosion can be a concern. An all plastic or all 316SS meter is the exception rather than the rule. As the application flow rate increases the size of the PD meter seems to increase by a square law! It is rare to find meters over 12-in in size although they exist at these elevated sizes for the prime reason of accuracy – frequently being utilised for custody transfer reasons.
In the author’s opinion, the most common PD meters are as follows:
  • Rotary Piston: As mentioned above these form the basis of domestic water measurement but the design of the rotary piston that oscillates in a circular chamber with a fixed web has been modified and extended to ultra low flows and high flows, as well as high pressures and for food applications. A good all-rounder.
  • Spur gear: The fluid rotates two gears and is forced around the outside of the gears and the inside of the chamber. Depending on the location of the sensor these can yield very high pulses per litre values useful in batching and fast acting processes.
  • Diaphragm (or bellows meter): These are common in many people’s home as their domestic gas meters. When the gas flows through it alternately fills and empties bellows causing levers to crank a shaft providing an output. Very useful for wide-ranging gas totalisation.
  • Oval Gear: Quite similar to the spur gear where two oval gears mesh together and sweep the chamber. The volume displaced is much larger than the round gear. Fairly low cost and some designs available in plastic.
  • Nutating Disc: This meter is the hardest to understand but is effective. The rotor is a circular disc attached to a ball. The shaft on the ball is inclined. As the disc rotates in a spherically sided chamber the disc and therefore the shaft wobble creating an output.
  • Helical Screw: Possibly the most accurate PD: meter two intersecting cylindrical bores are fitted with 2 interlocking helical screws. As the fluid passes through they rotate. On standard applications the author has observed differences of just ±0.37 per cent of reading over 50:1 turndown over annual recalibrations over 10 years – quite an achievement. Also common nowadays fitted on petrol pumps.
  • Slide Vane: Historically the most accurate of PD meters with the rotating element having a number of moving blades that rotate about a fixed cam. Linearities have been claimed of ±0.02 per cent.
  • Others: If we go back to Felix Wankel’s seminal work on rotary machines we see that there are as many designs for PD meters as there are pumps. He explored in a rational way the various shapes of rotor and chamber. While the majority don’t see the light of day in the marketplace this brief essay illustrates the variety in general use, and this is without discussing the Roots meter, wet gas meter and multi rotor designs.
Two decades ago the PD meter was considered to be old technology and likely to be overtaken by more modern electromagnetic and ultrasonic devices. Nowadays the PD meter still represents good value and can provide excellent measurement in a wide variety of duties.

Product labelling – QR codes, documentation access

We also unveiled a new electronic project documentation system. Details of each flowmeter supplied as part of a single project – for example, all the flow meters supplied for a particular chemical injection skid – will be held at a unique URL. The website will include items such as calibration certificates, PMI certificates and material certificates as well as specifications, manuals and instructions.

The address of the website will be printed on a chemical-resistant and wear-resistant label securely attached to each meter. The label will also carry a QR code linking to the website which will make it easy for service personnel on site to call up all the documentation on a smart phone by simply pointing it at the label. We believe most customers will note down the simple address and access it from a control room.

LM QR code
Litre Meter QR code allows remote access to documentation

Flowmeter selection and calibration – How well do you need to know it?

What factors should you look for in flowmeter selection relating to the output or display?

  1. Precision. Often misunderstood, but in the most part, it’s what matters in measurement. It’s the degree to which repeated measurements under unchanged conditions show the same results.
  2. Accuracy. The degree of closeness of measurements of a quantity to that quantity’s actual (true) value.
  3. Linearity. For flowmeters it’s the curve of accuracy compared against flow rate.
  4. Resolution. If the digital display only has 3 digits then selection of the units has more effect than the accuracy (etc.) of the meter itself. For example, set up with a maximum of 110 US gallons/min the resolution of ±1 US gallon per minute is 1% at best and 5 or 10% or worse at lower flows. Changing over to litres improves the resolution by a factor of 4. More importantly this shows the value of having enough display digits to match the users requirements and, probably, the accuracy of the meter.
  5. Traceability. So the supplier gives you a set of data, a claim of performance. All meaningless without reference to something solid, something comparable like a National Standard.
  6. ISO 5725. According to ISO 5725-1, the terms trueness and precision are used to describe the accuracy of a measurement. Trueness refers to the closeness of the mean of the measurement results to the actual (true) value and precision refers to the closeness of agreement within individual results. Therefore, according to the ISO standard, the term “accuracy” refers to both trueness and precision.
  7. ISO17025. Simply a laboratory standard: General requirements for the competence of testing and calibration laboratories.
  8. Repeatability. Another word meaning precision but often taken as the closeness of one set of results to some more with conditions unchanged. Probably should include some reference to time and:
  9. Hysteresis. In some systems the precision varies according to whether the flow is increasing to the measurement point or decreasing. In particular near the start-up flow rate it may be found that, with the flow increasing from zero, the meter provides an output at ‘x’ whilst, when the flow decreases the meter may continue to provide an output at lower than ‘x’.
  10. Long term accuracy. This could be restated as: will it measure the same tomorrow as it does today and what about next year?
  11. Recalibration. The periodicity at which the meter should be recalibrated is not set in stone. Some meter types are less stable than others. Where the meter is used to calculate tax or fiscal amounts then a daily recalibration is sometimes necessary. In a benign fluid, with flow rates kept within bounds then others might need checking every 10 years. Litre Meter recommend a yearly check at first, analysis of the results, then an increased period depending on the customers needs.
  12. On site calibration. Whilst every flowmeter that Litre Meter manufactures is calibrated in laboratory conditions on a similar fluid and at a steady flow rate there are differences such as meter orientation and pressure pulsation.  Whilst pressure pulsation won’t affect positive displacement meters it can have a severe effect on turbines, for example. So Litre Meter recommend that each meter is calibrated in-situ. Various techniques are described in the flowmeter manual.

Expert recommendations form Litre Meter – the flow measurement specialists.