SAW technology for Bürkert flowmeter

This review of Surface Acoustic Wave (SAW) techniques was first published in my regular column in the December issue of the journal “South African Instrumentation and Control” (SAIC), published by TechNews.co.za

The SAW (surface acoustic wave) technique offers fascinating opportunities for many different styles of monitoring sensor. The first example seen many years ago really impressed me: it was called TorqSense, a torque measurement sensor applied onto a drive shaft, with no external electrical connections to the shaft needed. This used a SAW device mounted on a quartz substrate: the input and output sensors for the acoustic waves are separated by the length of this substrate, which changes as the quartz is deformed by the torque. Feedback creates a high-Q resonant circuit, and the resonant frequency changes as the quartz is distorted. RF excitation and monitoring of this resonance from an external unit gives a measure of the torque: this has been offered commercially by the UK based Sensor Technology for over 10 years.

Since then SAW techniques and sensors have been studied and researched by many universities, and sensors have resulted that measure temperature, pressure, viscosity, humidity, and even chemical concentrations. The idea is to choose a substrate or acoustic delay-line material between the acoustic transducers that is influenced by the environment to be monitored, such that it is stretched, or the acoustic path length changes in some other way. A recent market status report, by Mordor Intelligence, suggests that the total market for all such SAW sensor systems will be almost $4Bn by 2018.

The clever part in creating a sensor is to modify the acoustic properties of the piezoelectric material between two sensors in some way. Chemical and biochemical sensors for monitoring liquids have been created using a lithium tantalate piezoelectric with a micron thick coating of PMMA or cyanoethyl cellulose, which is sensitive to the chemical target, and keeps the surface waves near the surface, which are therefore influenced by the liquid properties.

Industrial flow applications

After collaborating with such university research for some years, in 2014 Bürkert saw the opportunity to develop a liquid flowmeter using SAW transducers, which could give major advantages particularly in hygienic applications – one of its key market areas. In this case, the SAW transducers were to be used to launch the ultrasonic pulse into the pipe wall of the flowmeter, which then leads to transmission of the signal diagonally across the fluid flow. The pipe wall and the moving liquid create the variable length acoustic delay line between opposing pulsed sensors, and fluid movement creates the change in this delay.

Burkert261_FLOWave_SAW_flow_sensor_pic1_PR2548_58253Effectively, Bürkert was using the SAW transducers as the upstream and downstream sensors for a time of flight type ultrasonic flowmeter. But also there is no intrusion into the flow tube, so the meter is suitable for ultra-pure applications like pharmaceuticals, water for injection and so forth, as well as food and beverage applications.

Development and field testing has covered the last two years, with a careful product release for suitable applications – typically initially used on low conductivity clean liquids, such as water for injection (WFI) in the pharmaceutical sector. Indeed one field test unit was installed in the supply line of a production filling system for infusion bags. Now, the Bürkert FLOWave range of flowmeters, covering DN15 to DN50 pipe sizes, is fully available for sale. This range of sizes covers the smaller bores typical of industrial requirements, in contrast to the larger ultrasonic flowmeters available from other suppliers. FLOWave is designed for hygienic use, and certified to EHEDG and 3A standards. The pipe has hygienic style end connections, and is internally finished to 0,8 or 0,4 microns: it is fully CIP and SIP tolerant, and indeed has been used to control CIP cycles, as the unit also provides a temperature measurement of the flowing liquid. It uses four SAW transducers, two on each side of the sensor pipe section, therefore acting as a dual path flowmeter. Flow measurement performance over the range 1-10 m/sec flow velocity is 0,4% of reading.

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The latest development work has introduced density measurement and an acoustic transmission monitor parameter, which allow indications of the viscosity, bubble and suspended solids content of the liquid. This is useful in CIP process control, and also for monitoring milk in the dairy, during filtration. Bürkert claim an advantage over other styles of flowmeter, in that the unit is small and light in weight when used on a skid. Other applications now being investigated are for wort concentration monitoring in breweries, and homogenisation control in paint manufacture. Highly viscous liquids, such as glue, are also being monitored, where the full bore obstruction-free design is important.

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Modern trends in long distance power links

Many of the changes in the way the world works lead to new opportunities for different technologies. This has led to a new approach to electricity distribution using HVDC – High Voltage Direct Current – transmission lines, operating at up to 800 kV. Such power transfer lines are now installed particularly around Europe, and across China.

When power stations were smaller, and based near the major population centres, they tended to serve a local area with electric power, and this was best delivered using AC transmission, via local transformers, to produce the 110–240 VAC power distributed to each street. (As an aside, even more locally around the power station, district heating schemes could distribute some of the power using thermal transmission.) To provide the electrical energy transmission further afield, higher voltage AC transmission lines were used to feed a major substation, then distributing the power to local transformers, creating local networks – like the branches of a tree.

Currently, the new solar farms and wind power sources have been built well away from the major centres of population, where the land (or sea) space is available, and the conditions are right. Plus, hydroelectric plants are necessarily placed near the river or water flows, naturally located in the hills. All these sites are at the end of the thinnest branches of the old ‘distribution tree’, so new transmission lines are needed to take the power back to the population centres.

Long distance transmission

China also faced this problem, with economic development and a growing demand for power by the population in the west of the country, with the major new power stations and hydro plants located in the east. For transmission of power over distances like 500 km or more, the reactive power flow due to the large cable capacitance limits the maximum possible transmission distance, as the power loss becomes high. The installation and maintenance costs for the necessarily taller and wider dual pylon AC overhead transmission lines, also becomes excessive.

For such long distance transmission, HVDC comes into its own economically because the line losses are much lower, as are the line installation and maintenance costs, since HVDC (at around 600 kV) can use a single overhead pylon carrying just two conductors, or can use a buried cable. The higher costs of the HVDC terminal equipment, needed at both ends to convert the power back to AC for local distribution, are more than offset by the savings in the transmission line costs. Plus the environmental impact of the HVDC underground cables is insignificant, compared to overhead AC transmission. The possibility of using underground cables means HVDC links can deliver power into cities and urban areas where the use of pylons and overhead cables would not be tolerated.

So, over the last few years China has installed 24 projects using HVDC power transmission: one of these used a 1670 km line carrying 8000 MW of power to the east. The supplier for 19 of these projects, including the largest one, was ABB Power Systems. ABB also claims to be the major supplier of recent HVDC power transmission projects throughout Europe, and the rest of the world.

Undersea links

In Europe there are many power networks, based around different standards that were developed by the different countries: these AC networks can run at different frequencies, and are not often synchronised. It makes sense to wish to trade power between networks, to make use of surpluses when these are available, and cover for power outages or other unforeseen events. Transferring power using HVDC links makes sense, firstly because the receiving terminal can convert the DC to an AC power source running at the same frequency as the receiving network, plus the local ­engineers can phase synchronise the generated AC power with their other sources.

The second big advantage of HVDC links is that they can run in economically constructed underwater cables, to islands and across major sea routes, such as from the UK to France, or Norway and Sweden to Denmark, Germany and Finland. The NorNed link, from Norway to the Netherlands, is the world’s longest submarine power cable, at 580 km length. Similar HVDC links are used to supply power from hydro schemes and wind farms in the north of Scotland, across the estuary of the Moray Firth to the heavily populated Inverness/Aberdeen area.

The growth of offshore wind farms has led to this green energy being sent onshore using an HVDC submarine cable, and also vice versa, in the sense that offshore oil production platforms are now being supplied with power from onshore, delivered by cable, and just converted to AC power on the platform – saving weight and complexity offshore. Plans are being made to extend this European network, with possible hydro-electric power being delivered by cable from Iceland to Scotland, and from Norway via the Shetland Islands, then also to Scotland.

More importantly, in an African context perhaps, solar farms in North Africa will be able to transmit power to Europe via Spain from Morocco and to Italy from Tunisia and Libya.

This article first appeared in my column in the South African Journal of Instrumentation and Control, November 2017 issue. SAIC is published by Technews in South Africa.

Confusion over radar level measurement

We have learned not to get too confused over suppliers using buzz-words and clever marketing names, but recently it seems the major level measurement system vendors have been introducing new and higher radar frequency systems as their latest development – and therefore, by implication, maybe the best. We were used to 6 GHz, and then 26 GHz radar frequencies, but why should we suddenly go to 80 GHz? Then, perhaps just to add a little excitement to the mix, Endress+Hauser started talking about 113 GHz!

113GHz_Key_Visual

The E+H radar line up that offers 113GHz!

This article was first featured in the journal South African Instrumentation & Control in September 2017, a journal published by Technews

Let’s dispel a few myths. Firstly, in the same way that lasers for fibre-optic communications systems made the technology available to create infrared optical systems for process gas analysers, and mobile phone technology possibly provided the hardware for the first radar level measurement systems; the 80 GHz versions are a result of measurement technology made commercially viable on the back of production investment in the distance measurement systems and parking sensors used in modern cars. So the suppliers take the available sensors and chipsets to create a new industrial product, and then have to find the best applications – in this case, the ones that might benefit from the 80 GHz.

Secondly, E+H do not have a 113 GHz system, this is a marketing statement, made to catch attention – ‘with a wink’ is their expression. They claim a ‘complete radar competence of 113 GHz’ because this is the sum of the many different frequencies their different sensors use! These are 1, 6, 26 and 80 GHz.

So why have different frequencies?

Possibly the best explanation for the applications suited to the different frequencies has been provided by the Rosemount measurement division of Emerson, in their “Engineer’s Guides”. The Emerson expertise stretches back many years, having acquired the Saab Tank Radar business. Per Skogberg, from the Gothenburg HQ in Sweden, separates the devices into low, medium and high frequency, to generalise.

Radar signals are attenuated, i.e. they lose signal strength as they pass through the air, or vapour, above the liquid. High frequencies are more severely affected than lower. When the air has moisture, steam or liquid droplets (from spray or filling) present, the attenuation is higher. Equally in solids applications, dust particles have the same effect. So low and medium frequency radar are best when there is dust or moisture present.

At lower frequencies, the wavelength is longer (30-50 mm), so surface ripples in a tank have a small effect. At higher frequencies, surface ripples and foam on the surface can be a problem. But the shorter wavelength of the high frequency units (4 mm) allows accurate operation over short ranges, for example in small tanks. The higher frequency units can use a smaller sensor construction, so the unit is easier to install. The beam angle is narrower, so it can be aimed at a smaller target area and therefore can be positioned more easily to avoid any obstructions in the tank. But even this can be a disadvantage, as the installation needs to be exactly vertical and any turbulence of the surface during filling or stirring can cause the signal to be lost temporarily, in larger tanks.

When reading these suggestions, it is important to remember that Emerson does not offer an 80 GHz unit yet, so their marketing approach would naturally bias users to look at low and medium frequency units. The suppliers of high frequency units (Vega, Krohne and E+H) would point out that in many liquid storage tanks the surface is undisturbed, since any foam, turbulence and significant ripples (>2 mm) caused by filling or liquid transfer will only cause short-term interference. Plus the small antenna size and short range performance make 80 GHz units very useful for smaller process vessels and tanks.

Radar system types

There are two types of radar systems, Guided Wave Radar (GWR) and Free Space Radar. The GWR systems use a conducting rod, or similar, extending down into the liquid, often working in a stilling chamber attached to the main process tank. These operate at low microwave frequencies, and are independent of surface turbulence and foam. They are useful for shorter range measurements and interface measurement between liquids, as well as long ranges.

The Free Space Radar systems are more widely used, since they are top-mounted with nothing in the tank: indeed, some can operate through non-conducting windows in the tank roof. Low and medium frequency radar systems generally transmit a signal pulse and measure the liquid distance by the time delay for the returned pulse. High frequency (80 GHz) systems use an FMCW radar measurement, where the frequency of the transmission is swept, and the frequency difference of the returned signal is measured to assess the distance. The FMCW technique is also used at 26 GHz in some recently launched sensors.

Radar systems can transmit their measurement data using 4-20 mA, fieldbus systems like HART, FF, Profibus PA and Modbus, or indeed via wireless systems like Bluetooth. The low and medium frequency pulsed radar systems generally operate over a two-wire interface: some of the higher frequency FMCW systems require more power and use a separate power connection.

Major applications

Simple low-cost radar level measurement sensors have been specifically designed for water industry use, in sewage sumps and flume flow measurement, by Vega and Endress+Hauser. Vega suggest that 40,000 such sensors are now in use in the water industry, mainly in Europe, and claim their total output of such sensors exceeds 550,000 units over the last 25 years.

Several of these devices use simple Bluetooth interrogation and programming from a handheld PDA: E+H demonstrates this at its facility in Maulberg, working on the stream that runs through the factory complex, as seen below.

Micropilot_FMR10_FMR20_on test stream at Maulberg, with operator using Bluetooth

Both E+H and Vega produce further industrial units for use on process vessels, and storage vessels for solids and liquids. Recently, E+H has extended its capability to add long-range units, such as the 80 GHz FMR62, working at up to 80 m range, with an accuracy of 1 mm. Other units work up to 125 m range, at 3 mm accuracy. These units will eventually be aimed at the large petrochemical industry storage tank markets, and specifically are working towards use for custody transfer duties.

Krohne have similarly announced a new range of its 80 GHz Optiwave sensors. Some of these can even operate at up to 700°C, for example for use on molten salt vessels in solar power plants. Lower specification units rated at up to 150°C can be used through a tank roof made of plastic, or similar materials. Suitable for small or narrow tanks, the unit can measure ranges of up to 100 m. Krohne also offers lower frequency Optiwave systems for use on solids and powders, or to electronically monitor the float position in magnetic level indicator columns attached to process vessels.

Postscript: Krohne is organising a webinar with the title “80 GHz Radar Level – Allrounder or Overrated?” to discuss their recent developments with such systems. This webinar will take place on 18th October 2017 at 3pm London time/10am New York time.

Protect your flowmeter IP

trevor-forsterThe following comments come from Trevor Forster, the MD of Titan Enterprises, a specialist flowmeter manufacturer based in Dorset, UK. He recounts his experiences over the development of a new style of time-of-flight ultrasonic flowmeter, later called the Atrato, in their latest newsletter, called the Titan Flowdown. It is an interesting experience and maybe holds some lessons for all.

“A few years ago, Titan Enterprises filed a patent application for some new ultrasound technology we had been developing over the previous 12 months. On examination by the patent authority it transpired that someone else had the exact same idea and had filed some three months before us. Annoyingly this competitive filing was nine months after we had our first thoughts and six months after our first successful experiments. There were two valuable lessons here:

  1. File your ideas as soon as possible.
  2. Do not waste time in developing a completely viable idea before protecting the intellectual property behind the innovation.

As a consequence of this setback we had to revisit what we wished to achieve with our ring technology development. This project involved development of an ultrasonic device which was tolerant to variations in tube diameters due to the material, temperature or pressure. Our new idea was to section the device annulus into several segments which where independently acoustically coupled to the pipe but joined electronically. The benefit of this innovation is that it would provide us with a “flexible” crystal which can accommodate variations in the tube diameter as well as having a consistent acoustic connection.

Our developmental options were to make drawings, get the specially shaped crystals manufactured and then perform the tests. Alternatively we chose to get some miniature diamond cutting saws with appropriate boring burrs and make our own segmented crystals from existing larger crystals which we use on another ultrasonic meter. This enabled us to prototype and test our idea much more quickly.

The initial tests on the new device were extremely promising which gave us sufficient confidence to file our patent application while more accurate components were being manufactured and tested. This technology has formed the basis of our soon to be released Metraflow ultra-pure flowmeter and our developments with a new 1350 bar flow device.

The initial disappointment was a valuable lesson in getting intellectual property registered as quickly as possible especially with any rapidly developing technology.”

ENDS

Editor’s comment:

From previous discussions about this development, the initial research and testing of the flowmeter concept was undertaken in co-operation with a University, using a research student, so the development was not completely ‘under wraps’, under the control of the company. Nevertheless in a fast developing technology area, many minds are grappling with similar perceived problems and solutions, so parallel work would have been going on elsewhere: an early patent filing is very important under such conditions! The ultra-pure nature of the Metraflow flowmeter arises as the flow tube is a simple straight glass or similar tube, and the ultrasonic transducers are all external. To register to receive further info on the Metraflow, please email Titan.

600,000 flowmeters measure beer and lager flow

Titan Enterprises has established a long-standing working relationship with Vianet plc (formerly Brulines) for the supply of beer flowmeters for pub and bar automation projects. Over the last 20 year period Titan have delivered, and Vianet has installed, over 600,000 of these meters for beer and other bar flow measurement and automation applications.

Brulines, was formed in 1993 with the intention of providing pub chain owners with data on their bar activity via an electronic point of sale (EPOS) system. After trialling several other flowmeters, the company sought a solution to resolve flowmeter bearing lifespan problems and to overcome the unreliability of the optical detection method in beer.

beer-meter

The beer flowmeter

Following a collaborative approach to developing the solutions needed for the Vianet customer base, Titan Enterprises proposed an adapted version of its 800-series turbine flowmeter as the design included durable sapphire bearings proven reliable for many thousand hours operation, and a Hall effect detector which was not subject to problems with discolouration inside the pipe. After successful tests, a trial order for 400 units was placed in 1997, which after the subsequent field trials, was followed by an order for >5000 meters which were all delivered to the clients required timescales.

To ensure the flowmeter was ‘fit for purpose’, Titan additionally adapted the cable type as well as the body and increased the length to 10 metres. These adaptions enabled Brulines installations to be maintained in beer cellars with differing wire runs to the control panel without any junction boxes.

Twenty Years of Collaboration

With the widespread reliability of this product, Vianet turned again to Titan Enterprises in 1999 to develop for them an “intelligent” flowmeter (IFM) for their enhanced iDraught retail product. The specification for the IFM required that it should additionally measure temperature as well as determining the type of fluid in the line to detect line cleaning cycles which are essential for the dispensing of a good pint.

At the time, Titan did not have the technology to provide sensing electronics at a reasonable price so we produced a revised version of the beer flowmeter with the capability of being matched to a PCB designed, manufactured and installed by a third party.

After trialling and testing, this new IFM was introduced in June 2000 and supplied to Vianet at the rate of up to 3500 units a week. Mark Fewster, product manager at Vianet commented “Titan’s supply chain has always delivered to our quality and timescale needs”.

IoT Developments

ifm latest

An intelligent flowmeter design

Since this first IFM introduction, close collaboration between the two parties has resulted in 5 iterations of the product with revised features as end user requirements have developed and evolved with the growth of the IOT (Internet of Things). Drawing upon this close working relationship, over a long period of time, Titan continue to work with Vianet on new solutions and offerings as the Vianet customer offering further develops.

This Titan Enterprises application story is based on a report in the Autumn issue of Flowdown, the regular news bulletin published by Trevor Forster, MD of Titan, from their Dorset, UK base.

Yokogawa acquires FluidCom chemical injection valve technology

Yokogawa has announced the acquisition of TechInvent2 AS, a Norwegian enterprise
that holds the rights to FluidCom, a chemical injection metering valve (CIMV). The FluidCom CIMV prevents blockages and corrosion in oil wells, pipelines, and other facilities and employs a patented technology for thermal control. It incorporates the functions of a mass flowmeter, control valve, and valve controller and has very few moving parts. FluidCom systems have already been delivered to several international oil and gas majors. With TechInvent2 joining the Yokogawa Group, Yokogawa will now target delivery of this solution to the oil and gas upstream and midstream sectors, thereby helping to improve operational efficiency, reduce operational costs, and enhance health, safety and the environment (HSE).

Background Information

Based on its Transformation 2017 mid-term business plan, Yokogawa will continue to focus on the oil and gas industries, and will strive to strengthen its solutions targeting the upstream and midstream sectors, in addition to its forte downstream sector businesses.

Following its April 2016 acquisition of KBC Advanced Technologies, a provider of consulting services that are based on its own advanced oil and gas simulation technologies, the company has been striving to work with its customers to create
value through the provision of solutions that address every aspect of their business activities. At oil wells and pipelines, efforts to ensure a secure oil flow path (flow assurance) play an important role in maintaining production efficiency. The adherence of various chemical substances to the inside walls of a pipe can reduces its internal diameter and causes corrosion. To prevent the accumulation of substances and corrosion, certain chemicals must be injected in the pipes. Improving the efficiency of this process is a major challenge in the upstream and midstream sectors.

The FluidCom CIMV

FluidCom

Chemical injection valves have traditionally been manually operated in the upstream sector, although there are cases where chemical injection has been automated using an actuated solution. In the former case, the valves must be frequently opened, closed, and adjusted by plant personnel. This is costly as it necessitates the hiring of additional staff, and it is work that must be done under very harsh environmental conditions in the field.

It is also a well-known problem that inaccurate and unstable dosing of chemicals leads to additional operational costs and challenges with specific processes. To address and resolve such problems, there is an increasing demand for integrated automatic injection solutions that perform stably and offer a high level of precision in the dosing. The FluidCom CIMV has a unique design which is based on a patented technology, providing integrated flow control and metering using a unique combination of material and thermal effects.

FluidCom is a fully automated and reliable device with a simple design that performs autonomous valve control and continuous flow metering. The device is able to stably inject chemicals in the required small amounts. It has few moving parts and has proven to be an accurate, reliable solution for the control of chemical injection applications. No regular maintenance is required and remote control features are provided.

The device features a self-cleaning mechanism that reduces maintenance workload, and the automatic injection of chemicals in the correct amounts eliminates the need for manual interventions by plant operators and maintenance workers, thereby enabling personnel to lessen their exposure to harsh environmental conditions in the field.

Chemical injection valves have traditionally been operated as manual systems in the upstream sector under harsh conditions. The FluidCom can automate chemical injection operation and reduce times that plant operators and maintenance workers go to field and operate in harsh environments. So using FuidCom improves healthy and safety.

FluidCom is also a valuable solution for downstream operations, where corrosion prevention is always a pressing concern. An ISA100 Wireless version is planned. The ISA100 Wireless technology is based on the ISA100.11a standard. It includes ISA100.11a-2011 communications, an application layer with process control industry standard objects, device descriptions and capabilities, a gateway interface, infrared provisioning, and a backbone router.

Commenting on the acquisition of this company, Shigeyoshi Uehara, head of the Yokogawa IA Products and Service Business Headquarters, said: “FluidCom will improve flow assurance, which is a key concern of our customers in the oil and gas industry, and it will make a major contribution to their operations by helping them not only improve production efficiency and reduce operational costs, but also enhance HSE. The combination of FluidCom, KBC simulation technology, and Yokogawa field devices will allow us to expand the range of our upstream and midstream solutions and enable the delivery of value in new ways to our customers.”

About TechInvent2

TechInvent2 is a fully owned subsidiary of TechInvent AS, a Stavanger, Norway-based company founded in 2008. TechInvent is owned by the founder and CEO Alf Egil Stensen, the venture capital firm Statoil Technology Invest AS, Aarbakke Innovation AS, and Ipark AS. The company has been supplying its FluidCom chemical injection technology to major oil companies since 2016. Alf Egil Stensen will continue as CEO of the company now that it is part of Yokogawa.

The mystery of intelligent sensor diagnostics

The fashion, or trend, that has developed over the last few years for process and analytical instrumentation sensors is to use their on-board intelligence to monitor their own performance status. They achieve this by monitoring and tracking various diagnostic measurements – secondary parameters where consistent values are said to indicate the sensor is working as it should, and has not been subject to any changes since leaving the factory.

This approach is easily understood if you consider the possible effects of exposure of a sensor to excessive temperatures, which might soften the potting or glues holding a sensor to a ‘window’ – and it can be expected that this would be detectable. The addition of a diagnostic sensor, such as a temperature probe, within the sensor housing, could also be an option for checking the sensor condition, and alarming if the sensor exceeds a high or low set-point.

But how else do sensors check their own performance, and how relevant are these “checks”? This topic was discussed in the latest issue of the South African Journal of Instrumentation and Control, August 2017 issue: SAIC is a journal produced by technews.co.za.

Modern (intelligent?) sensors

So, over the past two years of attending and listening to presentations, and reading relevant articles describing the advantages of self-monitoring systems and sensor diagnostics, waiting for an engineer’s explanation as to how the clever monitoring system actually tells the factory instrument engineer anything, it is a bit of a disappointment to report that there seem to be no suppliers that actually make any significant disclosure. This applies across sensors ranging from ultrasonic and Coriolis flowmeters, electromagnetic flowmeters, level measurement systems using radar or ultrasonics, and level alarms. Obviously all the major suppliers are involved in such equipment, and compete with each other, but this secrecy seems a little extreme.

The problem is possibly that until a manufacturer can point to a failure that was detected – or anticipated – using their diagnostics, and decides to publish it, the user population has no idea what systems might actually work. But equally, by publishing a success for the diagnostics, the same manufacturer is saying that one of his sensors failed – and that is a very unusual event, these days. Plus also maybe not something they would wish to publicise.

The older approaches

The whole idea of diagnostics and sensor monitoring has been around for a long time. From personal experience with Bestobell Mobrey, in the 1980s, Mobrey launched an ultrasonic version of a float switch, the ‘Squitch’, which switched a two wire mains connection through a load circuit. When not alarmed it just sat there taking a small control current. For customer reassurance that it was operating in this quiescent state, there was a blinking red LED to show that the sensor was ‘armed’ and operating normally. Mobrey called that a heartbeat indicator, a term that is now used more widely.

For custody transfer flowmeters, the classic approach to validate confidence in the reading is to use two meters in series, and check that both give the same answer. This has progressed to having two separate ultrasonic flowmeters mounted in the same flowtube, on some installations.

For the more safety conscious plant there are often requirements for duplicated sensors for such duties as high level alarms, where two different technologies are used by the sensors – e.g. by mixing float, capacitance or ultrasonic level alarms.

The modern approach

It seems that the ultimate approach is to let the sensor supplier link into your plant automation and data system to interrogate the sensor, and he will verify the measurement and performance diagnostics on a regular basis. With many and varied sensors, this leads to a lot of external interrogation of your plant assets, and possible worries over losing control of your plant.

Overall, it begins to look as though it is becoming impossible for a discerning plant engineer to decide which supplier has the best performing diagnostic system to monitor the relevant sensor’s performance. Rather like opening the bonnet of a modern car, and deciding it would be best to take it to a garage!

At a recent lecture on this subject, held by the InstMC Wessex section in co-operation with Southampton University, a detailed discussion concluded that the sensor suppliers now have all the real expertise in-house and a normal plant engineer could not be expected to cover the depth of this technology for all the many sensors and other equipment within his control. In the end the decision as to ‘which supplier to use’ returns to your own previous experience, including the service and support that has been and is now on offer, and the suitability of the product for the money available for that sensor task.