New design philosophy results in ground-breaking rugged gas analysers

Paul Boughton

Right from the outsetthe aim was to create a family of portable gas analysers that would use common components – and technologies carried across from some of the company’s other product ranges.

So because of the requirement to develop different variants out of the one conceptcombined with the need to keep pace with changing customer needsthe project team adopted a new platform design philosophy. This led to the gas analysers being based upon a shared set of softwareelectronics and sensorswhich enabled faster development times and the ability to tailor each new model to meet specific market requirements.

By starting the digital sensor design ahead of the analyser platformthe team was able to present the certification bodies with its novel designs early in the development processthereby ensuring that any issues could be resolved before the sensor was built into the analyser platform.

The first model of this new rangethe Servomex 5100i.s. portable oxygen analyserwas launched at Achema2006 as a replacement for the company’s reliable 570A hazardous area analyser (Fig.1). In additionan infrared version is the first intrinsically safe infrared portable analyser from Servomex.

While the 5100i.s. has been designed for use in AtexFM or similar land-based hazardous areasparallel developments have ensured that a model will soon be available for marine applications that require the analyser to meet tough marine certifications. Additionally there will be a safe area modelthe 5200HDwhich will be a tough hybrid model combining the case of the 5100 with the electronics from the successful 5200Multi Purpose benchtop gas analyser.

Tim Wilkesmarket manager for the 5100 series of instrumentscomments: “A detailed market analysis was used to create the functional design specification for the 5100 model. Important points on the specification were that the new analyser should be around half the weight and size of the models being supersededbut as easy to useand the sensor technologies should be non-depleting in order that users can always be certain that the analyser will workeven after long periods of inactivity.”

To develop this digitalcompactlightweightrugged and versatile rangethe design team had to overcome many difficulties – especially with respect to meeting the tough new Atex regulations.

Servomex has a wealth of in-house expertise in the design and manufacture of paramagnetic sensors for measuring oxygen (Fig.2) andas a resultthe development team was able to combine an existing sensor cell with new electronics and magnets to create an exceptionally robustaccurate and intrinsically safe new transducer in record time. This was no mean featas it can take considerable resource to overcome the challenges faced by using paramagnetic sensor technology in a portable application (for examplethe paramagnetic properties of oxygen change with temperatureand the measuring suspensions could be affected by gravity and movement/vibration).

Another area of expertise for Servomex is infrared gas analysis. Againthe team was able to reuse proven core technologysuch as infrared source and optics knowledgeas the basis of a new family of CO2CO and CH4 sensors. Extensive initial research into end-user applications and profiling all possible future hosts for the sensorssuch as 19inch rack or wall-mounted analysershelped reduce the level of technical risk in the project and ensure that other instruments can be quickly brought to market once the initial portable units are launched.

Atex compliance

While one team of engineers focussed on developing suitable transducersanother concentrated on the challenges presented by the need to design a portable instrument capable of meeting the stringent requirements of the Atex Directive and IEC Ex hazardous area regulations. Thesealong with FM Approval in the Americas and certifications such as TIIS in Japanare the technical requirements set down to ensure that a piece of portable equipment can be safely taken into an area where there is a risk of explosive atmospheres formingsuch as might be encountered in an oil refinery or on a gas production platform.

CK Pangthe project managerexplains: “Traditionally the housing for a hazardous area approved product would be a flameproof casting and/or machined metal boxbut this would have made the new portable analyser heavier than the target weight. Howeverthe concept of using intrinsically safe transducers – where power is reduced and managed so that even under fault conditionselectrical energy is limited to prevent a source of ignition – allowed us to move away from using metal cases or heavy encapsulated sensors. Insteadwe were able to investigate plastic cases and we therefore approached RTP Company in the USAa firm that specialises in polymer-based compounds for difficult applications. One of the challenges of using plastics in potentially explosive atmospheres is the risk of static build upwhich on discharge can generate sparks with sufficient energy to ignite the hazardous gas mixture. RTP recommended a grade of polypropylene loaded with stainless steel fibres that gives the necessary surface and volume resistivity to ensure that the housing will not hold a static charge. This grade of material already had the necessary approvalswhich saved us valuable time in the development programme and helped us to get the new product to market as soon as possible.”

Not only did RTP’s recommended material have the necessary electrical propertiesbut it was also tough enough to withstand the drop tests demanded by the Atex Directive. These include six drop testsfrom a height of one metreonto concrete. Moreoverthis test has to be undertaken at -20°Ca temperature at which most polymers are brittle. Designing the housing itself to withstand the drop test is one matterbut it has to be remembered that it contains some extremely sensitive electronic and mechanical systems that must also survive. Additional measures were therefore taken to provide further protection: a neat design of rubber overmoulding on the front and rear faces of the analyser features elegantly protruding corners that offer cushioning regardless of which side of the analyser lands first (Fig.3).

“This overmoulding presented its own challenges due to the waxy surface finish on the moulded polypropylene casing” comments CK Pang. “Following a series of moulding trials and an iterative design processwe selected a grade of carbon-loaded butyl rubber that adheres to the polypropylene in addition to providing the desired material propertieswhich again included static dissipation for safe use in hazardous areas.”

Industrial design

To assist with the exterior designServomex employed a firm of industrial design consultantsSatherley Design Associates. The designers helped turn what is basically a square box into a highly functional but visually attractive final productand were also able to assist with material selection and tool design for both the housing and rubber overmoulding (Fig.4). Satherley Design Associates has previously worked with Servomex on the successful 5200 series of analysers for use in environments such as laboratories. This prior relationship was fortunateas the 5100with its hazardous area requirements and the need to be super-rugged and weatherproofwas highly challenging for all involved

“Designing a product to be intrinsically safe needs a rigorous approach and close attention to detail” states CK Pang. “For instanceeven the keypad has a special layer to avoid any build-up of staticand the polycarbonate window for the display has an anti-static coating for the same reason. In terms of the electrical and electronic componentswherever possible we used pre-specified intrinsically safe components and sub-systems in order to minimise the technological risk and the time required to gain the all-important approvals.”

Howeverto complicate matters furtherthe analyser's power consumption had to be minimised in order to achieve the target life for the onboard rechargeable batteries. CK Pang again: “With a pumptransducerLCD display and printed circuit boardsall of which have their own power requirementswe went through several iterations of the designoptimising various parameters to achieve the desired result: at least eight hours of operation at -10°C from a single charge of the lithium ion rechargeable battery pack.”

Final approvals are expected imminently andjudging by initial customer feedbackit is anticipated that the analysers will be well received in the various target markets. Tim Wilkes describes the advances achieved by the new models of analyser: “They are 66percent smaller than the Servomex262the previous marine modeland 54percent smaller than the Servomex570. They also weigh 40percent less than the Servomex262 and a remarkable 70percent less than the Servomex570. And this comes despite meeting tougher mechanical criteriadelivering longer battery life and increasing functionality.”

Options and accessories for the 5100 series include an internal pumpa sample conditioning kit that is approved for use in hazardous areasa protective carry case (useful if the unit has to be checked in as luggage)a filter and probe holder assembly and a flexible sample tube (Fig.5).

Applications include inertingcombustion optimisationnatural gas processingrefineriescentrifugesHyCO applicationscatalytic cracker regeneration and process monitoringplus marine operations such as gas feeing and safe area applications where intrinsic safety is not normally required – for exampletransformer house entry or cylinder checks.

Tim Wilkes concludes: “Thanks to the use of an innovative modular designwith final stage tailoring to meet each application’s researched specific demandsthe vast majority of customers will be able to purchase standardoff-the-shelf products with no need to incur the financial or time penalties associated with customisation. And for Servomexthe benefit of taking a modular ‘platform’ approachwhereby different components can be reused across several analyserswill be that in the next couple of years perhaps 10 to 15 other units will be launched based on the solid foundations of these first units – which is something that may have taken 20 years if each had been designed in isolation!” 

Graham Terry is with Servomex Group and is based in CrowboroughUK. www.servomex.com

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