Ergonomic and Dimensional Review of Liquid Fuel Measures
Posted / Last update: 23-07-2013
Liquid fuel measures are the most frequently used instruments for legal metrology worldwide. External developments in increased fuel dispenser flow rates and concern for occupational health hazards for operators have dictated another design review of the measure.
The resulting improved ergonomics allow the safe use of larger capacity measures and raise fundamental questions as to the appropriate size of samples for inspection and verification of dispensers and the validity of minimum delivery and slow flow rate tests. Are the use of 2, 5 and 10 litre measures, minimum delivery and slow rate tests relevant to the typical use of fuel dispensers and to consumer protection?
Current Practice of Testing
The detail of regulations governing inspection and verification of liquid fuel dispensers varies considerably between jurisdictions. National regulations and local procedures interpret OIML recommendations differently, and in so doing they frequently reflect long established local practices, and even the historic presence of old small copper measures. However, whether the testing is by the officers of a unitary national metrology authority, or by local or regional bodies, or by accredited commercial service companies, internationally the actual physical process at the fuel dispensers/pumps is fundamentally identical. A sample volume as registered by the meter is dispensed into a liquid volume measure of fixed volume capacity, the variation is observed, the fuel sample is returned to the tank from which it came, and an adjustment made to the meter, if necessary. Despite the universality of the service station phenomenon, the size of vehicles, the performance of dispensers, etc, a surprising lack of uniformity exists in national regulations for testing dispensers in use.
It is difficult to minimise systematic and random errors in the measurement process, particularly when one recognises how variable and even hostile the environment at the service station can be, compared with the environment of the laboratory or factory at which the measures were calibrated. The two largest absolute errors were addressed in the first fundamental redesign of the liquid fuel volume measure (OIML Bulletin, Vol XXXVI, No. 3 April 95), and the resultant “PUMPWATCH Integrated Measure” eliminated the systematic temperature error by automatic temperature correction and minimised the random observer error by greatly increased resolution in an integrated tall parallel measurement tube fed from the narrow neck of the measure.
Simultaneously, the ergonomics of the testing process were addressed and the 20 litre integrated measure was made transportable on wheels across the service station to the tank fill pipe.
Need for a Revised Design
As part of a compliance process of obtaining approval for the use of the “PUMPWATCH Integrated Measure” from statutory national metrology authorities in a number of countries, valuable international feedback has prompted another comprehensive design review. This has resulted in a much improved measure, manufactured to a simpler design but of a more sophisticated construction material, and the ergonomics of its use have received much greater attention. The choice of a new construction material is at the core of the redesign.
Carbon Epoxy Material
Instead of stainless steel, carbon fibre epoxy composite was selected as the new material, not just for its high strength to weight ratio but principally for its exceptional thermal dimensional stability. When a carbon fibre epoxy composite has a sufficiently high fibre content, ca. 50%, so that the properties of the resultant composite material can be described as “fibre-dominated”, the thermal expansion or contraction is so small as to be described as “within measurement uncertainties”, i.e. at least ten times less than stainless steel. This thermal stability property makes carbon fibre epoxy composite the preferred material for the structure of artificial satellites which are required to be dimensionally stable when oscillating between full solar radiation and the near absolute zero temperature of space in the shade of the Earth. Carbon fibre epoxy is also well known in many high performance consumer products such as fishing rods, golf clubs and tennis rackets. Epoxy coatings are well established for fuel storage tanks and have been specified for mild steel liquid fuel measures. Because of the high carbon content, the carbon epoxy composite is also electrically conductive and has been found to be satisfactory in tests for static electricity.
Improved Ergonomics and Bottom Discharge
Feedback from users of the earlier stainless steel integrated measure pointed to the need to improve the ergonomics. The first PUMPWATCH integrated design with wheels was already a major step forward in avoiding the lifting and carrying of 20 litre measures weighing approximately 20kg across a forecourt. Discharging into some tank fill pipes posed new demands. Many tank fill pipes are now up to 500mm overground instead of in a manhole. They are so placed in order to make it easier to connect the delivery hoses from tanker lorries and they are usually capped with a 45o bend. This configuration has caused problems by requiring the lifting of all measures, including traditional ones, to discharge them, and a conventional funnel will tend to fall out of such a fill pipe.
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