Water. Desalination + reuse
Issue link: https://fhpublishing.uberflip.com/i/513849
| 32 | Desalination & Water Reuse | May-June 2015 TECHNOLOGY _________ Richard Lowrie, Water and wastewater industry manager, Krohne Inc. ___ The bigger the water distribution network the more costly any metering inaccuracy. But the bigger the meter, the greater the diffculty in ensuring its accuracy. The author explores the surety offered by various measures taken to calibrate large fow meters. WATER IS growing in economic value in developed and developing countries worldwide as resources become stressed through drought and as environmental regulations become more demanding. With the increasing value comes a greater need for water suppliers and customers to know that fow measurements are accurate. In large distribution networks fow meters over 120 cm in diameter measure tremendous volumes of water and with this size small errors can become costly. Meters in these large diameters are a challenge to calibrate because there are few suitable facilities able to accommodate them. It is therefore important to understand different types of calibration and accreditation methods and how volumetric fow measuring accuracies are achieved and certifed (see Calibration methods box). AccReditAtion Measuring the fow throw through a device on an accredited calibration rig is the best potion in most instances. Accreditation – the independent evaluation of conformity against recognized industry standards – for a given calibration rig must be certifed by a recognized entity to ensure compliance. Each country has its own organization responsible for accrediting calibration facilities. Many people assume that the accreditation body for the US is the National Institute of Standards and Technology (NIST) but that is not the case. Actually, the NIST does not accredit calibration facilities but it oversees a separate programme known as the National Voluntary Laboratory Accreditation Programme (NVLAP) that provides third-party accreditation to testing and calibration laboratories. NVLAP accredited laboratories are assessed against the management and technical requirements published in the International Standard, ISO/IEC 17025:2005. While accreditation to ISO/IEC 17025:2005 for calibration rigs is claimed; this requirement only certifes the competence of laboratories and personnel but does not actually accredit or certify the calibration rig itself: "ISO/IEC 17025:2005 is for use by laboratories in developing their management system for quality, administrative and technical operations. Laboratory customers, regulatory authorities and accreditation bodies may also use it in confrming or recognizing the competence of laboratories. ISO/IEC 17025:2005 is not intended to be used as the basis for certifcation of laboratories," says the International Standards Organization. Depending on the location, the actual calibration rig accreditation comes from that individual country's metrological organization. These accreditation bodies established in many countries are subject to oversight by a long-established international grouping of laboratory and inspection accreditation bodies, the International Laboratory Accreditation Cooperation (ILAC). Each of its member organizations is evaluated by peers for acceptance to ensure conformity of products and services to support international trade. For example, the United Kingdom Accreditation Service (UKAS) is equivalent to the Swiss Accreditation Service, and both are comparable to the American Association for Laboratory Accreditation in the US and the Raad Voor Accreditatie in the Netherlands. So how would a fow meter user know if its measuring device has been calibrated to the stated accuracy by an accredited calibration facility? A user can determine this by looking at the fow meter manufacturer's calibration facility accreditation organization to see if it is registered as a member of ILAC. Krohne's accreditation details are, for example, available on its website. Then the user must also verify that all of the manufacturer's available meter sizes have been calibrated. Recognized accreditation agencies will list in their certifcates the total volume and the sizes of the calibration rigs. The certifcate also lists the verifed uncertainty of the calibration rig. Table 1 gives a brief summary of the published accreditation certifcates provided by several unnamed manufacturers, and Krohne, for large diameter fow meters. They show the fow velocities available to calibrate large diameter meters. Larger fow meters speak volumes