QMT Features: April 2013
The unbelievable impact of calibration

A single National Physical Laboratory (NPL) density measurement calibration can impact hundreds of companies and thousands of products. By Graham Topley, NPL.



A study of the interaction between NPL, UKAS-accredited laboratory H&D Fitzgerald, and one of their customers Radius Systems revealed the ‘fan-out’ effect of calibration. This identified that a single National Physical Laboratory (NPL) density measurement calibration impacted hundreds of companies and thousands of kilometres of pipes.

Calibration is the act of comparing measuring equipment against a standard of higher accuracy. If this reveals errors, then corrections can be evaluated and applied.

For a calibration to be considered traceable, there must be an unbroken chain of measurements, relating the calibration result to a national or international standard (such as the metre); and the calibration result must be accompanied by a statement of uncertainty.

Although traceability and calibration may seem mundane, they can in fact have a real impact on a huge number of people, products, and businesses.  A good example is the interaction between NPL, a UKAS-accredited density measurement laboratory called H&D Fitzgerald, and just one of their many customers – Radius Systems. Companies such as Radius Systems who manufacture all their products under ISO EN 9001: 2008 conditions are obliged to use UKAS-accredited laboratories.

Radius Systems manufactures and supplies polyethylene (PE) pipe systems and services to over 300 customers across the following industries: gas, water, nuclear and civil construction. In 2011 alone they manufactured 12,000 km of pipe work – almost the same distance as Earth’s polar diameter. As these pipes transport essential utilities the material they are made from must be able to withstand the pressures of the liquid/gas they are transporting.

To confirm that pipe material is fit for purpose Radius Systems test its density using a calibrated float supplied by H&D. NPL provides H&D Fitzgerald with mass calibrations directly by calibrating the stainless steel weights that H&D Fitzgerald use on their own balances to measure the weight of their floats. Primary standards of temperature at NPL are also used by H&D.

Radius Systems use the floats to check that the density is correct for PE raw materials they use to make their pipes and fittings. Two checks are conducted – first, the incoming resin to ensure it is the correct type (its density tells us this), and after processing to verify that processing the material has not affected its physical properties.

When you realise that Radius Systems is just one of H&D Fitzgerald’s 700+ customers, a conservative estimate of the true number of end-user customers impacted by the density standard maintained at NPL and disseminated by H&D Fitzgerald is undoubtedly in the thousands. And then when you consider that H&D Fitzgerald is just one of the seven accredited laboratories disseminating NPL’s density standard you get a real sense of how large the fan out is.

The fan-out effect also extends to the finances involved in calibration. H&D Fitzgerald value their annual sales of floats against the price they pay for NPL’s calibration services at 12:1.

Product quality is of paramount importance to Radius Systems, and precise density measurements enable them to identify whether they are using PE80 and PE100 grades of polyethylene. These materials are designed to withstand 50 years at either 8.0 or 10.0 MPa. Materials of a lesser density would be weaker, and therefore would need to be replaced sooner.

Safety is equally critical in Radius Systems’ products, nowhere more so than in their gas pipes. It is imperative that they can rely on the density measurement standards they buy from H&D Fitzgerald, as Martin Froggatt, Radius Systems’ quality manager explains:
“If the density was wrong the material type would be incorrect, for example, rather than a PE100 pipe grade material we may have been delivered a PE that is designed for milk bottles and would not withstand the operating pressures that our pipes and fittings are designed for. If a gas line was to rupture and gas escape, it could explode! Fortunately, a density check prevents this from happening. Calibration of measuring equipment and devices is critical to the quality of our products. We have to be certain that what we use to measure is correct.”
www.npl.co.uk
  
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