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NPL holds the UK Primary ozone standard reference photometer (SRP). This is a UV photometric device that precisely measures ozone concentrations in air up to 1000 nmol/mol by absorption at 253.65 nm.
NPL is accredited by UKAS to ISO 17025 to perform the calibration of ozone photometers up to 50 µmol/mol. The expanded uncertainty is 3.2 % for a confidence interval of approximately 95 %.
To evaluate the degree of equivalence of ozone photometers that are maintained as national standards for ambient ozone measurements, NPL has been participating in the intercomparison BIPM.QM-K1 with primary photometric systems deployed at the Bureau International des Poids et Mesures (BIPM) since 2004. The intercomparison takes place every four to five years.
The most recent comparison was in June 2022 following an upgrade of the SRP system. The overall uncertainty of NPL’s measurement was improved by more than 50 % for ozone amount fractions between 0 nmol/mol to 1 µmol/mol. Detailed results are available here.
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