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Traceable reference standards

Water vapour is one of the most difficult impurities to remove from gases, and it affects a number of manufacturing processes even at trace amount fractions. Instrumentation dedicated to measuring trace levels of water is therefore of paramount importance. The accuracy of these instruments can only be maintained through regular calibration to traceable reference standards.

Standards of water vapour provide an accurate method for calibrating instrumentation. Static reference standards of water (at amount fractions greater than 10 μmol/mol) in nitrogen are prepared in cylinders. Calibrations are also available at lower amount fractions using NPL's unique trace water vapour facility.

Static standards

NPL provides gas mixtures containing water vapour in a matrix of nitrogen. These mixtures are available with the following amount fractions:

Component Matrix gas Amount fraction range from Amount fraction range to
Water Nitrogen 10 μmol/mol 100 μmol/mol

Trace water vapour facility

We can generate an adjustable amount fraction of trace water (between 2 and 2,000 nmol/mol) by using continuous accurate measurements of mass loss from a permeation device coupled with a dilution system based on an array of critical flow orifices. This is achieved with a relative expanded uncertainty of less than 3%.

A major obstacle to introducing flexible organic devices into the commercial market is their limited lifetime when exposed to water. Ingress of water poses a particular challenge for flexible displays, since the polymeric substrates from which they are constructed are highly permeable. To reduce water ingress, thin inorganic coatings (approximately 10 nm in thickness) can be applied to the flexible polymeric substrates.

Water vapour transmission rate

To assess the efficacy of such barrier layers, measurements of water permeability are required. These are carried out using the calcium test, radioactive methods with tritiated water or, most commonly, the 'MOCON' test. The industry goal for water permeation is 10−6 g m−2 day−1 and, with recent reports of flexible barriers achieving 10−7 g m−2 day−1, the question of how to measure water permeation at such low levels is critical.

NPL has developed a new approach to measuring water vapour transmission rate directly, based on cavity ring-down spectroscopy. The unique facility provides accurate and traceable measurements with a detection limit below 5 x 10−5 g m−2 day−1.

The system operates with a dry chamber separated from a wet chamber of known temperature and relative humidity by the barrier material under test. Water vapour permeating through the film is collected by dry nitrogen and measured by cavity ring-down spectroscopy. The measurement cell uses a novel design to ensure optimum sealing conditions in order to eliminate water ingress to the dry chamber. The capability is underpinned by NPL's trace water vapour facility.

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