Primary reference standards of water vapour provide an accurate and cost-effective method for calibrating instrumentation for measurements of water vapour. Static reference standards are prepared in high pressure cylinders (at amount fractions greater than 20 μmol/mol). Calibrations are available at lower amount fractions using NPL's unique trace water vapour facility.

Static standards

NPL provides primary reference gas mixtures containing water vapour in a matrix of nitrogen. They are stable and provide a repeatable reference concentration that can be used for regular calibrations. Each mixture contains an accurately-weighed water component mixed with nitrogen. Mixtures are available with the following amount fractions:

NPL's gas standards are certified for a period up to three years depending on the stability of the mixture. NPL offers a cost-effective re-certification service after the initial certification period has expired, ensuring temporal consistency of gas traceability.

Trace water vapour facility

The NPL trace water vapour facility has a capability for generating an adjustable level of trace water (between 2 - 2000 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 (Measurement and Control, 40/9, 2007). This is achieved with a relative expanded uncertainty of less than 3%.

Trace water vapour facility: Valves A to E are switched to flow gas from the permeation source to either the output (O/P) or the vent.

Key elements of the facility:

• The source of ultra-pure nitrogen which uses an automated change-over regulator and a high-purity point-of-use getter to provide a continuous supply of nitrogen with sub-nmol/mol water vapour concentration.
• The customised permeation system which provides on-line measurements of the mass of a permeation source accurate to 1 µg. The required purity level is achieved by using magnetic suspension to hold the permeation source inside a temperature-controlled electropolished stainless steel chamber stabilised to ± 0.05 °C.
• The flow control system which uses critical flow orifices and pneumatically-controlled flow switches to blend controlled amounts of source and dilution flows without changing the permeation rate.
• The output manifold which allows up to five instruments to be connected in parallel to the calibration facility. The combination of the uncertainties of the statistically-varying components in the system leads to an overall expanded uncertainty of better than 3%.

Validation:

The facility has been validated by a recent international water vapour comparison with NIST, PTB and NMIJ. The comparison demonstrated excellent comparability within the stated uncertainty for all laboratories across the range of amount fractions from 10 to 2000 nmol/mol.

Contact

Customer enquiry tel: +44 20 8943 6372
E-mail: gases@npl.co.uk