National Physical Laboratory

Dynamic Generation and Dilution of Gases

Introduction

The dissemination of primary standards for reactive compounds (such as NO, NO2, SO2, H2O and CH2O at ambient amount fractions) by static methods is difficult because of their limited stability in high-pressure cylinders. Permeation and dilution methods are alternative approaches to establishing a stable primary standard.

Several novel systems have been developed at NPL and patented.

Gas Flow Dilutor using an Internally Calibrated Network of Critical Flow Orifices

The high accuracy dilutor uses a network of critical flow orifices. It is capable of stable and repeatable operation, which is not influenced by environmental conditions. When used with a novel self-calibration method it achieves dilutions of up to 31 to 1 with a relative standard uncertainty of ± 0.1%. (Measurement Science and Technology, 21, 2010, 115902). This approach avoids the uncertainty generated by setting and controlling the flow which is present in devices based on variable flow elements (Measurement Science and Technology, 13, 2002, 1138-1145). A patent application has been registered.

Total flow as a function of the dilution for all possible dilutions of a gas flow dilutor based on an internally calibrated network of critical flow orifices
Total flow as a function of the dilution for all possible dilutions of a gas flow
dilutor based on an internally calibrated network of critical flow orifices.
The filled circles represent the highest total flow achievable at each unique
dilution. Open circles represent duplicate dilutions with lower total flows.


Adjustable Gas StandardAdjustable Gas Standard

A method of diluting gases has been invented at NPL that takes a fixed 'charge' of gas and dilutes it with diluent gas (usually pure nitrogen). The device requires no external power and is stable over its operating temperature range to +/- 0.6%. A patent application has been registered.

Molbloc Dilution Facility

This dilution facility delivers dynamic ambient standards of SO2, NO and NO2 from stable, higher concentration reference mixtures. Molblocs (high accuracy mass flow meters) are used to measure the flow of the standard and diluent gas in the blended mixture. The facility has received UKAS accreditation for certifications of SO2, NO and NO2 at ambient concentrations (100 – 1000 nmol/mol).

Molbloc Dilution Facility

Permeation

NPL's Trace Water Vapour Facility is capable of generating trace water vapour standards at mole fractions between 2 and 2000 nmol/mol traceable to mass.

Contact us

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

Last Updated: 22 Sep 2016
Created: 13 Oct 2010

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