Static reference materials for hydrogen chloride

One of the main objectives of the MetAMCII project is to develop traceable static reference materials for hydrogen chloride (HCl) in nitrogen in the range of 1-10 µmol/mol with a stability of at least 1 year and an ambitious target uncertainty of better than 0.5% relative. Such reference materials are needed for the calibration of analysers and quality control and currently no NMIs are able to supply these mixtures. In order to reach these goals, novel passivation techniques to optimise the long-term stability of static reference materials are needed along with ways to reduce the water content in the cylinder as water will react with the HCl. Both VSL and NPL are preparing mixtures and performing stability tests.

At VSL, two types of cylinder passivation are currently under test. Eight mixtures have been prepared at 10 µmol/mol and also 8 mixtures at 1 µmol/mol. In two of the latter mixtures, a small amount of H2O has been added on purpose to investigate to the effect of H2O on the HCl amount fraction and mixture stability. The first results of the 10 µmol/mol stability study at VSL indicate only minor to no initial losses.

Figure 1. Preparation scheme of static reference materials in the MetAMCII project.


High-accuracy dilution device for dynamic generation of reference standards

The dilution system is based on the mass flow controllers traceable to Molbloc/Molbox laminar flow elements, traceable to the primary gravimetric flow standard (GFS).

One controller with a full scale range 30 cm³/min is used for the diluted mixture and second one with full scale range 750 cm³/min introduces the dilution gas. When operating both instruments at their full scale range, the dilution ratio is [30/(30+750)]³ = 1 : 17 576 ≈ 5,7·10⁻⁵.

Figure 2. Schematic of high-accuracy dilution device.

The 3-stage arrangement enable the wide range of dilution ratios while maintaining relatively high flowrates in all pipes of the dilution chain and therefore the acceptable equilibrium time. For the smaller dilution ratios, the 2-stage or 1-stage subsystem may be used to maintain the flowrates sufficiently high.

Generally, the 4-stage and higher n-stage systems could lead to much more short equilibrium times and much lower consumption of dilution gas, however the total uncertainty of the dilution ratio would increase with increasing number of stages.