Session F – Metrology, standardisation & certification

Session chair: Robert Wielgosz, BIPM (FR)

The session “Metrology, standardisation & certification” will take place on Wednesday 19 June 2019 with the following lectures:

F.01 – Challenges in development of NO2 primary reference materials in the range of 1 µmol/mol – 10 µmol/mol by Ewelina Zalewska, VSL (NL)

Nitrogen dioxide is one of the most harmful air pollutants. The ‘Air quality in Europe – 2016’ report of the European Environment Agency states that circa 70 thousand premature deaths were attributable to NO2 in Europe in 2013. As a result, nitrogen dioxide (NO2) is, only next to particulate matter, the deadliest air pollutant in Europe. Lower NO2 concentrations are needed to improve the quality of life for citizens, in particular in urban areas. To support accurate determination of the concentrations of NO2 in air, stable reference standards are needed. Due to its reactive nature existing NO2 reference gas standards have however a limited shelf life and relatively large uncertainty (e.g. 1 year and 2 % at 10 μmol/mol, respectively). To achieve reduced expanded uncertainty of 0,5 % and improved stability warranty of 2 years, static gas standards in the range 1 μmol/mol – 10 μmol/mol were developed. Therefore, several commercially available passivation treatments of cylinders have been tested and in addition the influence of the cylinder valve is currently under test. Moreover, methods to reduce the water content in the gas cylinders were examined as residual water leads to formation of unwanted reaction products such as HNO3. The formation of HNO3 was followed in time using cavity ring down spectroscopy. On the analytical side, the sampling system connecting the high-pressure gas standard to the analytical equipment was scrutinized to optimize time response and reproducibility of the measurement results. First results of the study will be presented, highlighting differences between commercially available types of reducers and tubes made of different materials and coatings. It can be concluded that for NO2 residual water inside the cylinder and sorption processes inside the cylinder as well as in the sampling system are the most important factors impacting both stability of the NO2 reference standards and the analytical performance.

F.02Highly accurate nitric acid (HNO3) in nitrogen standards based on permeation by Edgar Flores, BIPM (FR)

Nitric acid (HNO3) is a common impurity in nitrogen dioxide (NO2) in nitrogen (N2) gas standards and the ability to correctly quantify the impurity can strongly influence the accuracy of the NO2 mole fraction value assignments. For this reason important efforts have been done during last ten years in order to improve the HNO3 quantification. Since no reference standards of HNO3 were available in 2010 during the first CCQM-K74 international comparison, synthetic reference spectra created by the software MALT (multiple atmospheric layer transmission) and the high resolution transmission molecular absorption database (HITRAN) version 2004 were used, together with a fitting algorithm of the experimental spectra by classical least square (CLS), to determine the mole fraction of this impurity. The FT-IR measurements of HNO3 mole fractions at nominal values of 200 nmol mol-1 calibrated with synthetic spectra resulted in values with standard uncertainties of 23 nmol mol-1 (11% relative) with the dominating uncertainty in this case arising from the stabilization of the mole fraction value within the FT-IR gas cell. The requirement for improved accuracy of this measurement has led to the development and operation of a highly accurate primary gas facility at the BIPM for the dynamic production of mixtures of nitric acid (HNO3) in nitrogen (N2) based on continuous weighing of a permeation tube and accurate impurity quantification and correction of the gas mixtures using Fourier transform infrared spectroscopy (FT-IR). HNO3 gas mixtures in the range of 100 nmol mol-1 to 800 nmol mol-1 with a standard relative uncertainty of ~ 1 % can be produced with this facility. To achieve an uncertainty at this level, significant efforts have been made to reduce, identify and quantify potential impurities present in the gas mixtures, such as water and very low levels NO2. A complete uncertainty budget, based on the analysis of the performance of the facility, including the use of an FT-IR spectrometer will be presented during the presentation. The mixtures produced by this facility were validated and are being used to provide references values for the comparison of HNO3 measurement capabilities in National Metrology Institutes.

F.03Choosing between different dynamic gas standards generation methods of the ISO 6145 series by Daniël Calabrese, LNI Swissgas (CH)

For several applications gas standards are required at different concentrations. The typical way is to acquire a set of cylinders with different mixture concentrations. The ISO 6145 series provides a list of dynamic methods, namely as “preparation of calibration gas mixtures using dynamic volumetric methods” and describes well recognized solutions to generate accurate gas mixtures, either on the field or in the lab. The main benefit by using these techniques is to increase flexibility by generating any concentration on-demand and save costs by reducing logistics and quantity of gas cylinders having different mixtures. Nowadays these techniques are also providing good accuracy and precision, they can be seriously considered as complementary tools to gas cylinders standards. The lecture will start with the presentation of the different techniques listed in the ISO 6145 series. Special attention will be given by the most used methods. Discussion will follow about the criteria that can be used to select the most suitable method followed the advantaged and limitation of them.

F.04Development of primary measurement standards of trace moisture in argon, helium and oxygen using multi-gas trace-moisture generator by Minami Amano, NMIJ (JP)

In manufacture of semiconductor devices, many kinds of ultra-high purity gases are used. Residual moisture in these gases has negative effects on performance of the products even if the moisture concentration is lower than 1 μmol/mol (1 ppm). A lot of moisture analyzers are used in semiconductor manufacturing processes, but measuring such low level of moisture is a challenging issue. For obtaining reliable measurement results, it is necessary to develop trace-moisture standards and to periodically calibrate the moisture analyzers. The National Metrology Institute of Japan (NMIJ) developed a magnetic-suspension balance/diffusion-tube humidity generator (MSB/DTG) and established a primary measurement standard of trace moisture in nitrogen in the range of 10 nmol/mol (10 ppb)-1 ppm in 2007. Trace-moisture standards for other gases are also required because the performance of moisture analyzers differ depending on gas types. In this study, we developed a novel trace-moisture generator, “Multi-gas trace-moisture generator”, which is capable of generating trace moisture not only in nitrogen but also in argon, oxygen and helium. In this system, approximately 100 ppm water vapour in nitrogen is produced using the diffusion-tube method, and it is diluted to 1/100 – 1/10 000 in two stages. In the first stage, the produced gas mixture is diluted by dry nitrogen. After large portion of the gas mixture is vented, the remaining gas mixture is further diluted by dry target gas in the second stage, to obtain expected moisture concentration. Using this system, we developed primary measurement standards of trace moisture in argon, oxygen, and helium in the range of 10 ppb-1 ppm. The upper limit of trace moisture standard in nitrogen was extended up to 5 ppm. Uncertainty analyses for these standards are presented, focusing on stability of flow rate control of the gas which is the most important factor to realize two-stage dilution.

F.05A new preparation valve for high precision gravimetric mixtures by Gergely Vargha, EffecTech (GB)

17 years ago, while at NPL, I developed a new way of loop injection based on the use of a three-way Swagelok valve. This new method used only the vacuum in the cylinder to empty the loops, no purging gas was needed at all. In this way, the addition could be repeated over 20 times while no air could get into the cylinders during the whole process. With the new method, gases and liquefied gases could also be introduced up to 100 bar pressure. While at NPL, I took part in several successful CCQM inter-comparisons where our preparation was based on this new method. The three-way Swagelok valve had 3 drawbacks: the difficulties of releasing the excess amount of material from the loops when targeting, the limited pressure of use (100 bar) and the minute amount of material that it could trap and release when the handle was turned. That was especially annoying when preparing mixtures in the low ppb range. I had been thinking for at least a decade while finally I could develop a new preparation valve that eliminates all these problems. Pressure now can be as high as 200 bar, the valve has not got any memory effect at all, and using the new types of loops and micro-injectors targeting can be accurate down to sub mg levels. In my practice, 95 % of the PRGMs are prepared using only a single cylinder and even low ppb, multi-component mixtures can be prepared safely and accurately in two dilution steps using only two cylinders altogether.

F.06Industrial Emission Directive (IED) implementation updated with focus on calibration and instrumentation gases necessary to comply with the new regulatory requirements by Roberto Parola, Linde AG (DE)

The IED entered into force in November 2010 and covers some 50 000 installations across the European Union, e.g. to produce chemicals, power, petrochemical products, cement, glass, car/metal and to treat/incinerate waste. Requirements under the IED are included in the best available techniques reference (BREF) documents, agreed within each specific industry sector. BREF documents define the best available techniques (BAT) conclusions that are legally binding and they also include requirements in term of emissions monitoring. There are 32 BREF documents in total that should be revised under the IED by 2020, but currently only 13 BREF documents have been adopted. As an example, the large combustion plant (LCP) BREF will introduce new emissions monitoring requirements in 2021, including emission limit values, e.g. for NH3, Hg, HCl, HF and TVOC, as well as will tighten the existing requirements for SOx and NOx. This paper will provide an overview of the current legal framework, including some industry case studies and will look at the pure gases and gas mixtures, their packages and the gas supply systems, needed to ensure a reliable and accurate use of the emission measurement systems. This includes innovations in term of species to be monitored (e.g. HCl, HF, Hg), packages (e.g. use of VIPR in portable cylinders) and equipment (e.g. digital sensors). Moreover, aspects of technical compliance to applicable safety standards and metrological traceability requirements will be covered.

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