Hans-Joachim Heine

Final report on international key comparison CCQM-K53: Oxygen in nitrogen

Gravimetry is used as the primary method for the preparation of primary standard gas mixtures in most national metrology institutes, and it requires the combined abilities of purity assessment, weighing technique and analytical skills. At the CCQM GAWG meeting in October 2005, it was agreed that KRISS should coordinate a key comparison, CCQM-K53, on the gravimetric preparation of gas, at a level of 100 ?mol/mol of oxygen in nitrogen. KRISS compared the gravimetric value of each cylinder with an analytical instrument. A preparation for oxygen gas standard mixture requires particular care to be accurate, because oxygen is a major component of the atmosphere. Key issues for this comparison are related to (1) the gravimetric technique which needs at least two steps for dilution, (2) oxygen impurity in nitrogen, and (3) argon impurity in nitrogen. The key comparison reference value is obtained from the linear regression line (with origin) of a selected set of participants. The KCRV subset, except one, agree with each other. The standard deviation of the x-residuals of this group (which consists of NMIJ, VSL, NIST, NPL, BAM, KRISS and CENAM) is 0.056 ?mol/mol and consistent with the uncertainties given to their standard mixtures. The standard deviation of the residuals of all participating laboratory is 0.182 ?mol/mol. With respect to impurity analysis, overall argon amounts of the cylinders are in the region of about 3 ?mol/mol; however; four cylinders showed an argon amount fraction over 10 ?mol/mol. Two of these are inconsistent with the KCRV subset. The explicit separation between two peaks of oxygen and argon in the GC chromatogram is essential to maintain analytical capability. Additionally oxygen impurity analysis in nitrogen is indispensable to ensure the preparative capability. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

International comparison CCQM-P41 Greenhouse gases. 2. Direct comparison of primary standard gas mixtures

In this second part of this study, primary standard gas mixtures (PSMs) as used for calibrations for the greenhouse gases carbon dioxide and methane have been compared. The nominal amount-of-substance fraction levels were 365 µmol/mol for carbon dioxide and 1.8 µmol/mol for methane. The matrix was synthetic air, simulated by a mixture of nitrogen, oxygen (209 mmol/mol) and argon (9.3 mmol/mol). The measurements took place in the spring of 2003. In the protocol, it was foreseen to have the PSMs produced for this study in a small range, enabling regression analysis on the data to assess the consistency of the gas mixtures. The direct comparison of the PSMs shows agreement for carbon dioxide at 365 µmol/mol nominal within 0.52 µmol/mol (0.14% relative) standard uncertainty. One cylinder lies between the 95% and 99% boundaries of the regression line, and after removing this cylinder from the dataset the standard deviation of the x-residuals reduces to 0.18 µmol/mol (0.05% relative). This value for the standard deviation is substantially greater than the stated standard uncertainties for several cylinders. The standard deviation of the x-residuals for methane at 1.8 µmol/mol nominal is 0.011 µmol/mol. Main text. To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM.

International comparison CCQM-P41 Greenhouse gases. 1. Measurement capability

The measurement of greenhouse gases in the atmosphere is one of the activities to monitor changes in the global environment, as some of these gases are presumed to contribute to climate change. The CCQM Gas Analysis Working Group has organised a pilot comparison for carbon dioxide and methane at ambient levels to compare primarily the capabilities of national metrology institutes (NMIs) for measuring these species in air. The participation in this comparison was not limited to signatories of the MRA, as is usually the case. Laboratories from the World Meteorological Organisation (WMO) were invited as well, and two participated. The project has been split in two parts, a comparison of the (analytical) measurement capability, and a high-accuracy comparison of primary standard gas mixtures for greenhouse gases. In this first part, the measurement capability was compared between NMIs and WMO laboratories. The nominal amount-of-substance fraction levels are 365 µmol/mol for carbon dioxide and 1.8 µmol/mol for methane. The matrix was synthetic air, simulated by a mixture of nitrogen, oxygen (209 mmol/mol) and argon (9.3 mmol/mol). The measurements took place in the spring of 2003. The protocol used for this comparison was the same as used so far in the gas analysis area for key comparisons and studies of this type. The reference values were assigned to each gas mixture on the basis of the gravimetric preparation, taking into account the results from the purity verification. In particular for the value assignment of methane, traces of methane in the matrix gases (nitrogen, oxygen and argon) can influence the value assigned to the gas mixture appreciably. The results for methane agree within 4% relative, and for most participants even within 2% relative at an amount of substance fraction level of 1.8 µmol/mol. For carbon dioxide all results agree within 1% relative, and for most the agreement is even better: within 0.5% relative at an amount of substance fraction level of 365 µmol/mol. The methane data show a discrepancy of approximately 0.025 µmol/mol between the averages of the WMO laboratories and the NMIs, which confirms results of earlier comparisons. Measurement traceability is quite differently established in NMIs and WMO laboratories, and may account for this difference. Further work is needed to find out the causes of this difference. Main text. To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM.

Final Report on International comparison CCQM K23ac: Natural gas types I and III

At the highest metrological level, natural gas standards are commonly prepared gravimetrically as PSMs (primary standard mixtures). This international key comparison is a repeat of CCQM-K1e-g. The mixtures concerned contain nitrogen, carbon dioxide and the alkanes up to butane. The only difference with CCQM-K1e-g is the addition of iso-butane to the list. The results usually agree within 1% (or better) with the key comparison reference value. For ethane, nitrogen and carbon dioxide, the agreement is within 0.5% (or better), and for methane within 0.1% (or better) of the KCRV. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

International Comparison CCQM K23b-Natural gas type II

At the highest metrological level, natural gas standards are commonly prepared gravimetrically as PSMs (Primary Standard Mixtures). This international key comparison is a repeat of CCQM-K1e-g. The mixtures concerned contain nitrogen, carbon dioxide and the alkanes up to butane. The only difference from CCQM-K1e-g is the addition of iso-butane to the list. The agreement of the results in this key comparison is very good. For all parameters, with a few exceptions, the results agree within 0.5% (or better) with the key comparison reference value. For methane, the results are generally within 0.1% (or better) of the KCRV. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

International Comparison CCQM-K16: Composition of natural gas types IV and V

Natural gas is an important energy vector. The determination of its composition is often used as the basis for the calculation of the calorific value. The calorific value in turn is one of the two key parameters used in natural gas trade. In the first series of key comparisons (CCQM-K1e-g), natural gas was already included with three different compositions. These mixtures contained carbon dioxide, nitrogen, ethane, propane and n-butane in methane (matrix) and were only to a limited extent representative of real natural gas. In the past years, national metrology institutes have broadened the range of components by including, e.g., i-butane, neo-pentane, n-pentane, i-pentane and n-hexane. Based on this extended components list, two new mixtures have been defined, one characteristic for a low calorific mixture (type IV) and the other for a high calorific mixture (type V). In the low calorific mixture, helium was also present. Due to presence of the butane and pentane isomers, the mixtures of type IV and V are more demanding with respect to the separation technique than the mixtures used in CCQM-K1e-g. The measurements in this key comparison took place in 2001. There were eight participants and two coordinating laboratories. The key comparison reference value (KCRV) was based on the gravimetric preparation for all components. Even for the heavier hydrocarbons (pentanes and n-hexane) the effects of, e.g., adsorption can be controlled to such an extent that this approach is still valid. The uncertainty evaluation of the KCRVs reflected also the extent to which the preparation data could be demonstrated to be valid. The validity of the preparation data was demonstrated by comparing the composition of the mixtures prepared for this comparison with measurement standards maintained by the coordinating laboratories. The key comparisons demonstrated that the results of the laboratories agreed within 1% relative to the reference value for most components. Even better agreement was obtained for nitrogen in the low calorific mixture (0.5%), carbon dioxide (0.5%), ethane (0,5%), propane (0.5%) and methane (0.1%). In some cases, larger differences were observed, which then also exceeded the associated expanded uncertainty Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the Mutual Recognition Arrangement (MRA).

International comparison CCQM-K51: Carbon monoxide (CO) in nitrogen (5 µmol mol−1)

The first key comparison on carbon monoxide (CO) in nitrogen dates back to 1992 (CCQM-K1a). It was one of the first types of gas mixtures that were used in an international key comparison. Since then, numerous national metrology institutes (NMIs) have been setting up facilities for gas analysis, and have developed claims for their Calibration and Measurement Capabilities (CMCs) for these mixtures. Furthermore, in the April 2005 meeting of the CCQM (Consultative Committee for Amount of Substance) Gas Analysis Working Group, a policy was proposed to repeat key comparisons for stable mixtures every 10 years. This comparison was performed in line with the policy proposal and provided an opportunity for NMIs that could not participate in the previous comparison. NMISA from South Africa acted as the pilot laboratory. Of the 25 participating laboratories, 19 (76%) showed satisfactory degrees of equivalence to the gravimetric reference value. The results show that the CO concentration is not influenced by the measurement method used, and from this it may be concluded that the pure CO, used to prepare the gas mixtures, was not 13C-isotope depleted. This was confirmed by the isotope ratio analysis carried out by KRISS on a 1% mixture of CO in nitrogen, obtained from the NMISA. There is no indication of positive or negative bias in the gravimetric reference value, as the results from the different laboratories are evenly distributed on both sides of the key comparison reference value. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

A preparative comparison using the 'harmonization' method on standards of synthetic natural gas (CCQM-P87)

Twenty-two standards of synthetic natural gas standards were submitted from ten different NMIs for a comparison using the harmonization method developed by NPL. All standards had seven components. This method can remove the correlated variation in calibration data for multi-component mixtures and makes a significant improvement in fitting individual points to calibration curves. After removing the non-random variation from the data all the standards showed very good agreement with only a few outliers. The typical deviation from the calibration curves was around 0.05% (relative) for the main components and less than 0.2% for the butanes. Main text. To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM GAWG.

Final Report of CCQM P49: Natural gas types IV and V

During the protocol development phase of CCQM-K16, interest was shown by two more national metrology institutes (NMIs) in participating in the comparison, but rather as a study instead of a key comparison. Therefore it was decided to organize in parallel to CCQM-K16 a study, CCQM-K49, enabling NMIs that did not wish to participate in the key comparison to participate in a study with mixtures with the same nominal specifications. The natural gas mixtures type IV (low calorific mixture) and type V (high calorific mixture) have been prepared and checked in the same time frame as those for CCQM-K16. The protocol used for CCQM-P49 differed only in two aspects from that of CCQM-K16: (1) the results would be reported as results from a CCQM study, and (2) it was permitted that NMIs did not analyse all components present in the mixtures. The measurements in this comparison took place in 2001. There were two participants. The coordinating laboratories were the same as in CCQM-K16. The reference value for each component was based on the gravimetric preparation for all components. For most components, the results of the participating NMIs agreed within 1% relative of the reference value. Main text. To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the Mutual Recognition Arrangement (MRA).