Current measurement and calibration capabilities for mercury vapor in air are maintained at levels of 0.2–40 μg Hg m−3. In this work, a mercury vapor generator has been developed to establish metrological traceability to the international system of units (SI) for mercury vapor measurement results ≤15 ng Hg m−3, i.e. closer to realistic ambient air concentrations (1–2 ng Hg m−3) []. Innovations developed included a modified type of diffusion cell, a new measurement method to weigh the loss in (mercury) mass of these diffusion cells during use (ca. 6–8 μg mass difference between successive weighings), and a new housing for the diffusion cells to maximize flow characteristics and to minimize temperature variations and adsorption effects. The newly developed mercury vapor generator system was tested by using diffusion cells generating 0.8 and 16 ng Hg min−1. The results also show that the filter system, to produce mercury free air, is working properly. Furthermore, and most importantly, the system is producing a flow with a stable mercury vapor content. Some additional improvements are still required to allow the developed mercury vapor generator to produce SI traceable mercury vapor concentrations, based upon gravimetry, at much lower concentration levels and reduced measurement uncertainties than have been achieved previously. The challenges to be met are especially related to developing more robust diffusion cells and better mass measurement conditions. The developed mercury vapor generator will contribute to more reliable measurement results of mercury vapor at ambient and background air levels, and also to better safety standards and cost reductions in industrial processes, such as the liquefied natural gas field, where aluminum main cryogenic heat exchangers are used which are particularly prone to corrosion caused by mercury.
In the preparation of some types of reference gas mixtures, one of the starting materials is a liquid. The liquid is introduced in the cylinder by means of a syringe. The weighing of these syringes before and after injection is critical for the calculation of the gas composition. A robot has been developed to facilitate this weighing of syringes used in the gravimetric preparation of these gas mixtures. From the validation work, it is clear that the robot performs the measurements more reliably and with better repeatability than the manual method. The percentage of non-conforming gas mixtures dropped from approximately 5% to less than 1%. Subsequent experiments to revisit assumptions made in the preparation process have gained new insight into effects such as convection and evaporation of liquid. Additional preventive measures are taken, to further improve the weighing accuracy, and by implication, the accuracy of the gas composition data calculated from preparation. The gain in uncertainty is different for various types of gas mixtures and depends largely on the thermophysical properties of the liquid and the gas mixture being prepared.
It was decided at the EURAMET TC-M meeting in Torino in 2006 to realize a comparison in gauge and absolute pressure up to 200 kPa as it would allow establishing a link to the CCM.P-K6 and CCM.P-K2 comparisons. This project from the beginning interested a lot of laboratories with participants, 22 of which have submitted results. The circulation of the transfer standard began on July 2009 and lasted until January 2012. No major problem occurred during the transport.The mesurand of the comparison is the effective area of a piston-cylinder determined in gauge and absolute pressure from 25 kPa to 200 kPa with pressure steps of 25 kPa. The transfer standard is a gas lubricated tungsten carbide piston-cylinder with an effective area of ~9.8 cm 2 , fabricated by DH Instruments and compatible with a PG-7601 pressure balance. Some participants used their own pressure balance while a pressure balance with a reference vacuum sensor has been circulated for the participants not equipped with this system.One participant (SMU, Slovakia) has never provided the measurement results and another participant (FORCE Technology, Denmark) submitted a revised set of measurement results after the pilot laboratory mentioned that the equivalence was not met.After the determination of the reference value, all the 22 participants who delivered the results in gauge pressure demonstrated equivalence respective to the reference value on most of the range. In absolute pressure the equivalence is demonstrated, for all nominal pressures, by all 17 participants who submitted results.
In order to show equivalence of mass standard determination among NMIs of CIPM member countries, key comparisons of mass standards have been carried out under the auspices of the Comité Consultatif pour la Masse et les Grandeurs Apparantées (CCM). At each NMI, mass standards are derived from one kilogram by means of the multiples and submultiples methods. The pilot laboratory, NMIJ, prepared five sets of transfer standards. Any set of transfer standard consists of five pairs of mass standards with nominal values of 200 mg, 1 g, 50 g, 200 g and 2 kg. The nominated twenty participants have been divided into four groups and the corresponding four sets of transfer standards have been circulated within the groups simultaneously while remaining one set has been kept at the pilot laboratory for the stability evaluation. The pilot laboratory measured the volumes, the centres of gravity and the magnetic properties, susceptibilities or magnetism, of the standards before the circulation and has reported these values to the participants. The pilot laboratory has also verified the stability of all travelling standards in advance. Nineteen laboratories have reported final results to the pilot. Nine participants belong to EURAMET, three belong to COOMET but two of them also belong to EURAMET, four belong to SIM, and five belong to APMP. Table 2 shows all the results associated with their uncertainties reported by the participants. As shown in the table 3 the majority of the instabilities of the transfer standards during their circulation were less than claimed uncertainties of the participants except for few standards of small mass, 1 g and 200 mg. These instabilities are considered for uncertainty evaluation within the group. All reported results among different groups have been linked based on the average values of transfer standards before and after the circulation. Tables 10 to 19 show the differences 2 and associated expanded uncertainties referred to a confidence level of 95% between any combination of the laboratories in the form of matrices. They show few outliers, one at 2 kg and 200 g, three at 50 g, and two at 200mg.
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