Scale down of a dynamic generator of VOC reference gas mixtures

A system for the dynamic preparation of reference gas mixtures based on the diffusion technique has been developed by the National Metrology Institute of Italy i.e. the Istituto Nazionale di Ricerca Metrologica (INRIM). The gravimetric method used for the estimation of the diffusion rate and consequent concentration, gives the system the property to be a primary standard. The system can generate mixtures with low uncertainty and high stability in the 20 nmol.mol-1 - 2.5 µmol.mol-1 concentration range with a 5% (k = 2) expanded uncertainty for mixtures of acetone in air. Based on this system, a transportable device for the generation of VOC reference gas mixtures to be used as calibration standard was designed and developed. The methodology used for the scale down included several steps. An initial characterization and modelling of the primary device was done using computational tools. Based on the response of the computational model to the different physical quantities, a set of design parameters was identified. The thresholds for this set of parameters were established and translated into a set of design criteria to consider in order to keep the metrological performance target. After the design and development of the transportable device, a metrological characterization was carried out, to verify its capabilities. The metrological characterization of the generator was done in the Dutch National Metrology Institute i.e. the Van Swinden Laboratory (VSL) trough Cavity Ring Down Spectroscopy (CRDS) analyses to evaluate the linearity, the reproducibility and the short term stability. The results for the generation of methanol mixtures with molar fractions in the 80-150 nmol.mol-1 range, were 99.6% linear, with a reproducibility after 3 days within 2,9% and a short term stability better than 1% per hour. Repeatable measurements of the generated concentration were obtained for three different molar fractions, with the use of both CRDS (VSL) and the GC/FID (INRIM). A flow of the desired dry pure carrier gas can be connected to the device. The presence of water in the system has not been taken into account and further analyses should be done before introducing it to the system. Water presence might affect the adsorption rate, and consequently the flushing time before normal operation. This transportable device is able to perform in-situ calibration of instruments and has been designed to generate gas mixtures of up to four species at a time

Uncertainty of thermodynamic properties available via online data banks: Vapor pressure as case study

The aim of the work is to highlight the limitations of properties data commonly used in engineering estimations, as well as the importance of accounting for their uncertainty. Following established metrological guidelines, a full procedure have been developed, with particular considerations aimed to account for the ‘quality’ of the realization of the definition of the measurand as a source of uncertainty of the property. The case study considered was the vapor pressure, a property commonly used in processes involving vapor-liquid equilibrium, common in the industry and climate related calculations

Reproducibility of the Quantification of Reversible Wall Interactions in VOC Sampling Lines

Volatile Organic Compounds (VOCs) are widely measured at ppb and ppt level in many contexts, from therapeutic drug control in respiratory diseases to monitoring of climate change and indoor air quality. The need for accuracy is a common denominator in all these fields. The interactions between gas mixtures and solid surfaces in sampling lines and instruments play an important role in calculating the total uncertainty of the amount of VOC. The amount of substances in the gas mixture is affected by its reversible and irreversible interactions with the sampling line. The main aim of this paper is to propose and discuss a method to quantify the amount of substance segregated by reversible interactions on sampling lines. To validate the proposed method, the areic amount of a VOC (Acetone) is measured for a commercial test pipe (Sulfinert®) as the amount of substance per unit area of the internal surface of the test pipe segregated from the flowing gas mixture. Stainless steel coated by Sulfinert® was chosen as a test material because of its wide use and its limited irreversible and permeation effects. A certified gas mixture of Acetone in air with a nominal mole fraction of 10 µmol mol−1 was used for validation. Broad temperature control was used and the sensibility of the method to the temperature and the pressure has been evaluated to correct the bias due to physical condition. The sensitivity to the residence time and the Reynolds number of the gas flow has been evaluated to verify the reaching of equilibrium and the limits of the applicability of the method. The areic amount of Acetone at equilibrium on Sulfinert® coated pipe was measured as 40 nmol m−2, and an equilibrium constant value of around 0.2 m was calculated as the ratio between the superficial amount segregated on the wall and the amount concentration of Acetone in the mixture, both at the equilibrium. The observed reproducibility was better than 2.5%. This method is aimed to investigate VOC losses due to interactions for many VOC/material systems at a lower amount of substance levels

Modelling of a standard gas mixtures generator with computational fluid dynamics

Monitoring VOC for climate change and for indoor and outdoor air quality at trace level concentrations need reference standard materials at high metrological performance. To improve this performance, the description of phenomena involved in mixtures generation by rigorous models is mandatory. A model to describe diffusion cells of a primary generator was developed and validated with experimental data. A good agreement was found between the uncertainties of measurements and calculations

Calibration curve computing (CCC) software v2.0: A new release of the INRIM regression tool

The main features of the Calibration Curve Computing (CCC) Software, a tool for regression analysis developed at the INRIM, the Italian National Metrology Institute, are described in this paper. The first release of the software tool, version 1.3, has been available on the INRIM website for free download since 2015. It was developed under a Matlab environment and distributed as an executable file. The software tool featured a friendly graphical user interface, to encourage even non-Matlab users to use it. The software is able to perform regression fitting according to ordinary,weighted and weighted total least square models and it can be applied to datasets that can be characterized by uncertainties and covariances in both the dependent and the independent variable. The curves that can be fitted to the data are fractional polynomials with positive and negative exponents up to the fifth order. Recently, version 2.0 of the CCC Software has been developed and tested and will be uploaded early in 2020. The main improvements of this new version, concerning the graphical aspects and the calculation functionalities, are here described in detail, with a major focus on the enhancements obtained in the estimated accuracy of the weighted total least square regression. Validation results and the application to a real case study are also shown. The present paper is a significant expansion of a relevant abstract presented at the 'Mathematical and Statistical Methods for Metrology' Workshop, Torino, Italy, 30-31 May 2019 (Lecuna et al 2019 Mathematical and Statistical Methods for Metrology)

“VOC and wall surface interactions in sampling lines: Experiment Design”

Presentation in SESSION C. ANALYTICAL APPLICATIONS of GAS2015 about quantification of VOC losses due to wall interactions in sampling lines

Calibration curve computing (CCC) software v2.0: A new release of the INRIM regression tool

The main features of the Calibration Curve Computing (CCC) Software, a tool for regression analysis developed at the INRIM, the Italian National Metrology Institute, are described in this paper. The first release of the software tool, version 1.3, has been available on the INRIM website for free download since 2015. It was developed under a Matlab environment and distributed as an executable file. The software tool featured a friendly graphical user interface, to encourage even non-Matlab users to use it. The software is able to perform regression fitting according to ordinary,weighted and weighted total least square models and it can be applied to datasets that can be characterized by uncertainties and covariances in both the dependent and the independent variable. The curves that can be fitted to the data are fractional polynomials with positive and negative exponents up to the fifth order. Recently, version 2.0 of the CCC Software has been developed and tested and will be uploaded early in 2020. The main improvements of this new version, concerning the graphical aspects and the calculation functionalities, are here described in detail, with a major focus on the enhancements obtained in the estimated accuracy of the weighted total least square regression. Validation results and the application to a real case study are also shown. The present paper is a significant expansion of a relevant abstract presented at the 'Mathematical and Statistical Methods for Metrology' Workshop, Torino, Italy, 30-31 May 2019 (Lecunaet al2019Mathematical and Statistical Methods for Metrology)

Modelling of a VOC gas mixtures generator by Computational Fluid Dynamics

Monitoring VOC for climate change and for indoor and outdoor air quality at trace level concentrations need reference standard materials at high metrological performance. To improve this performance, the description of phenomena involved in mixtures generation by rigorous models is mandatory. A model to describe diffusion cells of a primary generator was developed and validated with experimental data. A good agreement was found between the uncertainties of measurements and calculations

Preparation of standard VOC mixtures for climate monitoring

European legislation regulates ambient concentrations of VOC key compounds relevant for indoor air monitoring and for air quality and climate monitoring programmes, e.g., the Global Atmosphere Watch (GAW) programme of the WMO. Data Quality Objectives refer to an expanded uncertainty lower than 10% for VOC measurement at concentration lower than 100 nmol/mol. Traceable and accurate standards and standardised dilution methods for VOC concentration are thus essential for the reliability and harmonisation of the measurement data provided. International research projects are ongoing to develop dynamic dilution systems from several sources (static cylinders and dynamic generation) to prepare standards at 1 - 100 nmol/mol VOC concentration with an uncertainty < 2-7 %. The uncertainty sources for VOC mixtures prepared by a single stage dynamic dilutor are here analyzed. Dilution flow rate, purity of the dilution gas, VOC losses and concentration variability due to imperfect gas mixing, wall-adsorption effects and leaks were considered as influence quantities. The dynamic dilutor performance was described by material balances on which a uncertainty budget was redacted. Mixing performances, wall interactions and leak tests were considered to design the dilutor. Design criteria for the dynamic dilutor and design specification for the air purification system were developed. This work contribute to develop standardized dilution methods for a better calibration of monitoring GAW instruments, it will helps to reduce uncertainty in climate change forecasting models

Stability of standard dynamic VOC mixtures for atmospheric measures

Traceable and accurate standard of Volatile Organic Compounds VOC at trace concentration level are necessary for a framework of quality assurance and quality control of atmospheric composition measurements. Dynamic preparation of standard mixtures can fulfill the target accuracy given by the framework. Short-term concentration stability is one of the sources of uncertainty of dynamic preparation and depends on sampling time of the mixture, stability contribution to total uncertainty must be calculated. A method for the calculation of the stability contribution is here proposed. The performance of a preparation device for accurate VOC standard mixtures is here discussed as an application of the method. Two threshold values for stability contribution were set to consider the contribution negligible and relevant respectively. Experimental stability of the dynamic preparation device was measured. The stability of one of the five influence quantities resulted to give the total stability contribution. Stability contribution to total uncertainty resulted to be negligible below 60 min, while its contribution resulted to be relevant over 400 min