Uncertainty analysis of the diffusion rate in the dynamic generation of volatile organic compound mixtures

Abstract

The measurement of volatile organic compound (VOC) concentration changes, in the range of 10 ppt–1ppb, is essential in several applications. The World Meteorological Organization has identified a set of VOCs that are critical because of their long-term instability. The measurement of such changes demands generation of reference mixtures at a high accuracy and stability level. The dynamic preparation of gas mixtures based on diffusion is a suitable way of generating accurate reference mixtures, particularly of unstable VOCs. Diffusion rate and dilution gas flow rate uncertainties are the main contributors to the overall VOC concentration uncertainty. This paper presents a comprehensive overview of the uncertainty of the diffusion rate. Stress is laid on the advantages offered by a modified measurand equation. The validity of this analysis and the modified equation has been confirmed experimentally. Temperature variability and mass difference uncertainty make the main contributions to the uncertainty: a 0.01 °C variability results in a 0.2% uncertainty. The new measurand equation provides both a better indication of the systematic effects at a narrow temperature variability and a more realistic calculation of the uncertainty. The modified equation enhances calculation of uncertainty at the low diffusion rates needed to generate very low VOC concentration mixtures.

Preparation of standard VOC mixtures for climate monitoring

“…Sonic nozzles were chosen instead of thermal mass flow controllers, since these are characterised by a worse calibration stability [14]. Mixtures of VOCs with vapour pressure between 100 Pa and 70 kPa can be generated, keeping the uncertainty of diffusion rate under 1% in the range 1-50 μg•min −1 [10,12]. Acetone was considered in this study as model VOC.…”

“…Dynamic generation has been proposed as a feasible option to generate standards [6,10]. Dynamic generation is widely used to produce standard materials for VOC at ppm level amount of substance fraction, as defined by ISO 6145 standards [11].…”

“…A single stage dilution of a single VOC stream generated by diffusion (acetone in purified dry air) that was conceived and designed to fulfil GAW requirement of 5% uncertainty for standard atmosphere [10,[12][13][14] was chosen as case study. This work is a contribution to the implementation of traceability chain for oxygenated VOC measurements at GAW monitoring stations and will help in uncertainty reduction for climate change models.…”

“…The uncertainty of diffusion flow rate was estimated in a previous work [10]. The dilution air flow rate uncertainty is the sonic nozzle uncertainty 0.5% mainly dependent on upstream pressure accuracy [19].…”

Uncertainty in cell confluency measurements

“…The variability of confluency and its uncertainty due to focus plane variation was calculated as type B, as indicated in [32], assuming a uniform distribution on the range. Significant index values of calculation method and variability contribution to uncertainty were calculated for each image as a ratio between squared contributions [33].…”