50 research outputs found
Probabilities of true and false decisions in conformity assessment of a finite sample of items
No full textProbability of conforming items of a finite sample of items is evaluated when conformity assessment (CA) of each item is based on comparison of measured item property values with their acceptance and/or tolerance limits. Two models of probabilities of false decisions on conformity of the whole sample are developed. The first model considers a specific sample of items that already underwent a CA process. A Poisson binomial distribution is applied to calculate the number of the sample items having good (conforming) true property values. The second model, applying a multinomial distribution, treats a generic sample of items potentially drawn from a common population. This model allows evaluating probabilities of false decisions on conformity of the sample items (false positives and negatives), as well as probabilities of correct decisions (true positives and negatives). Applicability of both the models is demonstrated using data of simulated and experimental case studies. These methods extend the existing framework for evaluation of probabilities of true and false decisions in the CA of individual items to the whole sample. The proposed approach provides quality indexes for such a sample, that can be applied in quality inspection and CA of products and objects in industry and trade, environmental monitoring, and other fields. In particular, this approach may be helpful for solving the inspection problem of a production lot, when the minimal number of non-conforming items in a sample for rejecting the whole lot is to be determined
Uncertainty propagation in field inversion for turbulence modelling in turbomachinery
No full textThe simulation of turbulent flows in turbomachinery requires to describe a wide range of scales and non-linear phenomena. Since the cost of scale resolving simulations is prohibitive for several configurations, turbulence closure models are still widely used in the framework of Reynolds-averaged Navier-Stokes (RANS) equations. In order to improve the prediction capability of these models, several machine learning strategies have been proposed. Among them, the field inversion approach allows to find a correction field which can be applied to the source term of the turbulence model in order to match experimental data: the correction field can then be generalised and expressed as a function of some flow features in order to extract modelling knowledge from the data.However, the reference experimental data are affected by uncertainty and this propagates to the correction field and to the final data-augmented model. In this work, the uncertainty propagation from the reference experimental data to the correction field is investigated. In particular, the flow field around a low pressure gas turbine cascade is studied in a challenging working condition characterised by laminar separation and transition to turbulence. The original RANS results are improved by the application of the field inversion algorithm in which the required gradients are computed by means of an adjoint approach. A sensitivity analysis is performed in order to provide a linearised propagation of the uncertainty from the experimental wall isentropic Mach number to the correction field
Bayesian analysis of repeated measurements affected by a systematic error and its application to conformity assessment
No full textThe Bayesian analysis of a series of correlated indications of an unknown quantity is here presented when they are modelled by a joint Gaussian distribution and their covariance is assumed to be the (known) squared uncertainty associated with a systematic effect common to all the indications. An interesting application of the obtained results to the conformity assessment of a series production is also presented. A criterion is derived so that at least a portion p1 of the series production shows to have a characteristic value below a prescribed limit, with a probability not less than p2
The use of dynamic dilution by Mass Flow Controllers in the environmental monitoring of CO2: calibration issues and strategies
No full textGaseous pollutants determination in atmosphere is usually carried out by comparison methods with dedicated analysers, which need to be calibrated with proper certified reference gas mixtures. D ynamic dilution allows the generation of gas mixtures which can be employed either to calibrate the analysers or to validate gas mixtures contained in high pressure cylinders. The dynamic dilution is often achieved by using Mass Flow Controllers (MFCs) due to their ease of use and good level of reliability. The MFC sensors are based on heat exchange equations, therefore the response of these instruments is sensitive to the nature of the flowing gas. The different response can be taken into account through a so-called Gas Correction Factor (GCF), which can be computed when the gas properties are known. Neglecting the use of proper GCFs may result in a bias in the assignment of the molar fraction of dynamic mixtures. The present paper deals with the calculation carried out at INRIM of the GCF in the generation of dynamic mixtures of carbon dioxide in synthetic air at ambient level. The results obtained without using the GCFs are compared with those obtained by application of the correction, showing the presence of a systematic non-negligible bias
EMUE-D2-3-TSPConcentration
No full textConformity assessment of mass concentration of total suspended particulate matter in air.
The example shows how to calculate risks of false decisions in the conformity assessment of test results, according to the framework of JCGM 106:2008, in the case in which a normal distribution is not a valid assumption for modelling prior information on the measurand. As a case study, test results of mass concentration of Total Suspended Particulate Matter (TSPM) in ambient air, collected in the proximity of three stone quarries located in Israel, are considered; for each quarry, a log-normal distribution is chosen as the prior distribution. [Activity A1.2.3]
Uncertainty evaluation of CTD measurements: a metrological approach to water-column coastal parameters in the Gulf of La Spezia area
No full textThe ENEA Marine Environment Research Centre of S. Teresa has been involved since the ‘70s in monitoring, analysis and comprehension of physical, chemical and biological processes in marine environment. The purpose of this work is to describe the recently-implemented metrological approach aimed at evaluating the uncertainty associated with measurements performed by a Conductivity-Temperature-Depth profiler (CTD) during routine coastal campaigns in the Eastern Ligurian Sea, close to the Gulf of La Spezia. Main effort of this work is focused on applying, to each involved parameter, the standard framework for uncertainty evaluation as prescribed by the Guide to the expression of uncertainty in measurement. To this aim, an appropriate uncertainty evaluation is performed by combining type A and B contributions, evaluated from experimental data obtained in reproducibility conditions and from calibration certificates periodically supplied by manufacturer, respectively. Concerning in situ measured practical salinity, probability density functions modelling water pressure, temperature and conductivity, from which salinity depends, are propagated by application of the Monte Carlo method for propagation of distributions, hence obtaining the salinity uncertainty
ARGO floats vs. Ship-based CTDs: An overall metrological comparison in the whole Mediterranean Sea
No full textThe widespread network of temperature/salinity profiling floats, known as Argo, has nowadays become a fundamental component of the ocean observing system. Since Argo floats usually are not recovered and should last up to five years without any re-calibration, their onboard sensors can reasonably suffer some drift and/or offset. During the years, very refined methods have been developed and implemented to post-process the Argo data, in order to correct the response of their profiling CTD (Conductivity - Temperature - Depth) sensors, in particular adjusting the salinity drift. The core of this delayed-mode quality control is the comparison of Argo data with reference climatology. At the same time, it's still considered metrologically fundamental the experimental comparison of Argo profiles with quasi-collocated in space and quasi-simultaneous in time ship-based CTD profiles. In this paper an overall comparison of Argo floats vs. shipboard CTDs was made, in terms of temperature and salinity profiles in the whole Mediterranean Sea and under strict space-time matching conditions
