344 research outputs found
A comparative study of techniques used for porous membrane characterization: pore characterization
A range of commerical UF membranes have been characterized by thermoporometry, biliquid permporometry and molecular weight cut-off experiments. A comparison of results from these three independent techniques for the same types of membrane shows an indication of the strength and weakness of the methods. MWCO values determined from actual rejection values using PEG and dextran were significantly lower than the manufacturer supplied data. The data obtained using the biliquid permporometry and solute rejection tests produced contrasting results for Amicon polysulfone (PM30) and regenerated cellulose (YM30) membranes. While MWCO determination resulted in sharper cut-off curves, the biliquid permporometry offered a broader size distribution with the PM30 and vice versa with the YM30. The pore sizes obtained by thermoporometry were significantly larger than those by the biliquid permporometry. The biliquid permporometry and thermoporometry give significantly higher values than the MWCO method. The closest comparison is obtained between the EM values and the MWCO method. This suggests that the controlling pore dimension for separation is the surface skin dimension
High-pressure Raman study of L-alanine crystal
Pressure-dependent Raman scattering studies in the range 0.0 -- 32 kbar were
carried out in L-alanine in order to investigate its external mode phonon
spectra in relation to the phase transitions in the crystal. A careful analysis
of the spectra shows that the low-energy Raman modes exhibit variation both in
frequency and in intensity and between 26 and 28 kbar it is observed a
splitting of a external mode, indicating that the D_2 normal phase undergoes a
transition. Pressure coefficients for external modes are also given.Comment: 11 pages, Latex, 2 figure
Resolving an apparent paradox in doubly robust estimators
Doubly robust estimators are an approach used for estimating causal effects, usually based on fitting 2 statistical models (1). As the initial motivating example, Scharfstein et al. defined a robust estimator of the causal effect of some exposure X on outcome Y using models for both X and Yâ ; they demonstrated that such an estimator is consistent if âat least one of the [fitted] models⊠is correctâ (2, p. 1142). Such estimators were later termed âdoubly robustâ (3, p. 6). Here, we demonstrate that using common (but incorrect) intuition about what makes a model âcorrectâ or âincorrectâ can turn doubly robust estimators into estimators that are inconsistent if at least one of the fitted models is wrong. We introduce and resolve this double-robust paradox, demonstrating what must be meant by âcorrect model.
The economic and accounting content of fixed assets
This book presents a mathematical methodology for image analysis tasks at the edge of current research, including anisotropic diffusion filtering of tensor fields. Instead of specific applications, it explores methodological structures on which they are built.DIPLECS, GARNICS, NACI
Adsorption of mono- and multivalent cat- and anions on DNA molecules
Adsorption of monovalent and multivalent cat- and anions on a deoxyribose
nucleic acid (DNA) molecule from a salt solution is investigated by computer
simulation. The ions are modelled as charged hard spheres, the DNA molecule as
a point charge pattern following the double-helical phosphate strands. The
geometrical shape of the DNA molecules is modelled on different levels ranging
from a simple cylindrical shape to structured models which include the major
and minor grooves between the phosphate strands. The densities of the ions
adsorbed on the phosphate strands, in the major and in the minor grooves are
calculated. First, we find that the adsorption pattern on the DNA surface
depends strongly on its geometrical shape: counterions adsorb preferentially
along the phosphate strands for a cylindrical model shape, but in the minor
groove for a geometrically structured model. Second, we find that an addition
of monovalent salt ions results in an increase of the charge density in the
minor groove while the total charge density of ions adsorbed in the major
groove stays unchanged. The adsorbed ion densities are highly structured along
the minor groove while they are almost smeared along the major groove.
Furthermore, for a fixed amount of added salt, the major groove cationic charge
is independent on the counterion valency. For increasing salt concentration the
major groove is neutralized while the total charge adsorbed in the minor groove
is constant. DNA overcharging is detected for multivalent salt. Simulations for
a larger ion radii, which mimic the effect of the ion hydration, indicate an
increased adsorbtion of cations in the major groove.Comment: 34 pages with 14 figure
Local linear regression with adaptive orthogonal fitting for the wind power application
Short-term forecasting of wind generation requires a model of the function for the conversion of me-teorological variables (mainly wind speed) to power production. Such a power curve is nonlinear and bounded, in addition to being nonstationary. Local linear regression is an appealing nonparametric ap-proach for power curve estimation, for which the model coefficients can be tracked with recursive Least Squares (LS) methods. This may lead to an inaccurate estimate of the true power curve, owing to the assumption that a noise component is present on the response variable axis only. Therefore, this assump-tion is relaxed here, by describing a local linear regression with orthogonal fit. Local linear coefficients are defined as those which minimize a weighted Total Least Squares (TLS) criterion. An adaptive es-timation method is introduced in order to accommodate nonstationarity. This has the additional benefit of lowering the computational costs of updating local coefficients every time new observations become available. The estimation method is based on tracking the left-most eigenvector of the augmented covari-ance matrix. A robustification of the estimation method is also proposed. Simulations on semi-artificial datasets (for which the true power curve is available) underline the properties of the proposed regression and related estimation methods. An important result is the significantly higher ability of local polynomia
Distinct respiratory responses of soils to complex organic substrate are governed predominantly by soil architecture and its microbial community
Factors governing the turnover of organic matter (OM) added to soils, including substrate quality, climate, environment and biology, are well known, but their relative importance has been difficult to ascertain due to the interconnected nature of the soil system. This has made their inclusion in mechanistic models of OM turnover or nutrient cycling difficult despite the potential power of these models to unravel complex interactions. Using high temporal-resolution respirometery (6 min measurement intervals), we monitored the respiratory response of 67 soils sampled from across England and Wales over a 5 day period following the addition of a complex organic substrate (green barley powder). Four respiratory response archetypes were observed, characterised by different rates of respiration as well as different time-dependent patterns. We also found that it was possible to predict, with 95% accuracy, which type of respiratory behaviour a soil would exhibit based on certain physical and chemical soil properties combined with the size and phenotypic structure of the microbial community. Bulk density, microbial biomass carbon, water holding capacity and microbial community phenotype were identified as the four most important factors in predicting the soilsâ respiratory responses using a Bayesian belief network. These results show that the size and constitution of the microbial community are as important as physico-chemical properties of a soil in governing the respiratory response to OM addition. Such a combination suggests that the 'architecture' of the soil, i.e. the integration of the spatial organisation of the environment and the interactions between the communities living and functioning within the pore networks, is fundamentally important in regulating such processes
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
Search for Higgs bosons decaying to tautau pairs in ppbar collisions at sqrt(s) = 1.96 TeV
We present a search for the production of neutral Higgs bosons decaying into
tautau pairs in ppbar collisions at a center-of-mass energy of 1.96 TeV. The
data, corresponding to an integrated luminosity of 5.4 fb-1, were collected by
the D0 experiment at the Fermilab Tevatron Collider. We set upper limits at the
95% C.L. on the product of production cross section and branching ratio for a
scalar resonance decaying into tautau pairs, and we then interpret these limits
as limits on the production of Higgs bosons in the minimal supersymmetric
standard model (MSSM) and as constraints in the MSSM parameter space.Comment: 7 pages, 5 figures, submitted to PL
Measurement of the photon-jet production differential cross section in collisions at \sqrt{s}=1.96~\TeV
We present measurements of the differential cross section dsigma/dpT_gamma
for the inclusive production of a photon in association with a b-quark jet for
photons with rapidities |y_gamma|< 1.0 and 30<pT_gamma <300 GeV, as well as for
photons with 1.5<|y_gamma|< 2.5 and 30< pT_gamma <200 GeV, where pT_gamma is
the photon transverse momentum. The b-quark jets are required to have pT>15 GeV
and rapidity |y_jet| < 1.5. The results are based on data corresponding to an
integrated luminosity of 8.7 fb^-1, recorded with the D0 detector at the
Fermilab Tevatron Collider at sqrt(s)=1.96 TeV. The measured cross
sections are compared with next-to-leading order perturbative QCD calculations
using different sets of parton distribution functions as well as to predictions
based on the kT-factorization QCD approach, and those from the Sherpa and
Pythia Monte Carlo event generators.Comment: 10 pages, 9 figures, submitted to Phys. Lett.
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