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At least 109 records · Page 6Linked to original sources
PubMed2004

Building reliable lattice Monte Carlo models for real drift and diffusion problems.

We revisit the well-known issue of representing an overdamped drift-and-diffusion system by an equivalent lattice random-walk model. We demonstrate that commonly used Monte Carlo algorithms do not conserve the diffusion coefficient when a driving field of arbitrary amplitude is present, and that such algorithms would actually require fluctuating jumping times and one clock per Cartesian direction to work properly. Although it is in principle possible to construct valid algorithms with fixed time steps, we show that no such algorithm can be used in more than two dimensions if the jumps are made along only one axis at each time step.

Journal Article↗
PubMed1993

Chironomid (Diptera: Chironomidae) larval occurrence and transport in a municipal water system.

Chironomid larval populations contaminating a municipal water system were monitored over a 24-month period between June 1987 and June 1989. Larval drift within distribution lines occurred throughout the year, and larval suspension was uniform at all depths within open reservoirs. Draining and cleaning reservoirs to rid them of chironomid populations was of questionable value because larval populations recovered fully within weeks, and larval drift occurred throughout the system.

Animals↗
PubMed1977

[Improvement of resolution of positron annihilation radiation energy spectrum measurement by data analysis (author's transl)].

Recently much attention is being paid to positron annihilation radiation energy spectrum measurement as a simple method that gives information about the momentum distribution of an annihilating pair. However, this method has a disadvantage that it is sensitive to drifts of the measuring system and the resolution is still insufficient. We have succeeded to reduce the influence of the drifts to a negligibly small degree by using, in addition to the necessary procedure for a temperature control and stabilizing of AC power lines, a compensation technique in data analysis. We have further attempted to improve the resolution by deconvolution processing. By these procedures, we have been able to separate a narrow component which has not otherwise been resolved directly from the spectrum. The resolution attained by this way is 0.43 keV (FWHM), and is two- or three-fold superior to those ever reported.

Models, Theoretical↗
PubMed1995

Clinical validation of the Deltatrac monitoring system in mechanically ventilated patients.

OBJECTIVE: To evaluate a monitor of pulmonary gas exchange (Deltatrac, Datex) in a clinical setting. DESIGN: After in vitro evaluation, comparison over 2 min between VO2 and VCO2 values measured by the Deltatrac and the Douglas bag technique. Comparisons were also achieved over 8 h periods between the Deltatrac and a system using a mass-spectrometer. SETTING: Polyvalent intensive care unit (ICU 15 beds) in a 1200 bed general hospital. PATIENTS: Comparison with the Douglas bag technique in 10 patients undergoing controlled ventilation. Comparison with the mass-spectrometer system in 25 other patients undergoing controlled or pressure support ventilation. MEASUREMENTS AND RESULTS: Compared to the results obtained by the Douglas bag technique, the bias (+/- 2 SD) for VO2 and VCO2 was -3.5 +/- 26.6 and 6.1 +/- 12.7 ml.min-1, respectively. By comparison with the mass-spectrometer system, the bias for VO2 and RQ was -5.8 +/- 16.0 ml.min-1 and 0.018 +/- 0.048, respectively. No drift between the two systems was observed over time. CONCLUSIONS: The Deltatrac appears suitable for VO2 and VCO2 measurements in ventilated patients and equivalent to a mass-spectrometer system for long term measurements.

Analysis of Variance↗
PubMedDate not supplied

Visualization experiment to investigate capillary barrier performance in the context of a Yucca Mountain emplacement drift.

The use of capillary barriers as engineered backfill systems to divert water away from radioactive waste potentially stored in a Yucca Mountain emplacement drift is investigated. We designed and conducted a flow visualization experiment to investigate capillary barrier performance in this context. A two-dimensional, thin slab, test system replicated the physical emplacement drift to one-quarter scale (1.4-m diameter) and included the simulated drift wall, waste canister, pedestal, capillary barrier backfill, and host-rock fracture system. Water was supplied at the top of the simulated drift and allowed to discharge by way of wicks located along the left wall of the cell (simulated fractures) or by a gravity drain at the bottom of the right side (simulated impermeable rock with floor drain). Photographs captured the migration of water and a blue dye tracer within the system, analytical balances measured the mass balance of water, while tensiometers measured the capillary pressure at numerous locations. Of particular concern to this test was the drainage of the capillary barrier, which terminates against the drift wall. We found that while the simulated fractures (left side) and drain (right side) each influenced the performance of the capillary barrier at early time, they had little differential affect at later times. Also of concern was the small disparity in capillary properties between the fine and coarse layer (limited by the need of a fine-grained material that would not filter into the coarse layer under dry conditions). While the capillary barrier was able to divert the majority of flow toward the edges of the system and away from the simulated waste canister, the barrier did not preclude flow in the coarse layer, which was noted to be visually wet next to the waste canister on day 92 and was continuing to take on water at termination on day 112.

Engineering↗
PubMed2001

Physiological and behavioral responses of stonefly nymphs to enhanced limestone treatment of acid mine drainage.

A new acid mine drainage (AMD) treatment system uses pulsed, fluidized beds of limestone, and carbon dioxide pretreatment of influent AMD, to enhance limestone neutralization of AMD. We conducted laboratory studies to evaluate the behavior and physiology of larval stoneflies (Pteronarcys proteus, Plecoptera) exposed to effluents produced by the treatment system. Survival, sodium balance, drift, and feeding responses by P. proteus to treated and untreated AMD were examined. P. proteus nymphs exhibited significant losses of whole body sodium in exposures to untreated AMD. Nymphs exposed to treated effluents experienced no loss of whole-body sodium. Nymphs exposed to untreated AMD showed elevated drift rates and depressed feeding rates relative to those of nymphs exposed to treated AMD, and to AMD-free controls. No significant differences in feeding or drift behavior occurred between nymphs exposed to treated effluents and those exposed to AMD-free controls. The treatment system, with and without CO2 pretreatment, provided water that was not toxic to the test animals, and that allowed normal behavioral and physiological function.

Acids↗
PubMedDate not supplied

Glacial sediment causing regional-scale elevated arsenic in drinking water.

In the upper Midwest, USA, elevated arsenic concentrations in public drinking water systems are associated with the lateral extent of northwest provenance late Wisconsin-aged drift. Twelve percent of public water systems located within the footprint of this drift (212 of 1764) exceed 10 microg/L arsenic, which is the U.S. EPA's drinking water standard. Outside of the footprint, only 2.4% of public water systems (52 of 2182) exceed 10 microg/L arsenic. Both glacial drift aquifers and shallow bedrock aquifers overlain by northwest provenance late Wisconsin-aged sediment are affected by arsenic contamination. Evidence suggests that the distinct physical characteristics of northwest provenance late Wisconsin-aged drift--its fine-grained matrix and entrained organic carbon that fosters biological activity--cause the geochemical conditions necessary to mobilize arsenic via reductive mechanisms such as reductive desorption and reductive dissolution of metal oxides. This study highlights an important and often unrecognized phenomenon: high-arsenic sediment is not necessary to cause arsenic-impacted ground water--when "impacted" is now defined as >10 microg/L. This analysis also demonstrates the scientific and economic value of using existing large but imperfect statewide data sets to observe and characterize regional-scale environmental problems.

Arsenic↗
PubMed2004

Mitochondrial DNA and Y-chromosome variation in the caucasus.

We have analyzed mtDNA HVI sequences and Y chromosome haplogroups based on 11 binary markers in 371 individuals, from 11 populations in the Caucasus and the neighbouring countries of Turkey and Iran. Y chromosome haplogroup diversity in the Caucasus was almost as high as in Central Asia and the Near East, and significantly higher than in Europe. More than 27% of the variance in Y-haplogroups can be attributed to differences between populations, whereas mtDNA showed much lower heterogeneity between populations (less then 5%), suggesting a strong influence of patrilocal social structure. Several groups from the highland region of the Caucasus exhibited low diversity and high differentiation for either or both genetic systems, reflecting enhanced genetic drift in these small, isolated populations. Overall, the Caucasus groups showed greater similarity with West Asian than with European groups for both genetic systems, although this similarity was much more pronounced for the Y chromosome than for mtDNA, suggesting that male-mediated migrations from West Asia have influenced the genetic structure of Caucasus populations.

Chromosomes, Human, Y↗
PubMed1983

Selective constraint in protein polymorphism: study of the effectively neutral mutation model by using an improved pseudosampling method.

To investigate the pattern of allelic distribution in enzyme polymorphism, with special reference to the relationship between the mean (H) and the variance (V(H)) of heterozygosity, we used the model of effectively neutral mutations involving multiple alleles in which selective disadvantage of mutant alleles follows a Gamma distribution. A simulation method was developed that enables us to study efficiently the process of random drift in a multiallelic genetic system and that saves a great deal of computer time. It is an improved version of the pseudosampling-variable (PSV) method [Kimura, M. (1980) Proc. Natl. Acad. Sci. USA 77, 522-526] previously used to simulate random drift in a diallelic system. This method will be useful for simulating many models of population genetics that involve behavior of multiple alleles in a finite population. By using this method, it was shown that, as compared with the model of strictly neutral mutations, the present model gives the reduction of both H and V(H) and an excess of rare variant alleles. The results were discussed in the light of recent observations on protein polymorphism with special reference to the functional constraint of proteins involved.

Alleles↗
PubMed2001

Genetic structure in natural populations of flukes and snails: a practical approach and review.

Several aspects of the coevolutionary dynamics in host-parasite systems may be better quantified based on analyses of population structure using neutral genetic markers. This includes, for example, the migration rates of hosts and parasites. In this respect, the current situation, especially in fluke-snail systems is unsatisfactory, since basic population genetics data are lacking and the appropriate methodology has rarely been used. After reviewing the forces acting on population structure (e.g. genetic drift or the mating system) and how they can be analysed in models of structured populations, we propose a simplified, indicative framework for conducting analyses of population structure in hosts and parasites. This includes consideration of markers, sampling, data analysis, comparison of structure in hosts and parasites and use of external data (e.g. from population dynamics). We then focus on flukes and snails, highlighting important biological traits with regard to population structure. The few available studies indicate that asexual amplification of flukes within snails strongly influences adult flukes populations. They also show that the genetic structure among populations in strongly affected by traits in other than snails (e.g. definitive host dispersal behaviour), as snails populations have limited migration. Finally more studies would allow us to deepen our current understanding of selective interference between flukes and snails (e.g. manipulation of host mating system by parasites), and evaluate how this affect population structure at neutral markers.

Animals↗
PubMed1982

Semiautomated system for simultaneous assays of serum vitamin B12 and folic acid in serum evaluated.

We evaluated performance characteristics of a semiautomated radioassay system (ARIA II, Becton Dickinson Immunodiagnostics) for simultaneous measurements of vitamin B12 and folic acid in serum. The system requires off-line boiling of samples, which are then processed on-line. The automated process includes dispensing of boiled samples, tracers, and binders (purified intrinsic factor and milk binder); incubation, separation of bound and free ligands; elution; counting radioactivity of 57Co and 125I; and data reduction. Results were compared with those obtained by a manual method (Diagnostic Products Corp.). Although the nonspecific binding of the ARIA II is more than double that of the manual system, the precision values are comparable (within- and between-assay CVs are less than 8.5%). Values obtained with these two systems correlated well except that folic acid concentrations obtained with ARIA II tended to be lower at high folic acid concentrations. We observed no significant total carryover or drift in the semiautomated system.

Autoanalysis↗
PubMed1993

Reproducibility of quantitative arteriographic assessment of atherosclerosis in the femoral artery.

Results from computer-analysed angiograms in the Probucol Quantitative Regression Swedish Trial (PQRST) were analysed to determine the reproducibility of the method and any drift in the analysing system. The precision index (P(mu)) for 2 angiography series, made at 10 min intervals, of the femoral artery in 276 patients was 10.5 for lumen volume and 21.9 for roughness (edge irregularity). No difference in reproducibility was found between patients with and without symptoms of peripheral atherosclerosis or when looking at the reproducibility over years. A drift of 0.67% per year in the radiographic equipment (but not in the analysis system) was found, confirmed by use of phantoms. Computer-based analysis of femoral atherosclerosis is a reliable method for follow-up trials, giving high reproducibility even if the trial spans over several years and involves different centres. The use of phantoms is essential for checking the method over time.

Adult↗
PubMed1981

Effect of test system on the ability of monoclonal antibodies to detect antigenic drift in influenza A(H1N1) virus haemagglutinins.

Results of analysing antigenic variation in the haemagglutinin (HA) molecule of naturally occurring influenza A (H1N1) viruses from 1977 to 1979 with monoclonal antibodies were found to be dependent in some instances on the test system used. In several instances A/USSR/90/77 HA-specific monoclonal antibodies had sharply reduced haemagglutination-inhibition (HI) titres with variant virus although they bound to the variant and A/USSR/90/77 HAs with similar efficiencies as judged by titration in a sensitive and accurate solid-phase immunofluorimetric assay. In another instance, the converse situation was observed: monoclonal antibodies having a reduced efficiency of binding to the HA of a variant virus nevertheless had comparable HI titres with the variant and with A/USSR/90/77. The chemical basis and epidemiological significance of these observations remain to be elucidated. Nevertheless, the finding that the reaction of monoclonal antibodies can, in some cases, be markedly dependent on the test system employed is of significance for the efficient design and correct interpretation of immunochemical studies which employ monoclonal antibodies to investigate the basis for variation in influenza strains.

Antibodies, Monoclonal↗
PubMed1998

Evaluation of dual-tip micromanometers during 21-day implantation in goats.

Investigative research efforts using a cardiovascular model required the determination of central circulatory haemodynamic and arterial system parameters for the evaluation of cardiovascular performance. These calculations required continuous beat-to-beat measurement of pressure within the four chambers of the heart and great vessels. Sensitivity and offset drift, longevity, and sources of error for eight 3F dual-tipped micromanometers were determined during 21 days of implantation in goats. Subjects were instrumented with pairs of chronically implanted fluid-filled access catheters in the left and right ventricles, through which dual-tipped (test) micromanometers were chronically inserted and single-tip (standard) micromanometers were acutely inserted. Acutely inserted sensors were calibrated daily and measured pressures were compared in vivo to the chronically inserted sensors. Comparison of the pre- and post-gain calibration of the chronically inserted sensors showed a mean sensitivity drift of 1.0 +/- 0.4% (99% confidence, n = 9 sensors) and mean offset drift of 5.0 +/- 1.5 mmHg (99% confidence, n = 9 sensors). Potential sources of error for these drifts were identified, and included measurement system inaccuracies, temperature drift, hydrostatic column gradients, and dynamic pressure changes. Based upon these findings, we determined that these micromanometers may be chronically inserted in high-pressure chambers for up to 17 days with an acceptable error, but should be limited to acute (hours) insertions in low-pressure applications.

Animals↗
PubMed1996

Dynamics and equilibrium behavior of cytonuclear disequilibria under genetic drift, mutation, and migration.

A simple two-locus drift model for cytonuclear systems is developed, in which the stochastic dynamics of cytonuclear genotypic frequencies are specified. Random union of zygotes is assumed. Trajectories for the first two moments of both genotypic and allelic disequilibria are given under three scenarios: (i) random drift alone; (ii) random drift with mutation; and (iii) random drift with migration. Steady state solutions for the cytonuclear disequilibria are reported. The utility of this simple two-locus drift model in testing the neutrality of mitochondrial DNA markers in artificial hybrid zones is briefly illustrated

DNA, Mitochondrial↗
PubMed1997

Effects of birds on the intensity of ant rain: a terrestrial form of invertebrate drift

Large numbers of wood ants, Formica aquiloniacrawl up trees to gather food (arthropods and honeydew), and most return to the ground the same way. A small but noticeable percentage, however, returns to the ground by falling. This continuous rain of free-falling ants, from trees to the ground, is analogous to the drift of invertebrates in freshwater streams, and provides ecologists with an easily replicated terrestrial system for studying the phenomenon of drift. In a field experiment in this system, when birds came to forage, the total numbers and percentages of ants entering the rain increased dramatically. Initial background levels of ant rain (before birds came to forage) ranged from 0 to 11.9% (X=4.0%). When birds came to forage in certain trees, the percentage of ants entering the rain in those trees ranged from 2.9 to 28.4% (X=9.1%). In trees not visited by birds, ant rain decreased slightly during the same period to a mean of 3.7%. As the total time birds spent foraging in a tree increased, ant rain there also increased.

Journal Article↗
PubMed2006

Fast fiber-optic tunable filter based on axial compression on a fiber Bragg grating.

We describe the design, fabrication, and performance of a fiber Bragg grating-based tunable optic filter. The filter, driven by two piezostacks, consists of a flexural hinge structure for displacement magnification and a fiber-ferrule assembly for axial compression of the fiber grating. Finite-element analysis was used to design the mechanical structure to achieve the required displacement magnification and the force for grating compression. A passive thermal compensation design was implemented to reduce thermal-induced wavelength drift. A feedback control system with a linear variable differential transformer was employed to control the displacement for accurate wavelength tuning and fine-tuning resolution. This tunable filter has achieved a closed-loop switching time of 17.3 ms, and a passive thermal compensation that reduced the thermal drift of the Bragg wavelength to 1.5 pm/C. The flexural-hinge structure that offers negligible backlash, noise-free motion, no need of lubricants, and no wear ensures its long-term reliability.

Journal Article↗