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871.
872.
Because a vascular aetiology has been suggested for the limb and oromandibular defects described after chorionic villus sampling (CVS), to determine whether transabdominal (TA) CVS causes noticeable changes in umbilical artery velocity waveforms in first-trimester pregnancies, the pulsatility index (PI) of the umbilical artery was evaluated before and after TA-CVS in 175 pregnancies sampled between 10·0 and 13·0 weeks' gestation. In 139 uncomplicated pregnancies, the mean PI values (with 95 per cent confidence interval) were before TA-CVS 2·751 (2·692–2·809), after 10 min 2·723 (2·697–2·809), and after 1 h 2·781 (2·722–2·840). There were no significant changes in PI relative to the CVS procedure either in pregnancies with an abnormal result or in those ending in spontaneous abortion. Our data do not support any statistically significant change in umbilical artery PI relative to TA-CVS in first-trimester pregnancies. This procedure, despite its invasive character, does not appear to affect the feto-placental circulation. 相似文献
873.
蒸发浓缩-火焰原子吸收光谱法测定水中铜、锌 总被引:1,自引:0,他引:1
通过蒸发浓缩-火焰原子吸收光谱法测定水中铜、锌研究结果表明,可有效扩大直接吸入-火焰原子吸收光谱法的测量范围,能充分满足地表水、地下水的检测。 相似文献
874.
875.
Wind Characteristics of Mesquite Streets in the Northern Chihuahuan Desert, New Mexico, USA 总被引:2,自引:0,他引:2
Past research has shown that the most important areas for active sand movement in the northern part of the Chihuahuan Desert
are mesquite-dominated desert ecosystems possessing sandy soil texture. The most active sand movement in the mesquite-dominated
ecosystems has been shown to take place on elongated bare soil patches referred to as “streets”. Aerodynamic properties of
mesquite streets eroded by wind should be included in explaining how mesquite streets are more emissive sand sources than
surrounding desert land. To understand the effects of wind properties, we measured them at two flat mesquite sites having
highly similar soil textures but very different configurations of mesquite. The differences in wind properties at the two
sites were caused by differences of size, orientation, and porosity of the mesquite, along with the presence of mesquite coppice
dunes (sand dunes stabilized by mesquites growing in the dune and on its surface) found only at one of the two sites. Wind
direction, u* (friction velocity), z0 (aerodynamic roughness height) and D (zero plane displacement height) were estimated for 15-m tower and 3-m mast data. These aerodynamic data allowed us to distinguish
five categories with differing potentials for sediment transport. Sediment transport for the five categories varied from unrestricted,
free transport to virtually no transport caused by vegetation protection from wind forces. In addition, “steering” of winds
below the level of the tops of mesquite bushes and coppice dunes allowed longer parallel wind durations and increased wind
erosion for streets that aligned roughly SW–NE.
U.S. Government right to retain a non-exclusive royalty-free licence in and to any copyright is acknowledged. 相似文献
876.
T.J. Wang Z.Y. Hu M. Xie Y. Zhang C.K. Xu Z.H. Chao 《Environmental geochemistry and health》2004,26(2):169-177
The regional acid deposition model system (RegADMS) was applied to simulate the air sulfur deposition onto different landuse types over China, in which the dry deposition velocities of SO2 and sulfate aerosol (SO
4
2–
) were estimated by use of a big leaf resistance analogy model and the wet scavenging coefficients were parameterized in terms of precipitation rate. Investigations show that the annual total sulfur deposition over mainland China is 7.24 mt (1 mt = 106 ton) , in which dry deposition and wet deposition accounts for 56 and 44%, respectively. The sulfur deposition onto agriculture land, grass land, and forest land is 1.09, 3.6 and 1.41 mt, respectively, which sums 6.1 mt and accounts for 84% of the total sulfur deposition. The modeled sulfur deposition was in agreement with the measurement conducted at farmland in Yingtan, a typical read soil region in Jiangxi province of China, during the period of November 1998–October 1999. The total sulfur deposition at the Yingtan site is about 10.3 gm–2 year–1 of which 83% is dry deposition. The modeling sulfur deposition at the same site is 8.4 gm–2 year–1. Furthermore, the comparison between RegADMS and RAINS-ASIA on modeling regional sulfur deposition shows the consistence of the two models. The correlation coefficient between the simulated sulfur deposition at the medium-large cities reaches 0.72. 相似文献
877.
Numbers of studies in externally fertilizing fish species provide evidence for an effect of ovarian fluid on sperm motility characteristics such as duration of forward mobility, velocity or percent motile sperm cells. Yet, because of variations among females in the quality of their ovarian fluid, such effects might differ between individuals. Additionally, ovarian fluid from different females could also be expected to affect each ejaculate differently, resulting in cryptic female choice. In this study on Artic charr (Salvelnius alpinus), sperm velocity from several males was measured in the diluted ovarian fluid of several females according to a fully balanced crossing design. This design allowed us to estimate variations among females in the effect of their ovarian fluid on the velocity of sperm from different males, and to detect variations among males in the ability of their sperm to swim in ovarian fluid. Sperm velocity was estimated by computer-assisted sperm analysis. Average velocity was found to vary among females, with some females having constantly higher velocity measurements in their ovarian fluid, and among males, indicating that some males had overall faster sperm in ovarian fluid than others. Moreover, variation in sperm velocity was shown to depend on individual female-male interactions. Our results document that females vary in the effect of their ovarian fluid on sperm velocity and that their ovarian fluid may stimulate sperm velocity according to individual characteristics of males. This latter result suggests a potential mechanism for cryptic female choice.
相似文献
Davnah UrbachEmail: Phone: +41-31-6313182Fax: +41-31-6314535 |
878.
879.
Dieter Lietze 《Journal of Loss Prevention in the Process Industries》1995,8(6):319-324
This paper describes the results of extensive research to determine the limit of safety against flame transmission for sintered metal flame arrester elements when stressed by a flashback in a fuel-gas/oxygen mixture and when stressed through a stabilized burning of a flowing mixture of fuel-gas and air and of fuel-gas and oxygen at the sintered metal element. On the basis of the results of these investigations, the limit of safety against flame transmission for sintered metal flame arrester elements can be estimated and the conditions for testing can be specified. An analysis of the protection of gas outlets on gas distribution lines for welding, cutting and allied processes has been carried out, resulting in recommendations for necessary changes to the regulations for testing of safety devices with sintered metal flame arrester elements, which should be made in the next revision of the corresponding technical regulations for acetylene installations and calcium carbide stores (TRAC), and the standards DIN 8521, EN 730 and ISO 5175. 相似文献
880.
Nabil G. Shafike Lucien Duckstein Thomas Maddock 《Journal of the American Water Resources Association》1992,28(1):33-43
ABSTRACT: Multicriterion decision making (MCDM) techniques were used to analyze a groundwater contamination management problem from the viewpoint of conflicting multiple objectives. The groundwater management model was used to find a compromise strategy for trading off fresh water supply, containment of the waste, and total pumping cost in a hypothetical confined aquifer affected by previous waste disposal action. A groundwater flow model was used to formulate the hydraulic constraints. A linear system model was used to describe drawdown and velocity as functions of the decision variables which were pumping rates. The model determines the pumping location and rates. A modified c-constraint method was used to generate the set of nondominated solutions which were the alternative compromise strategies. Three different MCDM techniques, Compromise programming (CP), ELECTRA II and MCQA II, were used to select a “satisficing” alternative. Analysis of the results showed that, although these techniques follow different principles, the same preferred strategies were reached. Also, it was noticed that maintaining high groundwater velocities is expensive and difficult. In order to meet a two year target date, large amounts of water had to be pumped. Therefore, rapid restoration results in large pumping volumes and high costs. 相似文献