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screeningthe use of academic qualifications as a means of selecting among candidates for employment, where it is the general level of academic qualification which is decisive rather than the particular content of the education. In this process, an employer may use educational qualifications, or sometimes also the type of institution attended, as a proxy for ‘general intelligence’, ‘perseverance and motivation’, or other 'social background’, instead of being interested in the specific content of the education received. see also CULTURAL CAPITAL.
According to the screening hypothesis, it is the screening process rather than any direct economic return on education which explains part of the correlation between level of education and level of income. This hypothesis provides an account of the effects of education which is at odds with other hypotheses (compare HUMAN CAPITAL). See also CREDENTIALISM, CULTURAL CAPITAL.
the sorting on screens of bulk materials according to particle size. Screening is used to separate coal, ore. building materials, and other bulk materials into fractions or to sort out particles of a given size. For example, in the production of crushed gravel (about 200 million cu m in the USSR in 1970). a mass of raw sand and gravel is sorted out into five fractions (70–40 mm. 40–20 mm, 20–10 mm, 10–5 mm, and less than 5 mm). During screening, the material is separated into layers as it moves along the sieve of the screen: the larger the particles, the higher the layer along which they move. Particles that are smaller than the screen openings (so-called lower grade) fall through the openings upon reaching the screen’s surface (screen underflow); larger particles (so-called upper grade) slide along the sieve and form the screen overflow. Because of limitations on the length of the screen, not all particles that are smaller than the sieve openings fall through; some remain in the screen overflow, contaminating it and reducing the quantity of the underflow.
The efficiency of screening depends on many factors: the size and shape of the particles in the initial material and the load it exerts on the screen, the type of screen, and the size and shape of the openings in the sieve, its length, and its angle of inclination. The maximum efficiency of trommels is 60–70 percent: of shaking screens. 70–80 percent; and of vibrating screens, 90–98 percent. Screening on sieves with openings of 3 mm and more is widespread in industry; openings of 1 mm are rarely encountered. Hydraulic classification or air separation is usually used to sort materials containing particles smaller than 1–3 mm. Since screening ensures high-quality sorting, its use is spreading, particularly the use of sieves with small openings.
A mechanical method of separating a mixture of solid particles into fractions by size. The mixture to be separated, called the feed, is passed over a screen surface containing openings of definite size. Particles smaller than the openings fall through the screen and are collected as undersize. Particles larger than the openings slide off the screen and are caught as oversize. A single screen separates the feed into only two fractions. Two or more screens may be operated in series to give additional fractions. Screening occasionally is done wet, but most commonly it is done dry.
Industrial screens may be constructed of metal bars, perforated or slotted metal plates, woven wire cloth, or bolting cloth. The openings are usually square but may be circular or rectangular. See Mechanical classification, Mechanical separation techniques, Sedimentation (industry)