Measurement: local practices

The laser-Doppler anemometer measures local fluid velocity from the change in frequency of radiation, between a stationary source and a receiver, due to scattering by particles along the wave path. A laser is commonly used as the source of incident illumination. The measurements are essentially independent of local temperature and pressure. This technique can be used in many different flow systems with transparent fluids containing particles whose velocity is actually measured. For a brief review or the laser-Doppler technique see Goldstein, Appl. Mech. Rev., 27, 753-760 (1974). For additional details see Durst, MeUing, and Whitelaw, Principles and Practice of Laser-Doppler Anemometry, Academic, New York, 1976. 

Time-resolved X-ray absorption is a very different class of experiments [5-7]. Chemical reactions are triggered by an ultrafast laser pulse, but the laser-induced change in geometry is observed by absorption rather than diffraction. This technique permits one to monitor local rather than global changes in the system. What one measures in practice is the extended X-ray absorption fine structure (EXAFS), and the X-ray extended nearedge strucmre (XANES). 

To be strictly correct, the velocity head should be multiplied by a to correct for the nonuniformity of the velocity. To do this precisely, it would be necessary to measure local velocities over the area, but this is not usually practicable, so a is usually assumed to be anything from 1.0 to 2.2, according to one s judgment as to the velocity distribution. As it is believed that the ratio of surface velocity to mean velocity is an important criterion for this case, it is customary to assume a as 1.0 to 1.3 for H equal to or greater than Z and to increase it up to 2.2 as the ratio HIZ decreases. 

The basic physical and chemical mechanisms of CMP are, however, poorly understood. The polishing process as used in semiconductor applications is the sum of a very large number of polishing interactions, with a range of conditions determined by the tools used. The sum of these individual interactions creates the polished surface. The specific conditions of these individual interactions are not well understood and it is quite difficult to set up experiments and measure local interactions which correspond meaningfully to those used in practice. This in itself has... 

Nonstate forms of measurement grew from the logic of local practice. As such, they shared some generic features despite their bewildering variety—features that made them an impediment to administrative uniformity. Thanks to the synthesis of the medievalist Witold Kula, the reasoning that animated local practices of measurement may be set out fairly succinctly. ... 

We have seen the idiosyncracies of metis at work in the historical vernaculars of measurement of area, weight, and volume. The aim was always to achieve a local purpose or to express an important local feature (such as a farm of two cows") rather than to accommodate some universal unit of measurement. Like Squanto s maxim, such vernacular measures apparently often conveyed more information than an abstract measure could. They certainly conveyed information that was more locally relevant. It was just this local, practical index, which varied from place to place, that ensured that metis would be confusing, incoherent, and unassimilable for purposes of statecraft. 

Requirements specified in this way are deemed-to-satisfy rules. Such rules cannot be used to quantify the performance of the structure in general, specific effects of additional measures (for instance increasing the cover to the steel), or the consequences of sub-standard practice (for example using a higher w/c). In this respect it is important to note that EN 206 also allows the use of alternative performance-related design methods with respect to durability that consider in a quantitative way each relevant deterioration mechanism, the service life of the element or structure, and the criteria that define the end of the service life. Such methods should draw a picture of the characteristics that the concrete must possess to protect the reinforcement for the service life requested from a predictive model of the corrosion attack. These refined methods (as opposed to standard methods) may be based on long-term experience with local practices in local environments, on data from an established performance test method for the relevant mechanism, or on the use of proven predictive models. 

Most measurements on the distribntion of a dispersed phase in a liqnid-Uqnid system have been made using the local techniques of measuring the conductivity or light transmittance. The probes used for these measurements are practically identical to those discussed in Sections 4-5.1.2 and 4-5.1.3 with reference to solid-liqnid mixing. The condnctivity-type probe can, however, snffer from the added complication of the electrode(s) becoming completely coaled with a layer of the nonconductive (organic) phase, which causes the instrument to fail. Sampling techniques are subject to the same problems discussed in Section 4-5.1.4. 

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

Projection radiography is widely used for pipe inspection and corrosion monitoring. Film digitisation allows a direct access to the local density variations by computer software. Following to a calibration step an interactive estimation of local wall thickness change based on the obtained density variation is possible. The theoretical model is discussed, the limitations of the application range are shown and examples of the practical use are given. The accuracy of this method is compared to results from wall thickness measurements with ultrasonic devices. 

These values are determined by experiment. It is, however, by no means a trivial task to measure the lamina compressive and shear strengths (52,53). Also the failure of the first ply of a laminate does not necessarily coincide with the maximum load that the laminate can sustain. In many practical composite laminates first-ply failure may be accompanied by a very small reduction in the laminate stiffness. Local ply-level failures can reduce the stress-raising effects of notches and enhance fatigue performance (54). 

A further objective is the evaluation in group (c) of the local polarization state by taking account of IR errors due to direct currents. Here Eq. (3-28) and the further explanations in the second half of Section 3.3.1 are relevant. In practical application, the error effect of A /<,ff must be estimated [2]. When foreign fields are present, it is necessary to substitute for the At/ value the average of the measurements made on both sides of the pipeline [2,52]. Figure 3-30 gives an example of... 

Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corrosion can occur in tidal regions, due to the intensive cathodic action of rust components [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, horizontal cells have practically no significance. 

The following is a brief introduction to making measurements that might be needed in the course of developing an lAQ profile or investigating an lAQ complaint. Emphasis has been placed on the parameters most commonly of interest in nonresearch studies, highlighting the more practical methods and noting some inappropriate tests to avoid. Most of the instruments discussed in this section are relatively inexpensive and readily available from many local safety supply companies. Air contaminants of concern in lAQ can be measured by one or more of the methods described below. 

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