Typing mechanism, GUIDs

NC Numerically controlled. Usually refers to a machine tool, in this case a drilling machine. The most basic type is one in which a mechanical guide locates the positions of the holes. NC machines are usually controlled by punched tape. 

The IEEE Guide to the Gollection and Presentation of Electrical, Electronic, Sensing Gom-ponent, and Mechanical Equipment Reliability Data for Nuclear Power Generating Stations (IEEE Std. 500-1984) compiles data from over a dozen other references and includes information for most types of components. 

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

Although the kinetic studies summarised here are useful guides to the gross features of mechanism it is evident from apparently closely related autoxidations, e.g. those of V(III) and U(IV), that subtle factors operate. Fallab has pointed out that these reductants give similar kinetics and possess similar reduction potentials, yet differ in autoxidation rate by a factor of 3 x 10 , and has discussed differences of this type in terms of the stereochemistry of the electron-transfer process in the coordination sphere. 

Simple kinetic measurements can, however, be an inadequate guide to which of the above two mechanisms, SN1 or SN2, is actually operating in, for example, the hydrolysis of a halide. Thus, as we have seen (p. 45), where the solvent can act as a nucleophile (solvolysis), e.g. H20, we would expect for an S 2 type reaction,... 

The interaction between experiment and theory is very important in the field of chemical transformations. In 1981 Kenichi Fukui and Roald Hoffmann received a Nobel Prize for their theoretical work on the electronic basis of reaction mechanisms for a number of important reaction types. Theory has also been influential in guiding experimental work toward demonstrating the mechanisms of one of the simplest classes of reactions, electron transfer (movement of an electron from one place to another). Henry Taube received a Nobel prize in 1983 for his studies of electron transfer in inorganic chemistry, and Rudolf Marcus received a Nobel Prize in 1992 for his theoretical work in this area. The state of development of chemical reaction theory is now sufficiently advanced that it can begin to guide the invention of new transformations by synthetic chemists. 

The use of HPLC in all its forms is growing steadily and may eventually exceed that of GC. This is because all four sorption mechanisms can be exploited and the technique is well suited to a very wide range of compound types including ionic, polymeric and labile materials. The most appropriate choice of mode of HPLC for a given separation problem is based on the relative molecular mass, solubility characteristics and polarity of the compounds to be separated and a guide to this is given in Figure 4.43. 

Durability in its broadest sense covers all aspects of irreversible property change with time and use. This includes all types of environmental agent that contribute to degradation and all aspects of mechanical action. This guide seeks to be comprehensive but concentrates on the most common environmental effects and the most important mechanical properties. More details of the test procedures used can be found in text books and the relevant international standards as referenced. 

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