Aging I C equipment

Key issues in determining the achieved life of equipment are the quality of maintenance, and environmental issues. Furlheimore, the age of equipment at which it changes from exhibiting random failure rates (i.e., the flat portion of the bathtub) to accelerating (or aging) failure rates will differ for different types of elecirical and I C equipment. 

By comparison with I C equipment, cable aging is dependent upon the type of cable (e.g., whether or not it is armored), the type of insulation (e.g., some types of insulation degrade more rapidly when exposed to sunlight), the potential for mechanical or thermal damage, and humidity. It is also a function of cable loading, and environment. Cable joints can be a weakness. However, 60-year-old operational cables are not unknown. 

So-called I C aging or obsolescence in practice is most often determined by spares availability. OEM contracts may expire after a given number of years, and spares become unavailable. However, I C equipment can in some instances, and with care and forethought, be operated safely well beyond this point. The key issues (Fig. 6.3) are as follows. 

Good maintenance management processes and practices are central to ensuring that aging safety-critical I C systems and equipment continue to operate reliably and... 

The samples were immersed in demineralised water for the ageing experiments and placed in an oven at 90°C + 2°C. After a certain immersion time, a sample was taken out of the oven and cooled in water at a temperature of 20°C. Subsequently, the sample was surface dried, connected with the measuring equipment and measured. This procedure was performed in about five minutes. A separate sample was used for each measurement. It is important to realise that although the samples were stored in water at 90°C, the resistivity measurement itself was performed at room temperature. The measuring procedure was kept as short as possible, j>(60) i.e. some time dependency of the measuring currents could stil be measured, to keep the moisture evaporation losses as low as possible. 

The solubility of amorphous silica in salt water solutions at 0 to 25 C has been the subject of much study in recent years, and it is interesting that determination of such an apparently simple nxunber can yield such a wide range of results (Table I). As an illustration of the problem, Willey ( 1) showed a plot of the solubility of amorphous silica in seawater at 0 C and at pressures from 1 to 1000 atmospheres pressure A later study using the same experimental apparatus ( ) reproduced the same plot. However, two months later during the next experiment with the same equipment and the same silica, a solubility decrease occurred at all pressures, and the pressure dependence became slightly different than in both previous studies. This aging effect caused a solubility decrease of approximately 20%. Two other pressure studies (3, , which used different experimental... 

Knowing k and the decay rates for the fresh sample and the old sample, we can calculate t, which is the age of the old sample. This ingenious technique is based on a remarkably simple idea. Its success depends on how accurately we can measure the rate of decay. In fresh samples, the ratio is about I/IO, so the equipment used to monitor the radioactive decay must be very sensitive. Precision is more difficult with older samples because they contain even fewer " C nuclei. Nevertheless, radiocarbon dating has become an extremely valuable tool for estimating the age of archaeological artifacts, paintings, and other objects dating back 1000 to 50,000 years. 

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