Data Acquisition and Storage

600 Hz. Since it is only the difference between these two frequencies that concerns us, and since the use of such large frequencies would unnecessarily occupy computer memory, it is convenient hrst to subtract the reference frequency (300,000,000 Hz in this case) from the observed frequency, and to store the remainder (0-3600 Hz) in computer memory. 

After each pulse, the digitizer (ADC) converts the FID into digital form and stores it in the computer memory. Ideally, we should keep sampling i.e., acquiring data till each FID has decayed to zero. This would require about 57 i seconds, where f is the spin-lattice relaxation time of the slowest-relaxing protons. Since this may often take minutes, it is more convenient 

F ure 1.26 Free induction decay and corresponding frequency-domain signals after Fourier transformations, (a) Short-duration FIDs result in broader peaks in the frequency domain, (b) Long-duration FIDs yield sharp signals in the frequency domain. 

Collecting such a large amount of data would require highly sophisticated and expensive instrumentation and is unnecessary, since we are really interested in the signals appearing in a rather narrow frequency range. 

The frequency at which the signals must be sampled is determined by the maximum separation between the signals in a spectrum, i.e., by the 

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With the availability of computerized data acquisition and storage it is possible to build database libraries of standard reference spectra. When a spectrum of an unknown compound is obtained, its identity can often be determined by searching through a library of reference spectra. This process is known as spectral searching. Comparisons are made by an algorithm that calculates the cumulative difference between the absorbances of the sample and reference spectra. For example, one simple algorithm uses the following equation... 

Historical data management—This includes the data acquisition and storage capabilities. Present-day prices of storage mediums have been dropping rapidly, and systems with 80 gigabyte hard disks are available. These disks could store a minimum of five years of one-minute data for most plants. One-minute data is adequate for most steady state operation, while start-ups and shutdowns or other non steady state operation should be monitored and stored at an interval of one second. To achieve these time rates, data for steady state operation can be obtained from most plant-wide D-CS systems, and for unsteady state conditions, data can be obtained from control systems. 

In the era of single-loop control systems in chemical processing plants, there was little infrastructure for monitoring multivariable processes by using multivariate statistical techniques. A limited number of process and quality variables were measured in most plants, and use of univariate SPM tools for monitoring critical process and quality variables seemed appropriate. The installation of computerized data acquisition and storage systems, the availability of inexpensive sensors for typical process variables such as temperature, flow rate, and pressure, and the development of advanced chemical analysis systems that can provide reliable information on quality variables at high frequencies increased the number of variables measured at... 

Allows for manual termination of data acquisition and storage. 

Two technological developments have begun to change this situation. Low-cost personal computers with sufficient memory and speed to accommodate the data acquisition and storage requirements are now available, and solid state detector arrays with near single-photon counting capabili-... 

Provide data acquisition and storage for different process measurements, such as temperature, flow rate, pressure, humidity, and moisture content... 

In addition, the popularity of operational modal testing and analysis of civil engineering structures has been favored also by the technological advances (i.e., the availability of data acquisition and storage systems, which are fully computer based) and by the large number of output-only modal identification techniques available in the literature (see, e.g., Magalhaes and Cunha 2011). 

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