Principles of automatic instruments

Besides understanding the philosophy and concepts of automatic instrumentation, it is necessary to make a clear economic assessment of the advantages to he gained from the introduction of automation. There can he no substitute for practical experience in solving such problems. The field is wide, so a complete review is not practical. Some developments are described here to stimulate the reader into deeper research. It is important to evaluate the various developments that have become available through both large and small instrumentation companies. 

In basic terms, all automated analyses of samples in the hquid state are performed by one of two methods, discrete or continuous, and occasionally by a combination of the two. There are several basic subdivisions in each group and some examples will be described here and throughout the other chapters of the book to provide a starting platform for the systems designer. 

In the design of an automated analytical instrument, or selection of the most appropriate commercially available type, the choice between the discrete and continuous approach is fundamental. Both types are considered in detail under the various technique 

The discrete method has the advantage that samples can he processed at a high rate. For example, commercial colorimetric analysers are capable of yielding between 100 and 300 measurements per hour, whereas for continuous analysers a processing rate of 20— 80 samples per hour is normal. However, the high-throughput discrete analysers are appreciably more expensive than the continuous analysers. 

Continuous analysis requires flexible tubes which are not attacked by the materials under examination, and this places certain limitations on the scope of the method. Certain reactive and corrosive materials cannot he satisfactorily pumped, although advances have been made in the development of inert plastics and other synthetic materials. Displacement pumping with the aid of a liquid compatible with sample and reagents provides an alternative, though generally inconvenient, approach. No such limitations arise in discrete analysers because there is no restriction on the choice of materials for sample and reagent containment. 

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