Analog Instrumentation

Peter T. Kissinger Purdue University and Bioanalytical Systems, Inc., 

Analytical techniques are conveniently discussed in terms of the excitation-system-response parlance described earlier. In most cases the system is some molecular entity in a specific chemical environment in some physical container (the cell). The cell is always an important consideration however, its role is normally quite passive (e.g., in absorption spectroscopy, fluorescence, nuclear magnetic resonance, electron spin resonance) because the phenomena of interest are homogeneous throughout the medium. Edge or surface effects are most often negligible. On the other hand, interactions between phases are the central issue in chromatography and electrochemistry. In such heterogeneous techniques, the physical characteristics of the sample container become of critical 

The counter electrode in the two-electrode system serves two functions. First, it completes the circuit, allowing charge to flow through the cell, second, it is assumed to maintain a constant interfacial potential difference regardless of the current. These two functions are mutually exclusive only under very restricted circumstances. Both needs are better served by two separate electrodes  

The reference electrode is used as a potentiometric (always zero-current) probe to monitor A w relative to its own A r. This value is compared with Ea and if a difference (i.e., an error signal) exists, the potential impressed across the cell by the potentiometer is adjusted until balance (i.e., no error signal) is achieved. A device that accomplishes this control function automatically is called a potentiostat for obvious reasons. Such behavior can be mimicked by the experimenter. Although this is assuredly almost never done these days, it is useful to think about a manual potentiostat as a pedagogical device. 

Notice (Fig. 6.5) that although the DCP and ACP are much closer together in the three-electrode approach, they are still not one and the same. There will be some iR drop error, albeit small. The total cell resistance has been divided 

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DCS sampling frequency too low or the response time for some analog instruments may be too slow for proper control of transient nature of batch processes and may lead to a process upset. 

The instrumental analytical techniques, developed in the last three or four decades, are almost all based on the limited signal and data processing capabilities of relatively simple analog instruments, and utilize a limited or simple theoretical basis for calculations. Apart from the rather advanced application of statistics, only a modest use of mathematical techniques in analytical chemistry has been used in these traditional analyses. 

Enke, C.G. (1971), Data Domains-An Analysis of Digital and Analog Instrumentation Systems and Components, Anal. Chem., 43, 69. 

Commercial processor-based electrochemical instruments are available in two forms. In the first configuration, a general-purpose laboratory or personal computer is interfaced to the analog instrumentation. In the other approach, the package is integrated in such a way that the processor is dedicated to the electrochemical experiments. Several dedicated, processor-based pulse polarographs... 

Today musicians take for granted that their instruments are polyphonic. Early analog instruments were monophonic, that is, they only could produce one note a time like a clarinet, which can only produce one note at a time. Musicians used to pianos expected to have arbitrary polyphony available since with a piano, a player could conceivably press up to all 88 keys simultaneously. 

This equation is easily implemented in an industrial plant using analog instrumentation. With computers becoming available in the plants, more accurate methods of obtaining Q (cal/s), can be employed. 

Ref [i] Kissinger FT (1996) Introduction to analog instrumentation. In Kissinger PT, Heineman WR (eds) Laboratory techniques in electroanalytical chemistry, 2nd edn. Marcel Dekker, New York, pp 165-194... 

Confirm that all analog instrument loops are correctly calibrated (over their entire measurement range) from the field instrument to the DCS displays any other indicating devices. Where process trending functionality is included this may also be verified during this test. 

Industrial Automation history in the Petrobras Downstream area can be traced back to 1986, when the company decided to replace old analog instrumentation by digital control systems (DCS), which, given the sheer size of the refining park, with 10 oil refineries, demanded a considerable investment. 

Case I uncertainties often appear in less expensive ultraviolet and visible specirophoiomeiers or photometers equipped with meters or digital readouts with limited resolution. For example, some digital instruments have 3 /i-digit displays. These can display the result to 0.1%r. Here, the readout resolution can limit the measurement precision such that the absolute uncertainty in T s the same from 1)%T to 100% T. A similar limitation occurs with older analog instruments with limited meter resolution. 

Several different methods have been used to obtain derivative spectra. For modern com iuter-controlled digital spectrophotometers, the differcjuiation can be performed numerically using procedures such as derivative least-squares polynomial smoothing, which is discussed in Section. SC 2. With older analog instruments, derivatives of spectral data could be obtained electronically with a suitable operational amplifier circuit (see... 

The above swii can be eliminated by controlling the preheater heat duty or, better still, the preheater heat duty per unit feed flow (362), instead of the preheater outlet temperature. For steam (or condensing vapor) preheaters, the duly per unit feed flow equals the ratio of the measured steam (or vapor) flow to the measured feed flow times a constant, the constant being the steam latent heat. For a sensible-heated preheater, the above ratio is multiplied by the measured hot-side temperature difference, and the constant is the average hot-fluid heat capacity. For two or more feed preheaters, it is best to compute their total heat duty on-line and ratio it to the feed (68, 259). The computation can be readily performed using conventional analog instrumentation. Similar techniques cured the above-cited swing problems (239,259). 

Aircraft and aerospace electronics Analog instrumentation Automotive electronics Business machines Communication systems... 

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