Chapter 23 Continuous analytical measurement

In the field of industrial instrumentation and process control, the word analyzer generally refers to an instrument tasked with measuring the concentration of some substance, usually mixed with other substances. Unlike the other “bulk” measurement devices for sensing such general variables as pressure, level, temperature, or flow, an analytical device must discriminately detect one specific substance while ignoring all other substances present in the sample. This single problem accounts for much of the complexity of analytical instrumentation: how do we achieve a high degree of selectivity in our measurement?

Analytical instruments generally achieve selectivity by measuring some property of the substance of interest unique to that substance alone, or at least unique to it among the possible substances likely to be found in the process sample. For example, an optically-based analyzer might achieve selectivity by measuring the intensities of only those particular wavelengths of light absorbed by the compound of interest, and absorbed by none of the other wavelengths. A “paramagnetic” oxygen gas analyzer achieves selectivity by exploiting the paramagnetic properties of oxygen gas, since no other industrial gas is nearly as paramagnetic as oxygen. A pH analyzer achieves hydrogen ion selectivity by using a specially-prepared glass membrane constructed to pass only hydrogen ions.

Problems are sure to arise if the measured property of the substance of interest is not as unique as originally thought. This may occur due to oversight on the part of the person originally choosing the analyzer technology, or it may occur as a result of changes made to the process chemistry, whether by intentional modification of the process equipment or by abnormal operating conditions. For example, a gas that happens to absorb some (or all!) of the same light wavelengths as the gas of interest will cause false measurements in an optical absorption analyzer. Nitric oxide (NO) gas is considered an interferent for paramagnetic oxygen analyzers, since this gas is one of the few gases besides oxygen also exhibiting significant paramagnetism. A pH analyzer immersed in a liquid solution containing an abundance of sodium ions may fall victim to measurement errors, because sodium ions also happen to interact with the glass membrane of a pH electrode to generate a voltage. These are but a few practical examples of analyzer non-selectivity.

For this reason, the student of analytical instrumentation must always pay close attention to the underlying principle of measurement for any analyzer technology, looking out for any ways that analyzer may be “fooled” by the presence of some other substance than the one the analyzer was designed to measure.

Some chemical analyzers are known for their unreliability, requiring a variety of conditions to be just right in order for to operate as designed. Proper conditioning of the sample to be analyzed (e.g. filtering, heating or cooling, drying) is one of the many points of failure potentially plaguing analytical instruments, as a typical sample conditioning system is a complex arrangement of tubes, valves, and instruments in its own right. Analyzers also tend to be expensive, both in their initial and consumable costs. These and other reasons are why analytical instrument maintenance is considered an advanced skill. Keeping analyzers in good working order is a challenging technical task for any technician, and usually requires special training above and beyond the knowledge and skill base required for general instrument maintenance.

An interesting historical reference on this point comes from the book Instrumentation and Control in the German Chemical Industry, written after the end of World War II as British investigators toured a variety of chemical manufacturing plants in Germany to learn about their process instrumentation. The instrument head at the I.G. Chemische Werke in the city of Hüls communicated the following sentiment to the authors of this text regarding the maintenance of analytical instrumentation by a special department called the Physical Laboratory:

[He] considers that the ordinary instrument man is not capable of giving the skilled maintenance necessary with this class of instrument. From a maintenance point of view he considers them in a different class to flowmeters, pressure gauges, etc. The section instrument manager is responsible for the routine day to day maintenance such as changing of charts and filling of pens, but special maintenance and calibration is carried out in the laboratory or in the plant by skilled men from the physical laboratory. In [his] opinion this is the only satisfactory way of maintaining complicated analysis instruments as only men who are having continuous experience can carry out the special maintenance, repair and calibration. (page 115)

Mind you, this was during a time when the state of the art for optical absorption analyzers required 2 to 3 days of labor to complete a full calibration, but in some very fundamental ways the challenge of analyzer maintenance remains unaltered. Chemically-selective measurements are by their very nature more complex than bulk measurements such as pressure, level, temperature, or flow, and as such they are prone to a greater number and more complex set of faults. If your career goal is to become as knowledgeable and skillful as possible in the field of industrial process measurement, analytical instrumentation is your specialty of choice!

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