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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 of little or no interest to the controlled process. Unlike the other “bulk” measurement devices for sensing such general variables as pressure, level, temperature, or flow, an analytical device must discriminately select one material over all others present in the sample. This single problem accounts for much of the complexity of analytical instrumentation: how do we measure the quantity of just one substance when thoroughly mixed with other substances?

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 as paramagnetic as oxygen. A pH analyzer achieves hydrogen ion selectivity by using a specially-prepared glass membrane intended 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 if not properly compensated for in the analyzer. Nitric oxide (NO) gas is one of the few gases also exhibiting significant paramagnetism, and as such will cause measurement errors if introduced into the sample inlet of a paramagnetic oxygen analyzer. 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.

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.

 

Conductivity Measurement

pH Measurement

Chromatography

Optical Analyses

Safety Gas Analyzers

 
 

References

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Novak, Joe, What Is Conductivity, And How Is It Measured?, Hach Company, 2003.

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Scott, Raymond P.W., Liquid Chromatography Detectors, Library4Science, LLC, 2003.

Scott, Raymond P.W., Principles and Practice of Chromatography, Library4Science, LLC, 2003. Sherman, R.E.; Rhodes, L.J., Analytical Instrumentation: practical guides for measurement and control, ISA, Research Triangle Park, NC, 1996.

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“Standard Operating Procedures – Thermo Environmental Instruments Model 43C Trace Level Pulsed Fluorescence Sulfur Dioxide Analyzer”, version 2.0, Environmental Protection Agency, Research Triangle Park, NC, 2009.

Theory and Practice of pH Measurement, PN 44-6033, Rosemount Analytical, 1999.

“XSTREAM Gas Analyzer Series Instruction Manual”, document HASX2E-IM-HS, Rosemount Analytical, 2009.

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