Continuous Analytical 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 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.
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- Machine Vibration Measurement - Vibration Physics
- Continuous Analytical Measurement - Safety Gas Analyzers
- Continuous Analytical Measurement - Conductivity Measurement
- Continuous Analytical Measurement - pH Measurement
- Continuous Analytical Measurement - Chromatography
- Continuous Analytical Measurement - Optical Analyses