Instruments such as pH meters, conductivity analyzers, dissolved oxygen meters, particle size analyzers using laser diffraction, and surface area (BET) analyzers are frequently placed here when they do not use optical radiation as their primary measurement principle and are not chromatographs or gas analyzers. The defining characteristic is that the instrument performs physical or chemical analysis through a method not covered by a more specific SITC heading.

Multi-principle instruments are classified by their principal analytical function. If the electrochemical measurement is the primary output the instrument is sold and used for, 87446 is likely correct; if optical detection dominates, 87443 or 87445 may apply. Importers uncertain about the principal function should seek a CBP binding ruling, particularly for high-value instruments where duty differences between headings are material.