An instrument that measures both temperature and humidity is classified according to its principal function if one function clearly predominates, or under the heading that appears last in the tariff schedule if both functions are equally important. A standalone hygrometer or psychrometer belongs in 902580, but a weather station combining multiple sensors may require analysis under the combination-instrument rules in the chapter notes.

Non-contact infrared thermometers are not liquid-filled, so they do not fall under 902511. They classify under 902519 as thermometers not combined with other instruments, provided they are not optical analysis instruments of another heading. The distinction between a simple infrared thermometer and an optical spectrometric instrument can be a classification edge case; the article's primary function and design determine the correct heading.