A datalogger that measures both temperature and humidity but is principally a humidity-sensing instrument may fall under 9025.80. However, if the device is primarily a data acquisition or recording system with temperature and humidity as secondary inputs, it could be classified under 9026 or 9032 depending on its principal function. CBP applies the essential character test; the dominant measurement function and the product's commercial marketing are key classification factors. A binding ruling is advisable for multi-parameter dataloggers.

The distinction is mechanical: a hygrograph produces a continuous record of humidity over time — typically via a paper chart or electronic log — while a non-recording hygrometer provides only a current or instantaneous reading. Instruments with data-logging capability are generally treated as recording instruments even if the recording function is secondary. Importers should confirm whether the device's specification sheet describes a recording or logging output to select the correct line.