Dedicated replacement probes designed exclusively for use with a specific pyrometer model are generally classifiable as parts under 9025.90. However, if a thermocouple probe is a standard industrial type sold for multiple applications beyond pyrometry, it may be classified under its own heading in Chapter 85 or 90 based on its principal design. The exclusivity of the probe's design to the parent instrument is the key classification criterion.

Both lines cover parts and accessories for the same range of instruments; the split typically reflects statistical reporting distinctions rather than a substantive product difference. Importers should consult the current HTS schedule notes and any applicable CBP guidance to determine which line applies to their specific part, as the operative distinction may relate to the type of parent instrument or other classification criteria in effect at the time of entry.