The key is whether the instrument's principal function is measuring or checking a process variable (flow, level, pressure) for control purposes, or performing a physical or chemical analysis. A flowmeter used to monitor pipeline throughput belongs here; a laboratory instrument that measures flow as part of a chromatographic or analytical process belongs in the physical/chemical analysis heading. The chapter notes and the instrument's design documentation guide the determination.

A pressure transmitter whose principal function is measuring or checking pressure in a liquid or gas system remains in this heading regardless of whether it incorporates digital output protocols (HART, Fieldbus, etc.). The communication capability is ancillary to the measurement function. Only if the article's principal function shifts to data processing or communication would reclassification to another heading be warranted.