NFPA 72 classifies duct-mounted smoke detectors and their sampling tubes as an initiating device that must be visually inspected once every 12 months. Table 14.3.1 (2022 ed.) lists “Duct smoke detector (including sampling tubes)” with a required inspection frequency of “1 year.” The same interval appears in earlier editions (e.g., 2019 Table 14.4.3.2). Therefore, the code-mandated inspection of sampling tubes is annual.

A. Monthly – NFPA 72 lists no monthly inspection requirement for duct detector sampling tubes; monthly is limited to some supervisory indicators.

B. Quarterly – Quarterly inspections apply to battery voltage and supervisory signal devices, not duct detector tubes.

C. Semiannually – Semiannual checks are required for certain waterflow and valve supervisory devices; sampling tubes are explicitly annual.

1. NFPA 72: National Fire Alarm and Signaling Code, 2022 ed., Table 14.3.1 “Inspection Frequencies,” row “Duct smoke detector (including sampling tubes)” – frequency: 1 year (p. 725).

2. NFPA 72, 2019 ed., Table 14.4.3.2 “Inspection Requirements,” row “Duct smoke detector and sampling tubes” – frequency: Annual (p. 760).

3. University of Maryland, Dept. of Fire Protection Engineering, “FPE 624 – Fire Detection Systems” lecture notes, Week 6, slide 18: cites NFPA 72 Table 14.3.1 annual requirement for duct detector sampling tubes.