A UV lamp is a scheduled replacement, not a failure item. Its UV-C output falls steadily through life while the lamp continues to light normally, so a reactor with visibly working lamps can be well below its dosing duty. Replacement is made on elapsed operating hours against the measured intensity trend, not on the lamp going out. Order against the reactor model, and handle the new lamp by its ends only — skin oils fired onto the envelope at operating temperature devitrify the quartz and cost output permanently. This lamp is 843 mm long, 15 mm in diameter, and fitted with a 4-pin base. The same envelope and base are used across the Berson 42/S, B42V and 80VIK positions, so dimensions alone do not identify the lamp — order against the reactor model.