🔍 Key Concepts

• Flow paths through the reference tube and measuring tube and what they are comparing • The role of the lamp and mirrors in the center section of the device • Why a fan would be needed in this type of monitoring or detection device

💭 Think About

• Look at what is happening in both the reference tube and the measuring tube – what property would be compared between two gas/air streams? • Ask yourself which of the listed devices would use light (a lamp and mirrors) to detect a change in what is flowing through it. • Consider which option logically requires continuous circulation of air or gas past a sensing element, rather than one‑time measurement or fuel/air distribution.

✅ Before You Answer

• Identify which choice would reasonably have both a reference tube and a measuring tube doing a side‑by‑side comparison. • Confirm which listed device would realistically include a photo‑electric cell (P.E.C.), lamp, and mirrors as its primary sensing method. • Eliminate any options that do not need a fan-driven air flow path through two tubes and an optical chamber.