🔍 Key Concepts

• Note the labels REFERENCE TUBE and MEASURING TUBE and how air flows through both paths toward the fan. • Pay attention to the lamp and mirrors in the center and what kind of measurement system needs a light source and optical path. • Think about which option’s name best fits a device that compares two streams using light, rather than directly sampling exhaust smoke or condensing vapors.

💭 Think About

• Ask yourself: why would an instrument need both a reference tube and a measuring tube—what is being compared? • Which listed devices normally rely on an optical system (lamp, mirrors, photo‑electric cells) to sense density of mist or particles in an airstream? • How does a Ringelmann indicator usually work, and does it look like this kind of enclosed, ducted system with a fan?

✅ Before You Answer

• Verify which choice specifically describes a device that uses two parallel paths (reference and measuring) to detect a difference. • Confirm which instrument is associated with detecting mist or opacity in air using photo‑electric cells or light beams. • Eliminate any option that mainly deals with exhaust gas color grading or cooling/condensing vapors, since the diagram shows neither a stack nor a heat‑exchanger arrangement.