🔍 Key Concepts

• Note the two parallel passages labeled reference tube and measuring tube, and how air is drawn through them by the fan. • Look at the lamp and mirrors between the two tubes – what kind of instrument uses light to compare the clarity or opacity of two gas/air samples? • Consider what marine safety device uses a rotary valve to take small samples from several crankcase spaces or enclosures on a large diesel engine.

💭 Think About

• Which option describes a device that needs both a clean reference stream and a sampled stream in order to work properly? • Which of these choices is normally connected by small pipes from multiple engine spaces, and is used to detect a dangerous condition before an explosion can occur? • Ask yourself: is this drawing showing something installed in an exhaust uptakes, in a lube oil lab bench, in an air-start system, or on crankcase sampling lines?

✅ Before You Answer

• Verify which listed device commonly has reference and measuring tubes compared optically using a lamp and mirrors. • Check which option is specifically associated with monitoring crankcase conditions on large diesel engines for safety. • Eliminate any answer that would not reasonably require a fan to pull vapor/air samples through tubes for continuous comparison.