🔍 Key Concepts

• Notice the reference tube and measuring tube and how air flows through them. • Focus on the lamp, mirrors, and P.E.C. (photoelectric cells) and what they are typically used to detect in engine safety systems. • Think about which devices need to compare a clean sample with a sample from the engine to detect a dangerous condition.

💭 Think About

• What is the purpose of having both a reference tube and a measuring tube with light passing between mirrors? • Why would a rotary valve sample from multiple points and send them past photoelectric cells – is that more consistent with gas concentration, crankcase mist, or liquid level? • Which of the listed devices would most likely use optical (photoelectric) detection rather than chemical or mechanical measurement?

✅ Before You Answer

• Match the use of photoelectric cells (P.E.C.) with the most appropriate option in the choices. • Identify which option involves comparing two paths or samples, as suggested by the terms reference tube and measuring tube. • Eliminate any answer choices that normally would NOT use a lamp and mirrors as their primary sensing method.