Which of the following is an example of a passive detector?

• A. Multi-gas monitor
• B. Colorimetric detector tube
• C. Photoionization detector (PID)
• D. Infrared spectrometer

Answer: (B)

A colorimetric detector tube is considered a passive detector because it relies on a chemical reaction that occurs when a gas comes into contact with a reactive material in the tube. The change in color indicates the presence and potentially the concentration of specific gases or vapors. This type of detection does not require an electronic signal or active electronic components, meaning it functions passively by interacting with the surrounding environment.

In contrast, multi-gas monitors, photoionization detectors, and infrared spectrometers are all examples of active detection devices. These instruments typically utilize electrical sensors and other components that actively sample the environment and analyze gas concentrations, providing real-time readings. Thus, the nature of how a colorimetric detector tube operates is what makes it fitting for the classification of a passive detector.