Which of the following can be considered a passive detector?

• A. Multi-gas meter
• B. Colorimetric detector tube
• C. Pitot tube
• D. Thermal imager

Answer: (B)

A colorimetric detector tube is considered a passive detector because it does not require any external power source or active sampling to function. Instead, it relies on the chemical reaction that occurs within the tube when specific gases come into contact with the reactive materials. When a gas enters the tube through diffusion, a visible color change occurs, providing a qualitative indication of the presence and concentration of that gas. This simplicity and reliance on natural diffusion processes exemplify the defining characteristics of passive detection methods.

In contrast, a multi-gas meter actively samples air and requires power to operate, measuring and displaying the concentrations of various gases in real time. A pitot tube is designed to measure fluid flow by determining the pressure difference between two points in a fluid stream and is not oriented towards gas detection. A thermal imager detects infrared radiation, providing visual imagery based on temperature differences rather than directly indicating the presence of specific hazardous materials. Each of these lacks the passive characteristics distinctive to colorimetric detector tubes.