Air pollution monitoring: the smoke density (opacity) of flue gas leaving a chimney is commonly measured using which instrument principle?

Introduction / Context:
Stack emissions compliance often requires a measure of smoke density (opacity), which reflects particulate content affecting light transmission. This is a standard parameter in combustion systems and environmental control engineering.

Given Data / Assumptions:

• Flue gas contains particulate matter that scatters/absorbs light.
• We seek an on-line, non-invasive method through the stack cross-section.
• Opacity correlates inversely with light transmittance.

Concept / Approach:
A photoelectric smoke (opacity) meter transmits a light beam across the stack; a photodetector measures received intensity. The Beer–Lambert-type attenuation due to aerosols gives percent opacity. Thermal conductivity meters measure gas composition (e.g., H2 detection), polarographs measure current–voltage behavior of electroactive species in solution, and chromatographs separate components—not used for direct opacity.

Step-by-Step Solution:
Install light source and detector across stack path.Measure transmitted light intensity I relative to incident I0.Compute opacity = (1 − I/I0) * 100%.Report smoke density per regulatory standard and calibrate with neutral density filters.

Verification / Alternative check:
Correlation of opacity with gravimetric particulate sampling (isokinetic sampling) validates instrument readings and supports emission factor calculations.

Why Other Options Are Wrong:

• Thermal conductivity: unrelated to optical scatter/absorption.
• Polarograph: solution electroanalysis, not flue opacity.
• Chromatograph: composition analysis, not direct optical density.

Common Pitfalls:
Window fouling and misalignment cause false opacity changes; regular purge-air systems and auto-zero routines are essential.

Final Answer:
Photoelectric cell (smoke meter/opacity meter)