Gas analysis detector principle: A thermal conductivity cell (katharometer) is commonly used as the primary sensing element in which type of gas analyser for process streams?

Introduction / Context:
Gas analysers exploit different physical properties: paramagnetism (O2), non-dispersive infrared absorption (CO2 and CO), electrochemistry (O2, CO), ultraviolet absorption (SO2), and thermal conductivity differences (katharometer). A thermal conductivity cell measures how readily heat is removed from a heated element by the surrounding gas, which depends on the gas’s thermal conductivity and composition. This question targets recognition of a classical pairing used in many exam contexts.

Given Data / Assumptions:

• Binary or pseudo-binary gas mixtures where one component’s concentration changes.
• The analyser compares sample and reference gases across matched cells.
• Small composition changes cause measurable bridge output changes.

Concept / Approach:
Historically, thermal conductivity cells have been applied to CO2 analysis in flue gases and process off-gases where composition varies around a known background (e.g., CO2 in air or N2). Although modern plants favor NDIR for CO2 due to superior selectivity and sensitivity, the katharometer remains a valid, well-known approach for CO2 when cross-sensitivities are acceptable and calibration is frequent. Therefore, among the listed single-species analyzers, “carbon dioxide analyser” best matches the conventional textbook association with thermal conductivity cells in process measurements.

Step-by-Step Solution:

Verification / Alternative check:
Many legacy CO2 analyzers (and hydrogen purity monitors) used TCDs; modern replacements often adopt NDIR, but the principle remains academically correct.

Why Other Options Are Wrong:

• O2: Paramagnetic or zirconia sensors are primary; TCD is less selective.
• CO: Prefer NDIR or electrochemical cells for specificity.
• SO2: UV absorption is standard due to strong spectral features.
• Hydrogen purity only: TCD is indeed used here, but the option says “only,” which is too restrictive compared to broader CO2 use in teaching examples.

Common Pitfalls:
Ignoring cross-sensitivity to other gases and flow/temperature effects; TCDs need stable conditions and frequent calibration.

Final Answer:
Carbon dioxide analyser