Process instrumentation — thermal conductivity principle Which instrument classically operates on the principle of measuring thermal conductivity differences of gases?

Introduction / Context:
Industrial gas analysis can use a variety of physical principles. Thermal conductivity measurement (katharometry) is a simple, robust method for distinguishing gases based on their ability to conduct heat.

Given Data / Assumptions:

• A katharometer uses heated elements whose temperature (and resistance) changes with the thermal conductivity of the surrounding gas.
• CO2 is often measured against a reference gas with known conductivity.

Concept / Approach:
Gases have characteristic thermal conductivities. By comparing the rate of heat loss from a heated filament in the sample versus a reference cell, concentration of components like CO2 can be inferred. While modern CO2 analyzers may also use infrared absorption, the classical thermal conductivity analyzer remains a recognized category.

Step-by-Step Solution:
Identify which option aligns with thermal conductivity → katharometer-style CO2 analyzer.Eliminate instruments based on optical rotation (polarimeter), spectral dispersion (spectrometer), and separation platforms (chromatograph).

Verification / Alternative check:
Gas chromatography detectors include thermal conductivity detectors (TCD), but the instrument itself is a chromatographic system; here the standalone analyzer is more directly tied to the principle.

Why Other Options Are Wrong:

• Polarimeter: measures optical rotation in liquids, not gas conductivity.
• Spectrometer: optical spectra, not thermal conductivity.
• Chromatograph: separation device; only the TCD detector uses the principle, not the whole instrument by itself.

Common Pitfalls:

• Assuming all CO2 analyzers are IR; many are, but katharometers are a valid classical type.

Final Answer:
CO2 analyser (katharometer type)