Limits of non-contact thermometry — where can’t radiation thermometers be used? A radiation (optical/infrared) thermometer measures temperature from emitted radiation without touching the target. In which of the following cases is such a thermometer not suitable?

Introduction / Context:
Radiation thermometers infer temperature from thermal radiation. Their major requirement is an unobstructed optical path between the instrument and the target. If that line of sight is absent, the device cannot sense the target’s radiation, no matter how hot or cold the process is.

Given Data / Assumptions:

• The thermometer is non-intrusive and requires clear optical access.
• Target emissivity is acceptable or compensated.
• No specialised windows or viewports are assumed unless stated.

Concept / Approach:
The only scenario that fundamentally disables a radiation thermometer is the absence of a radiative path. A sealed pressure vessel with no optical window prevents any radiation from reaching the sensor, making measurement impossible. Conversely, measuring moving hot objects, or even cryogenic surfaces (with suitable calibration and emissivity handling), is feasible provided the instrument can “see” the target.

Step-by-Step Solution:

Verification / Alternative check:
Industry practice uses sight-glass or sapphire windows to enable radiation measurements into vessels; without such viewports, measurement is not possible, confirming the limitation.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming low temperatures or movement disqualify radiation thermometers. It is the line-of-sight that is truly critical.

Final Answer:
inside a pressure vessel (no line of sight)