Thermometer applicability at sub-zero conditions: Which thermometer type cannot reliably measure sub-zero temperatures (below 0°C) in standard practice without special modifications?

Introduction / Context:
Selecting a thermometer for cold-region or cryogenic-adjacent service hinges on whether the sensing medium remains liquid and responsive below 0°C. Many industrial and laboratory questions stress that simple mercury-in-glass instruments are not suitable for routine sub-zero measurements, whereas alcohol or gas systems are commonly preferred.

Given Data / Assumptions:

• “Standard practice” implies typical, unmodified instruments.
• Sub-zero means temperatures below 0°C.
• Mechanical integrity and readability are part of suitability, not merely theoretical range.

Concept / Approach:
Mercury freezes at around −39°C, and even above that, mercury-in-glass thermometers require special design (e.g., nitrogen-filled) for reliable sub-zero operation. In practical exam contexts, the accepted guidance is that mercury-in-glass is unsuitable for sub-zero work; alcohol-in-glass, vapour-pressure, gas-filled, or RTD devices are used instead because their working medium performs better below 0°C and the readout can be remote or electrical as needed.

Step-by-Step Solution:

Verification / Alternative check:
Instrument handbooks prescribe alcohol-filled or gas-filled devices for low temperatures and caution about mercury separation and readability issues below 0°C unless specially constructed.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming any mercury thermometer works fine below freezing; in practice, specialised designs are needed, hence the general rule used in exams.

Final Answer:
Mercury-in-glass thermometer