Sub-zero temperature measurement: Which instrument from the list is generally not used for measuring temperatures below 0 °C (sub-zero work) under typical laboratory conditions?

Introduction / Context:
Choosing the right temperature sensor for sub-zero work depends on the sensor’s physical limitations and accuracy. While several devices function well below 0 °C, mercury-in-glass thermometers are constrained by the freezing point of mercury and are therefore unsuitable without special modifications.

Given Data / Assumptions:

• Temperature to be measured is below 0 °C.
• Standard, unpressurized mercury thermometers.
• Standard lab alternatives such as platinum resistance thermometers (PRTs), vapor-pressure thermometers, and non-contact methods are available.

Concept / Approach:
Mercury freezes at about −38.8 °C, and even near 0 to −10 °C mercury thermometers can suffer from column separation, wetting issues, and slow response. For reliable sub-zero measurements, PRTs (e.g., Pt100) remain accurate down to −200 °C; vapor-pressure thermometers with appropriate fluids function below 0 °C; radiation pyrometers can read cold surfaces if emissivity is known, though they are less common for low temperatures due to low radiance.

Step-by-Step Solution:

Verification / Alternative check:
Instrument datasheets confirm PRT operation to deep sub-zero temperatures; many vapor-pressure devices use propane or other fluids for low ranges.

Why Other Options Are Wrong:

Common Pitfalls:
Ignoring self-heating in PRTs or emissivity errors in pyrometry; always calibrate and account for environmental factors.

Final Answer:
Mercury-in-glass thermometer