Constant-volume gas thermometry below 0 °C: which fill gas is preferred for sub-zero temperature measurement to avoid condensation and ensure accuracy?

Introduction / Context:
Gas thermometers rely on the relationship between gas pressure/volume and temperature. For sub-zero measurements, the choice of fill gas is critical to avoid liquefaction and to maintain a nearly ideal-gas behavior over the range of interest. This question probes the practical selection of a fill gas for constant-volume thermometers operating below 0 °C.

Given Data / Assumptions:

• Temperature range: below 0 °C (e.g., −50 to 0 °C).
• Instrument: constant-volume gas thermometer.
• Goal: stable, non-condensing gas with good sensitivity.

Concept / Approach:
Hydrogen has a very low normal boiling point and remains gaseous at low temperatures encountered in typical sub-zero industrial measurements. It behaves close to ideal and provides adequate sensitivity to temperature changes. While helium also remains gaseous to extremely low temperatures, traditional constant-volume thermometer practice often specifies hydrogen for the sub-zero region, whereas nitrogen is common at moderate positive temperatures but can present larger non-ideality or risk of liquefaction at deeper cryogenic conditions.

Step-by-Step Solution:
Identify risk: gas must not liquefy within the operating range.Compare candidates: hydrogen and helium both remain gaseous; nitrogen is less favored for deep sub-zero.Select the conventional answer used in constant-volume gas thermometry for sub-zero: hydrogen.

Verification / Alternative check:
Handbook tables pair gas choices with ranges: nitrogen (around ambient and moderate highs), hydrogen (sub-zero), and helium (very low cryogenic work). The sub-zero selection commonly points to hydrogen as the standard fill gas.

Why Other Options Are Wrong:
Nitrogen: Widely used but less ideal for deeper sub-zero thermometry in this classical context.Helium: Technically suitable but more associated with extreme cryogenics and specialized laboratory setups.Argon/None: Argon is heavier and less favored; “none” ignores practical instrument design.

Common Pitfalls:
Assuming any inert gas suffices; overlooking boiling points and gas non-idealities at the targeted temperature and pressure.

Final Answer:
Hydrogen