Constant-volume gas thermometers for low temperatures: A constant-volume gas thermometer designed for measuring sub-zero (below 0°C) temperatures commonly employs which working gas?

Introduction / Context:
Constant-volume gas thermometers exploit the near-ideal relationship P ∝ T for a fixed amount of gas at fixed volume. Different gases are used to cover different temperature spans. Traditional laboratory practice assigns hydrogen to low temperatures below 0°C, while nitrogen or air serve near room and higher ranges, and helium covers cryogenic extremes.

Given Data / Assumptions:

• Goal: accurate measurement below the ice point (0°C).
• Gas remains in single phase over the operating span.
• Simple, classical assignment of gases by range is intended.

Concept / Approach:
Hydrogen has favorable thermophysical properties and a low condensation temperature at atmospheric pressure, allowing a usable sub-zero range without liquefaction under typical thermometer conditions. It was historically adopted for below-zero calibration segments. Helium is excellent at very low cryogenic temperatures, but standard exam convention usually associates “sub-zero” (not cryogenic) with hydrogen.

Step-by-Step Solution:

Verification / Alternative check:
Thermometry texts tabulate recommended gases by range: hydrogen for low temperatures, nitrogen for moderate to high, helium for the lowest cryogenic spans.

Why Other Options Are Wrong:

• Helium: More typical for deep cryogenics; not the classic choice for routine sub-zero measurements.
• Nitrogen and CO2: Condense or solidify within sub-zero spans under certain pressures, complicating calibration.
• None of these: Incorrect because hydrogen is a well-established choice.

Common Pitfalls:
Confusing “sub-zero” with “cryogenic.” Sub-zero refers to temperatures just below 0°C through tens of degrees; helium is reserved for much colder regimes.

Final Answer:
Hydrogen