Helium constant-volume gas thermometer: select the temperature range for which this primary standard instrument is suitable in practice, given helium’s stability at very high temperatures.

Introduction / Context:
Gas thermometers are primary temperature standards because the equation of state of gases is well understood. Constant-volume types infer temperature from pressure changes at fixed volume. Helium is particularly useful due to chemical inertness and stability at high temperatures, making it suitable for elevated-temperature calibration work.

Given Data / Assumptions:

• Working gas: helium (chemically inert, light, high thermal conductivity).
• Configuration: constant-volume gas thermometer (CVGT).
• Industrial/laboratory implementation with appropriate materials.

Concept / Approach:
With appropriate containment and materials, a helium CVGT can operate well above the ranges of liquid-in-glass or mercury thermometers, reaching very high temperatures for standards and calibration. While gas thermometers can also measure sub-zero temperatures, the question emphasizes suitability for “a temperature of … °C” and expects recognition of helium’s usability at very high temperatures, far beyond typical resistance thermometers’ upper limits.

Step-by-Step Solution:

Verification / Alternative check:
Reference practices in metrology employ noble-gas constant-volume thermometers for high-temperature points where other standard sensors face material limitations.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing general gas thermometry (broad range) with the question’s intent to highlight helium’s high-temperature capability.

Final Answer:
> 800 °C