Extending the upper range of a mercury-in-glass thermometer: a normal instrument reads to ~300 °C. How can the measurable range be increased to about 500 °C without changing the sensing fluid?

Introduction / Context:Mercury-in-glass thermometers are widely used due to mercury’s repeatable volumetric expansion and visibility. However, mercury boils at ~357 °C at 1 atm; practical glass constraints normally limit use to about 300 °C. Industry extends the range by modifying the gas space above the mercury column.

Given Data / Assumptions:

• Working fluid remains mercury.
• Target upper range ~500 °C.
• We may alter the construction of the stem/bulb but not the sensing fluid.

Concept / Approach:Filling the thermometer stem with an inert gas (e.g., nitrogen) at high pressure raises the boiling point of mercury and suppresses cavitation and separation of the mercury column. The pressurized nitrogen exerts counterpressure, allowing the device to operate safely to higher temperatures with proper glass selection.

Step-by-Step Solution:Recognize limitation: mercury boiling and column separation near upper range.Apply solution: backfill the stem with pressurized nitrogen to increase allowable temperature.Confirm practicality: this is standard practice for “nitrogen-filled” high-range mercury thermometers.

Verification / Alternative check:Catalogs list “nitrogen-filled mercury thermometers” rated to approximately 500 °C, demonstrating the effectiveness of the method.

Why Other Options Are Wrong:Larger diameter: Affects sensitivity/readability, not boiling/column separation limits.Steel tube: Mercury-in-glass relies on glass; steel is unsuitable and introduces different issues.Expansion correction: Computational correction does not extend physical limits.Alcohol fill: Alcohol thermometers cover low temperatures, not 500 °C.

Common Pitfalls:Confusing scale corrections with true range extension; underestimating the role of back pressure.

Final Answer:Fill the stem with nitrogen under pressure (gas-filled headspace).