Upper temperature range of mercury-in-glass thermometers: up to approximately what temperature (°C) can a standard pressurized mercury thermometer be used?

Introduction / Context:
Every temperature sensor has a practical operating range determined by materials, fluid properties, and construction. Mercury-in-glass thermometers are accurate and stable but limited by mercury vapor pressure, glass softening, and safety considerations.

Given Data / Assumptions:

• Standard laboratory mercury thermometer with nitrogen overpressure and suitable glass.
• No special fused-silica or gallium substitution assumed.

Concept / Approach:
Standard mercury-in-glass thermometers are typically serviceable up to roughly 350 °C. Above this, mercury’s vapor pressure grows significantly, and the glass envelope and sealing become limiting; readability and stability degrade, and safety concerns increase. Lower-range versions (e.g., 0–100 °C) are constrained by scale, not by physics; higher-range measurements are better handled by thermocouples or radiation pyrometers.

Step-by-Step Solution:
Consider fluid properties: mercury boiling point is 357 °C at 1 atm, but practical upper limit is slightly lower.Account for construction improvements (nitrogen fill) to extend range modestly.Adopt standard catalog value ~350 °C as practical limit.

Verification / Alternative check:
Manufacturer datasheets commonly list maximum scales near 300–360 °C depending on glass and pressurization, supporting 350 °C as a representative cap.

Why Other Options Are Wrong:
100 °C or 250 °C: Understate the achievable range of pressure-filled mercury designs.750 °C or 1200 °C: Far exceed mercury/glass limits; such temperatures require thermocouples or pyrometers.

Common Pitfalls:
Confusing the boiling point with the usable range; “usable” is set by accuracy and mechanical integrity, not just the fluid’s phase change temperature.

Final Answer:
350 °C