Filled-system expansion thermometers – lowest positive temperature capability Among the following filled-system thermometers, which type can measure the lowest (near-ambient or sub-zero) positive temperatures most effectively?

Introduction / Context:
Filled-system expansion thermometers use the volumetric expansion of a fluid to indicate temperature. The lower end of their measurable range is determined by the freezing point and expansion characteristics of the fill fluid. Choosing the right fill ensures reliable readings at low temperatures in laboratories and process environments (e.g., cold rooms, ambient outdoor monitoring).

Given Data / Assumptions:

• Options include common fills: mercury, alcohol, inert gas (nitrogen), and high-temperature liquid metals (Na/K) in steel systems.
• “Lowest positive temperature capability” asks which type functions best at temperatures just above 0 °C and below ambient.
• Accuracy and readability are within standard instrument specifications.

Concept / Approach:
Alcohol-in-glass thermometers remain fluid and responsive well below 0 °C; common alcohols (e.g., ethanol) have freezing points far below water, making them ideal for near-ambient and sub-zero service. Mercury freezes at about −38.8 °C; while that is low, alcohol thermometers typically offer better visibility at low temperatures and are preferred where very low positives and modest sub-zero ranges are expected. Fused metal fills target very high temperatures; nitrogen-filled systems cover broad ranges but are not optimized for the lowest positive temperatures compared to alcohol-in-glass instruments.

Step-by-Step Solution:

Verification / Alternative check:
Instrument catalogs list alcohol thermometers for low-temperature applications (often down to −70 °C), while mercury is recommended for higher-temperature precision.

Why Other Options Are Wrong:

• Mercury-in-glass: Usable, but not the best choice for the lowest positive/sub-zero range compared to alcohol.
• Fused metal (Na/K): Intended for high temperatures.
• Nitrogen in steel: Broad range but not optimized for the lowest positive end.

Common Pitfalls:
Confusing “lowest measurable” with “highest accuracy”; the former is dominated by the fill fluid’s physical limits.

Final Answer:
Alcohol-in-glass thermometer