Moisture content determination — which method gives the most accurate routine measurement of soil water content in laboratory practice?

Introduction / Context:
Moisture content w strongly affects compaction, strength, and compressibility. Several techniques exist to determine w, trading speed for accuracy. For quality control and index testing, the industry relies on a reference method that provides repeatable, unbiased measurements across soil types (with noted exceptions for organics and gypsum).

Given Data / Assumptions:

• Standard laboratory conditions available with thermostatically controlled oven.
• Typical mineral soils; special handling may be needed for organic/gypsiferous soils.
• Representative samples and airtight tins are used.

Concept / Approach:

The oven-drying method determines water content by removing pore water through evaporation at 105 to 110 degC until constant mass. The calculation is straightforward: w (%) = (M_wet - M_dry) / M_dry * 100. This direct gravimetric approach minimizes systematic error and is adopted in IS/ASTM standards as the reference for calibration of faster field methods.

Step-by-Step Solution:

Verification / Alternative check:

Replicate tests converge to the same w within small tolerance. Field rapid methods (calcium carbide) can be correlated but typically show greater scatter due to reaction completeness and sample representativeness.

Why Other Options Are Wrong:

Calcium carbide — fast but sensitive to fines and sealing; gives approximate values. Sand bath and alcohol methods can under/over-dry, leading to bias. Infrared lamp is quicker but less consistent without careful control.

Common Pitfalls:

Not drying to constant mass; leaving tins open to ambient humidity between weighings; overheating organic soils which decomposes solids and biases w high.

Final Answer:

oven-drying method