Hydrometer test in soil classification: Which corrections are typically applied to hydrometer readings in a sedimentation analysis of fine-grained soils?

Introduction / Context:
Hydrometer analysis estimates the particle-size distribution of fine soils (typically silt and clay sizes) by measuring suspension density at depth versus time. Accurate interpretation requires several standard corrections to the hydrometer readings before converting to equivalent particle diameter using sedimentation theory.

Given Data / Assumptions:

• Hydrometer measures relative density of suspension at a reference depth.
• Readings are influenced by instrument geometry and test conditions.
• Chemical dispersant is used to prevent flocculation.

Concept / Approach:
Three key corrections are usually necessary: (1) Temperature correction to account for water viscosity and density changes; (2) Meniscus correction because reading is taken at the top of the meniscus in opaque suspensions; (3) Dispersing agent correction to subtract the effect of the solution’s specific gravity from the reading. Applying all three yields corrected hydrometer values for calculating particle diameters and percentages finer.

Step-by-Step Solution:

Verification / Alternative check:
Perform a blank test (water + dispersant) to quantify the dispersant effect; compare viscosity–temperature tables to confirm applied temperature corrections.

Why Other Options Are Wrong:
Any subset omits at least one required correction, leading to biased results.

Common Pitfalls:
Ignoring flocculation (insufficient dispersant or agitation); not maintaining isothermal conditions; misreading the meniscus in turbid suspensions.

Final Answer:
Temperature, meniscus, and dispersing agent corrections