Which size particles does the hydrometer analysis specifically measure?

• A. Fine sand and silt-sized particles
• B. Silts and clay-sized particles
• C. Coarse gravel particles
• D. Sand and silt-sized particles

Answer: (B)

The hydrometer analysis is a laboratory technique used to determine the particle size distribution of soil, particularly focusing on the finer fractions of the soil sample. This method is specifically designed to measure silts and clay-sized particles, which are typically too small to be analyzed effectively by standard sieve methods. During the hydrometer analysis, the soil particles are suspended in water and allowed to settle; the rate of sedimentation is then measured, providing a means to determine particle size based on Stokes' Law.

This approach is particularly suited for fine-grained soils because these particles settle slowly in water, allowing for accurate measurement of their sizes over time. Coarse gravel particles and larger sand grains settle much more rapidly and are not adequately captured by the hydrometer techniques, making this method unsuitable for analyzing larger particle sizes. Therefore, the focus on silts and clay-sized particles accurately reflects the capabilities and design of hydrometer analysis.