Sedimentation Analysis – How W_d per mL of Suspension is Obtained In fine-grained soil particle-size analysis, identify how the weight of dry solids per millilitre of suspension (W_d per mL) is determined in common laboratory methods.

Introduction / Context:
Hydrometer and pipette analyses are classical methods for determining the distribution of fine particles in soils using sedimentation. A common point of confusion is how the weight of solids per unit volume of suspension is actually determined in each approach.

Given Data / Assumptions:

• Standard laboratory procedures for hydrometer and pipette analyses.
• Well-dispersed suspension, steady temperature control.
• Use of Stokes-based timing to sample at appropriate depths.

Concept / Approach:

In hydrometer analysis, readings reflect suspension density change, which is related indirectly to solids concentration through calibration. Thus W_d per mL is derived indirectly from hydrometer readings and correction charts. In pipette analysis, a known volume of suspension is withdrawn from a known depth at a specific time; this sample is oven-dried to obtain the dry mass of solids directly. Dividing by the sampled volume yields W_d per mL directly without relying on hydrometer calibration curves.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory manuals consistently describe pipette dried-residue measurements as direct quantification of solids per sampled volume.

Why Other Options Are Wrong:

(a) is false; hydrometer is indirect. (b) misstates pipette as indirect. (e) incorrectly claims both are direct.

Common Pitfalls:

Ignoring temperature and meniscus corrections in hydrometer method; not accounting for exact sampled volume in pipette analysis.

Final Answer:

Pipette analysis: W_d per mL is obtained directly