Hydrometer Method
Definition
A sedimentation technique that uses Stokes' Law to determine the proportion of silt and clay from the rate at which particles settle through a suspension, measured by a hydrometer at timed intervals.
- Physical basis
- Stokes' Law settling velocity of particles in suspension
- Measures
- Silt and clay proportion of a soil sample
- Instrument
- ASTM 152H-type soil hydrometer
- Reading points
- Timed intervals, typically minutes through 24 hours
Common questions
Why is the hydrometer method paired with sieving rather than used alone?+
Sedimentation only resolves the fine fraction; sand and gravel settle too fast for hydrometer timing to distinguish, so a sieve stack handles the coarse fraction and the two results are combined into one particle size distribution.
What can go wrong with the hydrometer reading in forensic soil work?+
Poor dispersion of clumped clay particles, temperature drift affecting fluid viscosity, and reading the meniscus incorrectly are the main sources of error, so calibration blanks and consistent timing are essential.
Related terms
- Laser Diffraction
- An instrumental method where a dispersed sample passes through a laser beam and the angular diffraction pattern is inverted to produce a...
- Particle-Size Distribution (PSD)
- The proportion by mass of soil particles falling within each size fraction. Expressed as a curve or a table of percentages, and...
- Ribbon Test
- A field method for estimating texture by wetting a small amount of soil and extruding it between thumb and finger. The length...
- Sieve Analysis
- Mechanical separation of soil particles by passing a dried, dispersed sample through a stack of sieves with calibrated apertures from 2 mm...
- Textural Triangle
- A ternary diagram with sand, silt, and clay at the three apices. A point plotted from the measured percentages of those three...