This Fun Fact Friday article will describe the procedure for hydrometer analysis of soils. This procedure measures the particle-size distribution of fine-grained soils and is one of the services offered by our Civil Testing Group to ensure the integrity of subject materials.

This test method begins with obtaining a representative sample of soil material from the construction site or desired area. The soil sample is passed through a sieve to ensure that it is the appropriate size for the test. This material is placed into a cylindrical container along with a fluid mixture to suspend the particles within the container. The mixture is then agitated using either an agitation machine or the tipping method, which involves placing a rubber stopper in the opening of the cylinder to prevent any leaks, and then turning the entire cylinder upside down and then back for a period of time.

After the sample mixture has been agitated, it is placed on a flat, stationary surface so that the contained material can be measured using a hydrometer. The hydrometer measures the position of the soil particles within the fluid suspension at a specific time. Hydrometer measurements should be taken and recorded at 1, 2, 4, 15, 30, 60, 240, and 1440 minutes.

The readings from the hydrometer are plotted to create a gradation curve, which sorts the particles by size in both time and position as they settle to the bottom of the container. According to Stokes’ Law, larger particles will fall through a fluid faster than smaller particles. Calculations are performed based on the gradation curve and other factors to determine the particle size distribution of the sample soil material.
The resulting particle size distribution calculation can be used to help determine other engineering properties at the construction site, such as hydraulic conductivity, compressibility, and shear strength.

If you need hydrometer analysis or other testing services, contact Civil Laboratory Supervisor Jeremy Lake at or (716) 592-3980 ext 133.