Atech Center - Hydrometer Procedure

6. Readings should be taken and recorded at the following times after the start of the test: 2 minutes, 4 minutes, 15 minutes, 30 minutes, 60 minutes, 250 minutes, and 1440 minutes. After each reading the hydrometer should be carefully removed from the sample and placed with a spinning motion into clear water in order to clean the hydrometer stem and bulb. Alternately, clean the hydrometer stem and bulb using a spray bottle (Figure 13).

Figure 13. Clean the hydrometer stem and bulb between uses.

The number of readings taken and the length of time necessary to take those reading will depend upon the specific gravity of the particles in the sample and the requirements for detrmination of the minimum particle size. Samples which necessitate a determination down to the 0.001 mm particle size. Readings are technically possible down to the 0.00024 mm size, below which partcles are colloidal (smaller than clay) and will remain in suspension indefinitely. Practically, readings below the 0.001 mm size using this procedure are not normally taken and are not usually required by specifications.

7. At the end of the hydrometer analysis, empty and thoroughly wash the contents of the sedimentation cylinder over a 0.075 mm (#200) sieve (Figure 14). Dry the retained material in an oven maintained at 110 ± 5 degrees C (230 ± 9 degrees F) to a constant mass in order to run a sieve analysis on materials passing the #10 and retained on the #200 sieve. Determination of particle percentages on particular sieve sizes should be specified by the specifying agency.