Grain-Size Analysis of Soil (Including Both Mechanical and Hydrometer Analyses) - Soil Properties: Testing, Measurement, and Evaluation - page 122

The portion of the original sample that passed through the No. 10 sieve
is tested for grain-size distribution using a hydrometer and later by
sieve analysis.

PROCEDURE FOR
HYDROMETER
ANALYSIS OF
PORTION PASSING
THROUGH NO. 10
SIEVE

Determination of Composite Correction
for the Hydrometer Reading

Prior to performing a hydrometer test, a “composite correction” for hy-
drometer readings must be determined to correct for three items that
tend to produce errors in hydrometer analysis. The first of these items
needing correction results from the fact that a dispersing agent is used
in the water, and this agent increases the specific gravity of the result-
ing liquid. The second is the effect of variation of temperature of the liq-
uid from the hydrometer calibration temperature, and the third results
from the fact that it is not possible to read the bottom of the meniscus
when it is in a soil suspension. Composite corrections can be determined
experimentally. Inasmuch as hydrometer analyses are performed often,
it is convenient to prepare a graph or table giving composite correction
as a function of temperature for a given hydrometer and dispersing
agent. For example, composite corrections can be determined for two
temperatures spanning the range of anticipated test temperatures, and
a straight-line relationship may be assumed for determining composite
corrections at any temperatures.
The procedure for determining a composite correction is as follows
(ASTM D 422-63 (Reapproved 2002) [1]):

Prepare 1,000 mL of liquid composed of distilled or demineralized
water and dispersing agent in the same proportion as will prevail
in the sedimentation (hydrometer) test. Place the liquid in a sedi-
mentation cylinder and the cylinder in the constant-temperature
water bath, set for one of the two temperatures to be used. When
the temperature of the liquid becomes constant, insert the hy-
drometer and, after a short interval to permit the hydrometer to
come to the temperature of the liquid, read the hydrometer at the
top of the meniscus formed on the stem. For hydrometer 151H the
composite correction is the difference between this reading and
one; for hydrometer 152H it is the difference between the reading
and zero. Bring the liquid and the hydrometer to the other temper-
ature to be used, and secure the composite correction as before.

Hygroscopic Moisture

Another preliminary task is to determine the hygroscopic moisture
correction factor. This is required because in the subsequent hydro-
meter analysis, the oven-dried mass of the soil used in the hydrometer
test is needed; however, the soil actually used in the hydrometer test
is air-dried soil and is not available for determining oven-dried mass.
To determine the correction factor, a portion of the air-dried sample
(from 10 to 15 g) is placed in a container, weighed, oven-dried at 230 ±
9°F (110 ± 5°C), and then weighed again. The hygroscopic moisture