Determining the Specific Gravity of Soil - Soil Properties: Testing, Measurement, and Evaluation - page 59

1. The pycnometer must be cleaned and dried, and its mass ( M p ) deter-
mined and recorded.

CALIBRATION
OF PYCNOMETER

2. The pycnometer is then filled with water that is approximately at
room temperature to the pycnometer calibration mark, and the mass
of the pycnometer plus water at the calibration temperature ( M pw,c )
is accurately determined and recorded.

3. The water temperature at calibration ( T c ) must be determined (and
recorded) to the nearest 0.1°C (0.2°F) by inserting a thermometer in
the water.

4. Compute the calibrated volume of pycnometer, V p , using the follow-
ing equation:

M pw , c M p
r w , c

V p (6-2)

where:
M pw,c mass of pycnometer and water at calibration tempera-
ture, g
M p mass of dry pycnometer at calibration, g
r w, c mass density of water at calibration temperature
(see Table 6-1).

TEST
SPECIMEN [1]

(1) The test specimen may be moist or oven-dry soil and shall be
representative of the soil solids that passes the U.S. Standard
No. 4 sieve in the total sample. Table 6-2 gives guidelines on rec-
ommended dry soil mass versus soil type and pycnometer size.

(1.1) Two important factors concerning the amount of soil solids
being tested are as follows. First, the mass of the soil solids divided
by its specific gravity will yield four-significant digits. Secondly, the
mixture of soil solids and water is a slurry not a highly viscous fluid
(thick paint) during the deairing process.

PROCEDURE The initial step of the procedure is to put the soil sample in a pycnome-

ter, which is then filled with water, taking care to eliminate air bubbles
(by methods described in detail shortly). The next step is to measure the
temperature of the soil-and-water mixture and determine the mass of
the pycnometer when filled with water and soil, at the test temperature,
M pws,t . If samples containing natural moisture content are used, the soil-
and-water mixture must be poured onto an evaporating dish and then
dried in an oven to determine the mass of solids, M s . With the tempera-
ture of the soil-and-water mixture known, the mass of the pycnometer
when filled with water at the test temperature, M pw,t can be found from
the calibration of the pycnometer. With these data now known, the spe-
cific gravity of soil can be computed by dividing the mass of solids, M s ,
by the mass of an equal volume of water, M pw , t 1 M pws , t M s 2 .