 Density
Testing
Density by Variable Density Gradient Column – ASTM D 1505
ASTM D 1505 uses a graduated glass column which is charged with two solutions, such that one is greater than and one less than the density of the material to be tested. The liquids are charged into the column by slowly adding varying amounts of each using a special charging system. The liquid at the bottom of the column is of the greater density and, as the column is filled, the density decreases until the top of the column contains only the lighter density liquid. The density gradient will be uniform through the length of the column. Standard glass spheres of known, but varying density are introduced into the column along with the specimens using a charging basket. This basket is slowly lowered through the column without disturbing the density gradient distributing the glass spheres and specimens according to their relative densities. Using the glass spheres as reference points, the density at the locations where specimens are neutrally buoyant is calculated by interpolation. ASTM D 1505 is probably the most sensitive, but also the most painstaking and expensive. Values to five decimal places are obtained. This method can be extended to high density materials if suitably dense liquids can be selected.
Reference: ASTM D 1505 Standard Test Method for Density of Plastics by the Density-Gradient Technique
A number of other methods for the measurement of density, including
the following:
Density by the Water Displacement
(ASTM C 693)
Density by water displacement (Archimedean density) since liquids
other than water can be used for materials with densities less than
1.0 grams/cm3. This method allows determination of the density in
air compared to its displacement in water or other liquid of known
density. Depending upon the nature of the specimen (e.g., open or
closed cell), the resultant value may deviate from the true mass.
A clean specimen is weighed accurately in air using a laboratory
balance. The same specimen is weighed while suspended in water or
other liquid of such density that the specimen will sink. Deducting
the mass of the suspension wire from the weight in liquid, the volume
of the specimen is calculated from the effect of displacement by
a liquid of known density (Archimedean principle). This allows the
determination of density of specimens with irregular shapes, uneven
surfaces, or porosity. Caution must be exercised to assure that
no air is trapped within the specimen. Placing the specimen in a
vacuum while submerged in the displacement liquid will usually avoid
error.
Dry Bulk Density (ASTM C 559,
C 838, C 830)
Several ASTM methods describe procedures for determining dry bulk
density. All use basically the same procedure where the apparent
volume of a regular shape (usually a rectanguloid) is determined
by measurement of its dimensions. The mass is determined using a
laboratory balance and the density calculated as: weight (mass)/unit
volume. Dry bulk density is used in the calculation of thermal conductivity
per ASTM C 177.
Pore Volume and Pore Size Distribution
by Mercury Intrusion
Non-wetting fluids, such as mercury can be forced into pores within
a solid material by application of pressure. The pressure required
is inversely proportional to the diameter of the pore or capillary
through which the mercury must pass. In this method, the total pore
volume and the distribution of pore sizes of a solid sample is determined
by incrementally increasing the pressure of a mercury column above
the sample and measuring the change in mercury height. Pressures
as high as 33,000 psi can be employed to examine pores from 360
(m down to 0.0055 (m in diameter. Total intrusion volume, total
pore surface area, mean and median pore diameter, material bulk
density, apparent (skeletal) density, and percent porosity can be
determined by this method for a variety of solid materials.
Sink Float Method (ASTM C 729-75)
This method is a variation on the density by displacement method
which incorporates a system to heat the liquid support medium such
that small incremental changes in density may be effected. It is
about as accurate as any other method for material in the density
range from 1.1 to 3.3 g/cm3, and quite convenient. The range may
be extended by using liquids other than water, such as maleate formate
(approximately to 5.0 g/cm3), but at the expense of accuracy. Multiple
samples of different densities can be measured using the same test
cycle if multiple test vessels can be placed within the heating
medium. Samples are placed in a test vessel which is, in turn, heated
in a water or mineral oil bath. As the liquid medium warms, its
density decreases. When the test specimen floats, the temperature
is recorded and the density value is calculated by comparison to
standard glass floats.
Specific Gravity of Liquids
using the Westphal Balance
This method uses a balance equipped with a gladd float and a sliding
weight. A sample of the unknown liquid is placed in a test cylinder
and the difference between the weight of the standard glass float
in air and in the liquid is used to calculate the specific gravity
of the liquid. This method is not as accurate as the pyncnometer
method, but is a convenient, quick way to determine specific gravity.
Accuracy is limited to the accuracy of the balance float and weight
and will typically be three decimal places.
True and Apparent Density by
Helium Pycnometry
This method provided a more rapid and more accurate measurement
of true densities that does the Archimedean technique. Ceramics,
metals, slurries, pastes, pigments, plastics, mineral matter, building
materials, and other materials can be examined by this method, which
has density resolution of 0.0001 g/cc. Helium, under precisely-known
pressure, is used to fill small voids within a specimen. The volume
change of helium in a constant volume chamber allows determination
of solid volume. The ratio of sample mass to its true (sold) volume
gives its true density. The volume of porosity open to the sample
surfaces can also be measured, allowing determination of apparent
(skeletal) density or analysis of open-cell foams. Sample volumes
of up to 100 cc can be examined and the data collected in minutes
through a RS-232 computer interface.
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