D1298_9 (12b)
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Bring the sample to
the test temperature,
which shall be such that the
sample is sufficiently fluid,
but not so high as to cause
the loss of light compounds,
nor so low as to result in
the appearance of waxy solids
in the test portion.
The density, relative
density, or API gravity
determined by the
hydrometer is most accurate
at or near the
reference temperature.
The volume and density,
the relative density,
and the API corrections in
the volume correction tables
are based on the average
expansions of a number
of typical materials.
Since the same
coefficients were used
in compiling each set
of tables, corrections
made over the same
temperature interval
minimize errors arising
from possible differences
between the coefficient
of the material
under test and the
standard coefficients.
This effect becomes
more important
as temperatures diverge from
the reference temperature.
The hydrometer
reading is obtained
at a temperature appropriate
to the physiochemical
characteristics of the
material under test.
This temperature is preferably
close to the reference
temperature, or when the
value is used in conjunction
with bulk oil measurements
within 3 degrees
Celsius of the bulk temperature.
See section 5.3.
For crude petroleum,
bring the sample
close to the
reference temperature,
or, if wax is
present, to 9 degrees
Celsius above it's expected
pour point, or 3 degrees
Celsius above its
expected cloud point,
or WAT, whichever is higher.
For crude petroleum, an
indication of the WAT
can be found using IP 389
with the modification of using
50 microliters plus or minus
5 microliters of sample.
The precision of WAT for crude
petroleum using this technique
has not been determined.
Bring the hydrometer
cylinder and thermometer
to within
approximately 5 degrees
Celsius of the test temperature.
Transfer the sample to the
clean, temperature-stabilized
hydrometer cylinder
without splashing
to avoid the formation
of air bubbles
and minimize evaporation
of the lower boiling point
constituents of more
volatile samples.
Warning-- extremely flammable.
Vapors may cause flash fire.
Transfer highly volatile
samples by siphoning or water
displacement.
Warning-- do not
siphon by mouth,
as it could result in
ingestion of sample.
Samples containing
alcohol or other water
soluble materials should
be placed into the cylinder
by siphoning.
Remove any air bubbles formed
after they have collected
on the surface of
the test portion
by touching them with
a piece of clean filter
paper or other suitable
means before inserting
the hydrometer.
Place the cylinder
containing the test portion
in a vertical position in a
location free from air currents
and where the temperature
of the surrounding medium
does not change
more than 2 degrees
Celsius during the time
taken to complete the test.
When the temperature
of the test portion
differs by more than 2
degrees Celsius from ambient,
use a constant temperature bath
to maintain an even temperature
throughout the test duration.
Insert the appropriate
thermometer, or temperature
measuring device, and
stir the test portion
with a stirring rod
using a combination
of vertical and
rotational motions
to ensure uniform
temperature and density
throughout the
hydrometer cylinder.
Record the temperature
of the sample
to the nearest 0.1
degree Celsius,
and remove the
thermometer or temperature
measuring device
and stirring rod
from the hydrometer cylinder.
If a liquid and glass
thermometer is used,
this is commonly used
as the stirring rod.
Lower the appropriate
hydrometer into the liquid,
and release when in a
position of equilibrium,
taking care to avoid wetting the
stem above the level at which
it floats freely.
For low viscosity transparent
or translucent liquids,
observe the meniscus shape
when the hydrometer is
pressed below the point of
equilibrium about 1 millimeter
to 2 millimeters and allowed
to return to equilibrium.
If the meniscus changes,
clean the hydrometer stem
and repeat until the meniscus
shape remains constant.
For opaque viscous liquids,
allow the hydrometer
to settle slowly
into the liquid.
For low viscosity transparent
or translucent liquids,
depress the hydrometer about two
scale divisions into the liquid
and then release it, imparting
a slight spin to the hydrometer
on release to assist
in bringing it to rest,
floating freely from the walls
of the hydrometer cylinder.
Ensure that the remainder
of the hydrometer stem,
which is above the liquid
level, is not wetted, as liquid
on the stem affects
the reading obtained.
Allow sufficient time
for the hydrometer
to come to rest, and for all air
bubbles to come to the surface.
Remove any air bubbles
before taking a reading.
See section 9.2.12.
If the hydrometer cylinder
is made of plastic,
dissipate any static charges
by wiping the outside
with a damp cloth.
Static charges often build
up on plastic cylinders
and may prevent the hydrometer
from floating freely.
When the hydrometer
has come to rest,
floating freely away from
the walls of the cylinder,
read the hydrometer
scale reading
to the nearest 1/5 or 1/10
of a full scale division,
depending upon the
number of subdivisions,
that is 5 or 10 respectively,
that exist between a full scale
division in accordance with
section 9.2.12.1 or 9.2.12.2.
For transparent liquids,
record the hydrometer reading
as the point on the
hydrometer scale
at which the principal
surface of the liquid
cuts the scale by
placing the eye slightly
below the level of the
liquid and slowly raising it
until the surface, first
seen as a distorted ellipse,
appears to become a straight
line cutting the hydrometer
scale.
See figure 1.
For opaque liquids, record
the hydrometer reading
at the point on the hydrometer
scale to which the sample rises
above its main surface by
observing with the eyes
slightly above the plane of
the surface of the liquid.
See figure 2.
This reading requires
a meniscus correction.
This correction
can be determined
by one of two methods.
One, observe the height
above the main surface
of the liquid to which the
sample rises on the hydrometer
scale when the
hydrometer in question
is immersed in a transparent
liquid having a surface
tension similar to that
of a sample under test.
Or two, uses the nominal
value shown in Table 1.
Document this value as
the meniscus correction.
When testing opaque liquids
using a metal hydrometer
cylinder, accurate readings
of the hydrometer scale
can only be ensured if
the liquid's surface is
within five millimeters of
the top of the cylinder.
Immediately after recording
the hydrometer scale reading,
carefully lift the
hydrometer out of the liquid.
Insert the thermometer or
temperature measuring device,
and stir the test
portion vertically
with the stirring rod.
Record the temperature
of the test portion
to the nearest 0.1
degrees Celsius.
If this temperature differs
from the previous reading--
see section 9.2.6-- by more
than 0.05 degrees Celsius,
repeat the hydrometer
observations and thermometer
observations until the
temperature becomes stable,
within 0.05 degrees Celsius.
If a stable temperature
cannot be obtained,
place the hydrometer cylinder
in a constant temperature bath
and repeat the procedure
from section 9.1.
If the test temperature is
higher than 38 degrees Celsius,
allow all hydrometers of
the lead, shot, and wax type
to drain and cool in
a vertical position.