The Determination of Specific Gravity
Hydrometers are used to determine the density of a liquid. The two scales
most commonly used are specific gravity and Baumé. Both are linear number
scales. The specific gravity scale is measured from 0.600 to 2.000 in increments
of 0.005. The Baumé scale is measured in degrees from 1° to 101° with
increments of 0.1°.
Water at 60°F has a density of 1.000 and is the basis of all
liquid specific gravity measurements. Therefore, all liquids are measured as
being lighter or heavier than water. The density of a liquid can be altered by
either the dilution of the liquid or increasing the percentage of dissolved
oxygen.
A sample of the test liquid is placed in a cylinder and the
hydrometer is then floated in the liquid. The level at which it floats indicates
the specific gravity or Baumé.
For Accurate Hydrometer Readings
The hydrometer should be clean and dry. A clear, smooth cylinder should be
selected and should also be clean and dry, or rinsed with liquid identical with
the sample. Liquid of the sample must be thoroughly mixed down to the bottom.
Temperatures of the hydrometer and liquid tested should be equal, and as close
to that of the surrounding atmosphere as conditions allow. The hydrometer must
be the proper one for the liquid, since unlike surface tensions, different
liquids will affect readings.
How To Use The Hydrometer
First be sure all conditions outlined above have been met. Immerse the
hydrometer slowly in the liquid to a point slightly below the point where it
naturally floats. Allow all air bubbles to disappear. Bring the eye, preferably
from below, the level of the surface of the liquid and take the reading at that
point on the side points and center of the mercury column in the horizontal
line; the line of sight is then normal to the stem.