Hydrometers
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Hydrometers
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The hydrometer is an instrument that is constructed on the principle
that a solid body displaces its own weight of the liquid in which
it floats. Hydrometers can be divided into two general classes;
namely, for liquids heavier than water and for liquids lighter
than water. The general standard hydrometer scale is known as
the "Specific Gravity Scale," in which distilled water equals
1.000 as the initial point. Liquids lighter than water are scaled
below 1.000 specific gravity and liquids heavier than water are
scaled above 1.000 specific gravity.
There are also other various scales used which are generally known
by the name of their originator such as: Baume, Balling, Brix,
etc., which are all convertible into specific gravity by formula.
Hydrometers are ususally calibrated at 60F/60F. To determine the
density of a liquid, the liquid should be 60F. If the temperature
varies, the liquid will either contract or expand, depending upon
the temperature. Therefore, the density fluctuates with the termperature.
Where there is a variation from the standard 60F, corrections
must be applied to the hydrometer reading. To assure proper corrections,
a separate accurate thermometer should be used, or a hydrometer
in combination with a thermometer (thermo-hydrometer).
The correct method of reading a hydrometer is illustrated in Figs.
1 and 2. Where the liquid is transparent, the eye should be placed
below the surface (Fig. 1), then raised slowly until the surface,
first seen as an ellipse, becomes a straight line (Fig. 2). The
point at which this line cuts the hydrometer scale should be taken
as the reading of the instrument.
If the liquid is not sufficiently clear for readings to be made
in this manner, read from above the surface and estimate as accurately
as possible, the point to which the liquid rises on the hydrometer
stem. Since hydrometers are calibrated to give correct indications
when read at the principal surface of the liquid, correct the
reading just taken at the upper edge of the meniscus by an amount
equal to this height above the principal surface of the liquid.
The amount of correction can be determined with sufficient accuracy
for most purposes by taking a few readings on the upper and lower
meniscus in a clear liquid of the same character as that being
tested and noting the differences.
Meniscus Corrections
In liquids lighter than water and read at the upper edge of the
meniscus, a specific-gravity hydrometer will read too low and
a Baume, A.P.I. (American Petroleum Institute), or percent alcohol
hydrometer will read too high. The correction for meniscus height
should, therefore, be added to a specific-gravity reading and
subtracted from a Baume, A.P.I., or percent alcohol reading. In
liquids heavier than water, a hydrometer will always read too
low so that the correction for meniscus height must be added.
The magnitude of the correction must always read too low so that
the correction for meniscus height must be added. The magnitude
of the correction must obviously depend on the length and value
of the scale subdivisions and must be determined for the particular
hydrometer.
Accuracy of Hydrometer Readings Depend on:
1. CLEANLINESS OF THE HYDROMETER, HYDROMETER JAR AND THE LIQUID IN
WHICH THE READINGS ARE TAKEN! For uniform and reproducible readings, the surface of the hydrometer
and especially of the stem must be clean so that the liquid will
rise uniformly and merge into an almost imperceptible film on
the stem. The readiness with which this condition is fulfilled
depends somewhat on the character of the liquid. Such liquids
as mineral oils and strong alcoholic mixtures readily adhere to
the stem. Weak aqueous solutions of sugar, salts, acids, and alcohol
require scrupulous cleaning of the hydrometer stem. Before a test
is made the hydrometer should be thoroughly washed, rinsed and
dried by wiping with a clean, lint-free cloth. The hydrometer
jar should be thoroughly washed and rinsed before the clean test
liquid is added.
2. TEMPERATURE: The hydrometer and liquid should be at nearly the temperature
of the surrounding atmosphere to prevent changes in density during
the observation as well a any doubt as to the actual temperature.
To insure uniformity of density and temperature, the liquid should
be completely stirred shortly before the observation is made.
Stirring is well accomplished with a perforated disk or spiral
on the end of a rod long enough to reach the bottom of the container.
Stirring from top to bottom disperses liquid layers of different
density. Readings should not be made until both liquid and hydrometer
are free of air bubbles and are at rest.
3. PROPER IMMERSION: Immerse the hydrometer slowly to a point slightly beyond that at which it floats naturally (not more than 1 or 2 scale graduations), then allow it to float freely.
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