Document Number: 117
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Balling, or percent of pure sucrose
(sugar) by weight.
A hydrometer is an instrument that is used to
determine specific gravity. It operates based on
the Archimedes principle that a solid body
displaces its own weight within a liquid in which
it floats. Hydrometers can be divided into two
general classes: liquids heavier than water
and liquids lighter than water. The standard
hydrometer scale is known as the specific
gravity scale in which distilled water equals
1.000, the initial point of measurement. Liquids
lighter than water are scaled below 1.000
specific gravity and liquids heavier than water
are scaled above 1.000 specific gravity.
Function, Scale Types and Use
The hydrometer is a thin glass tube sealed at
both ends and has a graduated or printed scale
calibrated to a specific gravity. One end of the
tube is bulb shaped and weighted with fine lead
shot or mercury. The lead shot or mercury
causes the instrument to float upright in a liquid
like a fishing bobber. A second glass tube,
commonly known as a hydrometer jar, is filled
with a liquid being measured. The hydrometer
is then placed in the hydrometer jar containing
the sample liquid. The specific gravity of the
sample liquid is indicated when the level of the
sample liquid in the jar aligns with a point on
the the hydrometer scale. Depending on which
scale is used, the number of times heavier or
lighter than water the sample liquid weighs can
now be recorded. The scale on the hydrometer
can be calibrated to Baume, Brix (or Balling), or
Brix or Balling Scale: A hydrometer
calibrated to read in degrees of Brix or
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Baume Scale: A hydrometer which is
calibrated to read degrees of Baume, or
percent of NaCl (salt) by weight.
Alcohol Scale: A standard “specific
gravity” hydrometer that is used to
measure specific gravity before and
aftera liquid fermented. The difference
of thetwo specific gravity readings is
referenced to an alcohol scale to
determine percent alcohol by weight.
All of these scales can also be converted back
to specific gravity using a formula. Some
hydrometers have one or more scales printed
on the hydrometer. To use the hydrometer, fill
the hydrometer jar with the sample liquid. Place
the hydrometer in the jar and give it a quick twirl
to dislodge any air bubbles. Once the
hydrometer has settled, take the reading from
the appropriate scale. In order for the
measurement to be accurate, the sample liquid
must be at 60°F. If the liquid is not at 60°F,
readings may not be accurate and the
measurement should be adjusted.
Method of Reading
When reading transparent liquids, the eye
should be placed slightly below the plane of the
surface of the liquid and then raised slowly until
this surface, seen as an ellipse, appears as a
straight line. The point at which the line sits on
the hydrometer scale should be recorded as the
reading of the hydrometer. When a liquid is not
sufficiently clear—as to allow the reading to be
made as described above—it will be necessary
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Document Number: 117
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to read from above the surface and estimate as
accurately as possible, the point to which the
liquid rises on the hydrometer.
+\GURPHWHUDccuracy depends on three main factors . . .
Cleanliness: The hydrometer,
hydrometer jar and the liquid in which
the readings are taken should be
cleaned properly—especially the surface
of the hydrometer and the stem—so that
the liquid can rise uniformly, merging
into an almost invisible film on the stem.
Temperature: The hydrometer and liquid
should be the same temperature of the
surrounding atmosphere. This will
prevent changes in density during the
Proper Immersion: A hydrometer jar
should have an inside diameter of
approximately 1 (25mm) greater
than the outside diameter of the
Commonly Asked Questions
Q. How do I convert degrees Baume (salt
scale) to a specific gravity reading?
A. At 60°F, specific gravity can be calculated
by using the following formulas:
Liquids lighter than water: Specific gravity
= 140 / (degrees Baume + 130)
Liquids heavier than water: Specific gravity
= 145 / (145 ± degrees Baume)
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