User:Milton Beychok/Sandbox

(CC) Drawing: Milton Beychok
A schematic of a specific gravity hydrometer

A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water with both at the same temperature. It is usually made of glass and consists of a small diameter cylindrical stem and a larger diameter section weighted with mercury or lead shot (sealed with wax) to make it float upright. The larger diameter section is necessary to provide the displacement volume needed for the proper buoyancy of the hydrometer

The stem contains a rolled paper marked with the scale being used. There are a great many different hydrometer scales commonly used to measure liquid densities in: petroleum crude oil marketing and refining; making wine, brewing beer and making whiskey; producing sulfuric acid and other industrial chemicals; and refining sugar.

Scales

The many hydrometer scales that are commonly used include:

Specific gravity

This scale is widely used in chemistry, engineering and physics ... and is the one shown in the adjacent drawing. It is expressed as:

$\mathrm {SG} _{4\,^{\circ }\mathrm {C} }^{4\,^{\circ }\mathrm {C} }={\frac {\mathrm {density\;of\;the\;tested\;liquid\;at\;4\,^{\circ }\mathrm {C} } }{\mathrm {density\;of\;water\;at\;4\,^{\circ }\mathrm {C} } }}$

Unless the two reference temperatures are explicitly stated, they are generally taken to be 4 °C.

API gravity

This scale was developed by the American Petroleum Institute (API) in 1921 for use in the petroleum industry. It is expressed as:

$^{\circ }\mathrm {API} ={\frac {141.5}{\mathrm {SG} _{60\,^{\circ }\mathrm {F} }^{60\,^{\circ }\mathrm {F} }}}-\mathrm {131.5}$     and     $\mathrm {SG} _{60\,^{\circ }\mathrm {F} }^{60\,^{\circ }\mathrm {F} }={\frac {\mathrm {density\;of\;petroleum\;liquid\;at\;60\,^{\circ }\mathrm {F} } }{\mathrm {density\;of\;water\;at\;60\,^{\circ }\mathrm {F} } }}$

Note: 60 °F is equivalent to 15.56 °C.

Baumé gravity

These two scales, one for liquids lighter than water and one for liquids heavier than water, was developed by the French chemist Antoine Baumé in 1768 and they are expressed as:

$^{\circ }\mathrm {B{\acute {e}}} ={\frac {140}{\mathrm {SG} _{20\,^{\circ }\mathrm {C} }^{20\,^{\circ }\mathrm {C} }}}-\mathrm {130}$     (for liquids lighter than water) and     $\mathrm {SG} _{20\,^{\circ }\mathrm {C} }^{20\,^{\circ }\mathrm {C} }={\frac {\mathrm {density\;of\;the\;tested\;liquid\;at\;20\,^{\circ }\mathrm {C} } }{\mathrm {density\;of\;water\;at\;20\,^{\circ }\mathrm {C} } }}$

$^{\circ }\mathrm {B{\acute {e}}} =\mathrm {145} -{\frac {145}{\mathrm {SG} _{20\,^{\circ }\mathrm {C} }^{20\,^{\circ }\mathrm {C} }}}$     (for liquids heavier than water) and     $\mathrm {SG} _{20\,^{\circ }\mathrm {C} }^{20\,^{\circ }\mathrm {C} }={\frac {\mathrm {density\;of\;the\;tested\;liquid\;at\;20\,^{\circ }\mathrm {C} } }{\mathrm {density\;of\;water\;at\;20\,^{\circ }\mathrm {C} } }}$

History

Knowledge of relative density or specific gravity has been with us since the days of Archimedes in 250 BC, with the observation that light objects can float while heavier ones will sink in water. Availa Hypatia (born ca. 350 and died 415 AD), a Greek scholar from Alexandria in Egypt, is reputed to have invented the hydrometer.