Editing Density (section)

== Measurement ==
A number of techniques as well as standards exist for the measurement of density of materials. Such techniques include the use of a hydrometer (a buoyancy method for liquids), Hydrostatic balance (a buoyancy method for liquids and solids), immersed body method (a buoyancy method for liquids), pycnometer (liquids and solids), air comparison pycnometer (solids), oscillating densitometer (liquids), as well as pour and tap (solids).<ref>{{cite journal |title=Test No. 109: Density of Liquids and Solids  |journal=OECD Guidelines for the Testing of Chemicals, Section 1 |volume= |issue= |pages=6 |date=2 October 2012 |doi=10.1787/9789264123298-en |isbn=9789264123298 |url= |issn=2074-5753 }}</ref> However, each individual method or technique measures different types of density (e.g. bulk density, skeletal density, etc.), and therefore it is necessary to have an understanding of the type of density  being measured as well as the type of material in question.

=== Homogeneous materials ===
The density at all points of a [[Homogeneous (chemistry)|homogeneous]] object equals its total [[mass]] divided by its total volume. The mass is normally measured with a [[weighing scale|scale or balance]]; the volume may be measured directly (from the geometry of the object) or by the displacement of a fluid. To determine the density of a liquid or a gas, a [[hydrometer]], a [[dasymeter]] or a [[Coriolis flow meter]] may be used, respectively. Similarly, [[hydrostatic weighing]] uses the displacement of water due to a submerged object to determine the density of the object.

=== Heterogeneous materials ===
If the body is not homogeneous, then its density varies between different regions of the object. In that case the density around any given location is determined by calculating the density of a small volume around that location. In the limit of an infinitesimal volume the density of an inhomogeneous object at a point becomes: <math>\rho(\vec{r}) = dm / dV</math>, where <math>dV</math> is an elementary volume at position <math>\vec r</math>. The mass of the body then can be expressed as
<math display="block"> m = \int_V \rho(\vec{r})\,dV. </math>

=== Non-compact materials ===
{{further|Bulk density|Particle mass density}}

In practice, bulk materials such as sugar, sand, or snow contain voids. Many materials exist in nature as flakes, pellets, or granules.

Voids are regions which contain something other than the considered material.  Commonly the void is air, but it could also be vacuum,  liquid, solid, or a different gas or gaseous mixture.

The ''[[bulk volume]]'' of a material —inclusive of the [[void space fraction]]— is often obtained by a simple measurement (e.g. with a calibrated measuring cup) or geometrically from known dimensions.

Mass divided by bulk volume determines ''[[bulk density]]''.  This is not the same thing as the material volumetric mass density.
To determine the material volumetric mass density, one must first discount the volume of the void fraction. Sometimes this can be determined by geometrical reasoning.  For the [[close-packing of equal spheres]] the non-void fraction can be at most about 74%.  It can also be determined empirically.  Some bulk materials, however, such as sand, have a ''variable'' void fraction which depends on how the material is agitated or poured. It might be loose or compact, with more or less air space depending on handling.

In practice, the void fraction is not necessarily air, or even gaseous.  In the case of sand, it could be water, which can be advantageous for measurement as the void fraction for sand saturated in water—once any air bubbles are thoroughly driven out—is potentially more consistent than dry sand measured with an air void.

In the case of non-compact materials, one must also take care in determining the mass of the material sample. If the material is under pressure (commonly ambient air pressure at the earth's surface) the determination of mass from a measured sample weight might need to account for buoyancy effects due to the density of the void constituent, depending on how the measurement was conducted. In the case of dry sand, sand is so much denser than air that the buoyancy effect is commonly neglected (less than one part in one thousand).

Mass change upon displacing one void material with another while maintaining constant volume can be used to estimate the void fraction, if the difference in density of the two voids materials is reliably known.