Density, True Density & Tapped Density

Different Methods of Density measurements

Density is defined as mass per unit volume.

Analysis of sample mass can be simply and accurately performed using a balance, but analysis of sample volume, particularly of powders or irregularly shaped materials can be difficult and can be perceived and measured in more than one way:

as bulk volume with corresponding bulk density - includes volume of solid phase, plus volume of inter-particular space and pores within particles
as apparent/envelope/geometric volume giving apparent/envelope density - includes volume of solid phase plus pores within the particle
as true/skeletal volume for true density measurement - includes volume of solid phase only and any closed pores within that are not open to the outside surface

Corresponding measuring methods are:

Bulk Density - volume determined by the use of graduated cylinder, the cylinder can be tapped giving a measure of the so called Tapped Density
Apparent/envelope Density - volume of large geometric shapes easy to measure (ruler, calippers), volume of irregular shapes measured by powder pycnometery using a free flowing powder, volume of small, irregular shapes measured by mercury displacement
True Density - For regular shapes this is easy to measure using an accurate balance and a ruler, for irregular or fractured shapes we employ the Archimedean principle of fluid/gas displacement to measure the volume. The gas pycnometry operating principle is Gas Law: PV = nRT Where P is the gas pressure, V is the volume containing the gas, n is the number of moles of the gas, R is the universal gas constant and T is the temperature of the gas. Introducing a sample into an analysis chamber of a known volume we determine the volume of a gas displaced by the sample and true volume of the analysed material. Most commonly used gas is helium but other gasses are also possible.

Applications:

True Density measurement is one of the simplest and most economical methods of Quality Control, the measurement takes few minutes and can be highly automated.
Pycnometric or True Density analysis combined with Mercury porosimetry can provide a measure of materials’ porosity.
Gas Pycnometry can be used to determine percentage of solids in the slurry
Specialist applications allow the determination of open and closed cell porosity of rigid and semi rigid foams.

For more information contact us.