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, calipers), volume of irregular shapes measured by powder pycnometry 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.
Common applications: cement, pharmaceutical powders, catalysts, dairy powder, ceramics, metallic powders, nanocomposites
- 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.
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