Density measurement

Density is defined as the mass per unit volume of a material. While the volume of a solid can be determined by measuring its dimensions, many materials possess surface irregularities, fractures, and pores—some accessible from the surface and others enclosed within the structure. These features influence the overall volume and lead to various definitions of density:

True (or Absolute) density

The ratio of a sample's mass to its true volume, excluding the volume of open pores, closed (blind) pores, and inter-particle voids.

Skeletal density

The ratio of the mass to the skeletal volume of particles, excluding inter-particle voids and open pores but including closed pores.

Bulk or Apparent density

The density measured after particles have freely filled a standard container under specific conditions, representing the mass per unit volume when loosely packed.

T.A.P. (Transverse Axial Pressure) density

A measure of compressed bulk density based on the external volume of a powder bed compressed under controlled force, relevant in packaging, handling, and shipping bulk granular products.

Envelope density

The ratio of the mass of a substance to the envelope volume defined by an imaginary boundary around its external surface, including both open and closed pores but excluding interstitial spaces between pieces.

A non-destructive technique that uses gas displacement to accurately measure volume, ideal for obtaining true, absolute, and skeletal densities. 

Inert gases like helium, nitrogen, or air fill the empty spaces within and between sample particles. The change in pressure after expanding the gas into a reference chamber allows for precise volume calculations. 

Our AccuPyc Gas Displacement Pycnometer offers the fastest, easiest, and most accurate measurements of true density.

At Malvern Panalytical, we offer advanced and reliable Micromeritics instruments that provide the most accurate density measurements for a range of applications. Explore these high-performance systems below:

The AccuPyc system utilizes gas displacement pycnometry to measure the true, absolute, and skeletal densities of a wide variety of materials with unmatched speed and precision. Using an inert gas, typically nitrogen or helium, the system precisely calculates the volume of even the smallest pores in a material by measuring the pressure changes in a sealed chamber. This method is ideal for solids, powders, and porous materials. With its automated operation and minimal sample preparation, the AccuPyc delivers fast, reproducible results with high sensitivity, making it the gold standard in density measurement for industries such as pharmaceuticals and materials science.

Key Features

• Fast, precise measurements of true and skeletal densities.
• Ideal for powders, granules, and solids.
• Fully automated, minimal sample preparation.
• Suitable for a wide range of applications, from industrial to research settings.

• Application note: Measuring Volume, Density and Porosity of Tablets for Pharma Process Control
• Application note: Volume Measurement Method Affects Density Result

Understanding density measurement techniques

Understanding density measurement techniques

Recorded webinar

We explore and compare Skeletal Density, Bulk Density, Envelope Density, and T.A.P. Density for common materials.