Wait a minute, there’s more than one density?


Density is the mass contained in a given volume. Anyway, it can be calculated in different ways.

  • Skeletal density is obtained measuring the volume occupied by the sample excluding accessible pores as well as the void spaces between particles within the bulk sample. Often the names True, Absolute or Real density are used, but they can be misleading – skeletal density can match the true, absolute or real density only if the samples doesn’t have closed pores (pores inaccessible to any testing medium).
  • Bulk density is obtained measuring the total volume occupied by a known mass of material.  This volume includes also inter-particle void volume and pores volume.
  • Tapped density is an increased bulk density obtained after mechanically tapping the container containing the powder sample.
How can I use them?

While skeletal density is an intrinsic property of the material composing the sample, bulk and tap densities reflect how sample particles behave collectively in specific conditions.

When analyzing powders, skeletal density is influenced by the chemistry and composition of the sample, while physical properties like particles size and morphology are less relevant.

On the other hand, bulk densities are strongly affected by changes in particles size and morphology, as well as by external conditions, or the ways the material is handled. These densities are linked to the rheological properties of the sample and can be used to understand different sample behaviors. The ratio between bulk density and tapped density is often used as an index of the ability of a powder to flow. This ratio is known as the “Compressibility Index or the Hausner Ratio“. It measures the propensity of a powder to be compressed and is influenced by the inter-particulate interactions that affect powder flow.

How are they measured?

Skeletal density: the best way to measure Skeletal density is using gas pycnometry, a non-destructive technique that relies on gas displacement to measure volume accurately. Inert gases, such as helium, nitrogen, or air are commonly used as the displacement medium for gas pycnometry.

A sample is placed in a chamber of known volume, which is sealed and pressurized. The gas fills the empty spaces within and between the sample particles. The sample chamber is then expanded to an adjoining reference chamber of known volume. The change in pressure is used to calculate the volume of the sample. Skeletal/True density is calculated from the sample mass and the volume it occupies.

In Alfatestlab we perform Skeletal/True density measurements using the Accupyc Gas Pycnometer by Micromeritics.



Bulk density (Apparent): it is often analyzed with a volumetric cylinder measuring the volume occupied by a bulk of powder of known mass (weight). The ratio of the volume read to the weight of the sample gives the bulk density value.

In Alfatestlab we analyze it using  the powder rheometer FT4 by Micromeritics with an “inverse” process compared to the traditional cylinder. We measure the bulk weight of powder placed and leveled in a known volume splitable vessel (the split is a procedure used to obtain a controlled volume of powder). The main advantage is that the instrument, before the split, performs a conditioning of the powder with the aim to disturb and gently drop each particle and build a homogenously packed powder bed, removing any pre-compaction or excess air and ensuring the results from the following test are independent of how the operator handles the powder and places it into the testing vessel. The measured bulk density is than called “Conditioning Bulk Density”.



Tapped density: the bulk density of the powder is measured following consolidation, performed by a controlled “tapping” process.

In Alfatestlab we perform this with the Copley JV1000 instrument. With the FT4 we perform the “Consolidation” test which in addition to the tapped density value also provides the tapped energy value required for powder flow.

Changes in flowability energy after consolidation on a relative scale are up to 300 times more relevant than the volume changes.

And of course, we can always go back to the cylinder where we measure the volume change as a result of tapping.

Find out how Alfatestlab can help you in your density analysis!

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AlfatestLab is a contract analysis laboratory that provides you with results you can trust, thanks to decades of experience on analytical techniques and their applications.

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