What the method measures, what it is for and where it stops being applicable
What ASTM D6393 is
ASTM D6393 is the standard test method covering apparatus and procedures for measuring properties of bulk solids, collectively designated Carr Indices.
Published by ASTM International and maintained by Subcommittee D18.24, the standard brings together eight measurements and two calculations. Each measurement, each calculation, or any combination of them may be used to characterize bulk material properties.
Essential point: Carr Indices describe relative handling behaviour, not absolute material properties. Their practical value comes from comparison between samples and from a history of correlation with the actual process.
Current version and scope of application
The current version is ASTM D6393/D6393M-21. The dual designation indicates the document carries both SI and inch-pound units.
The scope is explicit about its limits:
- It applies to free flowing and moderately cohesive powders.
- Particle size goes up to 2.0 mm (1/16 in.).
- The material must pour through a 6.0 to 8.0 mm (1/4 to 5/16 in.) diameter funnel outlet when in an aerated state.
Strongly cohesive materials that will not pour through the funnel under those conditions fall outside the scope. Applying the method to such cases produces a number, but not reliable information.
The measurements and calculations in the method
The method combines eight direct measurements and two calculated values. Among the parameters most used in industrial practice are:
- Carr angle of repose, indicating the material's tendency to form a stable pile.
- Aerated bulk density, measured with the powder in a loose state after controlled pouring.
- Tapped bulk density, measured after standardized tapping cycles.
- Compressibility, calculated from the two densities above.
- Angle of spatula, sensitive to cohesion and break behaviour.
- Cohesion, assessed by sieving under controlled vibration.
- Uniformity, derived from the particle size distribution.
The standard requires observed and calculated values to follow the significant digits and rounding guidelines set out in Practice D6026. This detail is frequently overlooked and produces disagreement between laboratories that, at source, measured the same thing.
Compressibility: the most used and most misread index
Compressibility derived from aerated and tapped densities is the parameter most often cited in raw material specifications, and also the most prone to misreading.
A high value indicates the material changes volume substantially under compaction, which usually correlates with poorer flow. But the index is sensitive to sample preparation: moisture, storage history, segregation during transport and even the pouring technique alter the result.
Comparing compressibility between laboratories that did not follow exactly the same preparation procedure means comparing two numbers that merely look like the same quantity.
What the method is for in practice
The standard is explicit about purpose: the measurements describe bulk properties and, combined with practical experience, allow relative ranking of handling behaviours for a specific application.
Typical applications include:
- Comparing batches of the same raw material in incoming inspection.
- Evaluating a supplier change without altering the process.
- Root cause investigation of flow problems in silos, hoppers or feeders.
- Formulation development, comparing the effect of excipients or additives.
- Assessing the effect of moisture, granulation or drying on powder behaviour.
The most productive use is comparative and longitudinal: tracking the same material over time under a stable procedure, and correlating index drift with process drift.
What ASTM D6393 does not do
This is the point separating competent use of the method from naive use.
Carr Indices do not size equipment. They do not supply the design parameters that bulk solids flow theory requires to calculate minimum outlet dimension, hopper angle for mass flow, or silo geometry.
For those calculations, the route is shear cell characterization, which measures flow function, internal friction angle and wall friction. Shear methods and Carr Indices answer different questions and do not substitute for one another.
The standard also explicitly notes that the quality of the result depends on the competence of the personnel performing it and the suitability of the equipment and facilities used.
Repeatability and good execution practice
Because the method is preparation sensitive, procedural consistency matters more than in many other tests. The controls that most affect reproducibility are:
- Standardized sampling, with attention to segregation in transport containers.
- Moisture and temperature conditioning before testing, with recorded conditions.
- Using the same pouring procedure and the same funnel across compared tests.
- A fixed number of cycles and fixed amplitude for tapped density.
- Testing in triplicate, recording dispersion and not only the mean.
- Periodic verification and calibration of the test equipment.
Documenting the internal procedure in an SOP and keeping it stable over time is what turns the test into a decision tool rather than an isolated number.
Equipment and laboratory execution
The method presumes a specific apparatus for measuring the indices, with a pouring funnel, a tapping device, a set of sieves under vibration and provisions for angle measurement. Dedicated commercial instruments run the full test sequence under standardized conditions.
For operations covering particle characterization and analytical instrumentation, the relevant decision is usually one of scope: which set of indices actually informs the process decision, and how often the test needs to run.
Relationship to other powder characterization standards
ASTM D6393 coexists with other methods describing different aspects of the same material:
- Shear cell methods, for silo and hopper design parameters.
- ASTM E11, for test sieves used in particle size analysis.
- Laser diffraction methods, for particle size distribution.
- Pharmacopoeial bulk and tapped density tests, with their own procedures and distinct criteria.
A common error is treating bulk density measured under D6393 as interchangeable with a pharmacopoeial measurement. The procedures differ, so the values are not directly comparable.
Revision in progress
ASTM maintains a work item registered under the code WK93761 for revision of D6393/D6393M-21, conducted by Subcommittee D18.24.
Technical specifications and SOPs should always cite the designation with its year, in the form D6393/D6393M-21, so the reference stays unambiguous once a new revision is published.