Understanding your powder flow test results
Choosing the right powder flow test based on your material and process.
What does each parameter measure?
Once you've identified which tests to run, this reference explains what each output parameter actually measures and what question it answers – useful both when planning your test programme and when interpreting your results.
|
Parameter |
Core question it answers |
What it is most useful for |
|
Cohesion Index (CI) |
How hard is it to get this powder moving? |
Assessing baseline resistance to flow initiation; identifying cohesive fines; screening for sticking and restart sensitivity. |
|
Bridging Factor |
How does this powder fail when it resists flow? |
Identifying arching, ratholing, and structure-driven discharge problems; diagnosing geometry-sensitive failures. |
|
Speed Sensitivity Ratio (PFSD) |
Does resistance change as speed increases or decreases? |
Understanding throughput effects; scale-up risk; explaining under- or over-filling at higher speeds. |
|
Flow Stability (PFSD) |
Does behaviour change with repeated handling? |
Identifying drift during long runs, recirculation effects, and sensitivity to handling history. |
|
Cake Height Ratio |
How much of the bed becomes a cake after storage? |
Assessing extent of set-up; likelihood of large lumps or incomplete discharge after storage or transport. |
|
Mean Cake Strength |
How hard is the cake locally? |
Understanding how easily formed cakes fracture; assessing lump hardness. |
|
Cake Strength (total work) |
How difficult is the cake to completely remove? |
Predicting restart difficulty, emptying problems, and need for mechanical agitation. |
|
Column Height Ratio |
How much does the bed settle and densify? |
Identifying packing sensitivity, bulk density drift, and fill-weight variability after handling. |
|
Work to Break After Dwell (Powder Consolidation & Caking) |
How difficult is it to restart flow after storage under load? |
Diagnosing "won’t start" behaviour after storage or shutdown; consolidation under self-weight; delayed or incomplete discharge from silos, bins, or hoppers. |
|
Compressibility % |
How much does the powder densify under load? |
Tablet/capsule filling, packing behaviour, formulation optimisation. |
|
Relaxation % |
How much internal stress dissipates after the powder bed is compressed? |
Identifying time-dependent structural rearrangement, stress relaxation, and delayed settling behaviour. |
|
Stiffness (MPa) |
How resistant is the powder bed to deformation under applied load? |
Assessing rigidity of compacted beds, mechanical stability during compaction, and stress transmission in storage or tableting |
|
Elastic Recovery (%) |
How much does the powder bed rebound after the load is removed? |
Identifying elastic behaviour that may cause die expansion, dimensional variability, or packing instability. |
|
Bulk Density (conditioned) |
What density does the powder adopt under controlled preparation? |
Fill-weight control, pack sizing, supplier comparison, QC trending. |
Note: Compaction and Cohesion Coefficients are measured and used to calculate some of the above parameters.
|
Parameter |
Core question it answers |
What it is most useful for |
|
Compaction Coefficient |
How much does the powder resist being pushed down and compressed? Measures the work required during the downward compression phase as the blade pushes through and compacts the powder bed. |
Assessing baseline resistance to flow initiation; identifying cohesive fines; screening for sticking and restart sensitivity. |
|
Cohesion Coefficient |
How strongly do particles stick together and to the blade when lifted? Measures the work required during the upward lifting phase as particles cling to each other and to the blade, reducing force on the vessel base. |
Identifying arching, ratholing, and structure-driven discharge problems; diagnosing geometry-sensitive failures. |
How long does each test take?
|
Test |
Typical duration |
|
Powder Flow Analyser tests |
|
|
Cohesion (1 speed) |
~4 minutes |
|
Cohesion (4 speeds) |
~10 minutes |
|
Powder Flow Speed Dependence (PFSD) |
~15–18 minutes |
|
Caking |
~5 minutes + dwell time (e.g. overnight for consolidation) |
|
Powder Consolidation and Caking |
~8 minutes + defined dwell time |
|
Compressibility |
~8–12 minutes |
|
|
|
|
Texture Analyser tests – available on the same instrument |
|
|
Uniaxial Powder Compression |
~1–2 minutes |
|
Powder Penetration/Hardness |
~1 minute |
|
Cake Strength/Break test |
~1 minute |
|
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|
Competitor instruments – for comparison |
|
|
FT4 Basic Flowability Energy (BFE) or stability/variable flow rate test |
10–30 minutes |
|
Ring Shear – complete yield locus |
10–15 minutes |
|
Brookfield PFT – flow function |
25–35 minutes |
PFA tests are significantly faster than most competitor instruments. A complete flow characterisation – Cohesion, Compressibility, and PFSD – can be completed in under 40 minutes. FT4 and Brookfield equivalents typically take 45–85 minutes for the same breadth of measurement. Texture Analyser tests add as little as 1–2 minutes per parameter.
