What problem? What test? Your guide
Matching powder flow problems to the most appropriate test methods.
The most common mistake in powder test selection is choosing a test based on the material rather than the problem. This guide works the other way: start with what is actually going wrong, and the test selection follows naturally. Identify which of the five complaint categories below best describes your situation. Each leads to a specific test recommendation and explains what the results will reveal.
This guide covers two complementary instruments that work together to give you a complete picture of your powder:
The Powder Flow Analyser (PFA) is best for understanding how powders behave in motion – dynamic, conditioned testing that captures speed sensitivity, aeration effects, consolidation history, and flow instabilities directly relevant to hopper discharge, conveying, feeding, and dosing.
The Texture Analyser (TA) is best for understanding how powders resist, deform, and fail under load – controlled static testing that measures compaction strength, cake breakability, shear failure, and penetration resistance, ideal for tabletability, storage stability, and structural integrity.
The PFA sits directly on the Texture Analyser to operate. Remove the PFA attachment and you have a fully equipped materials testing platform – so every PFA purchase automatically includes both testing capabilities. Together they cover flow behaviour, storage stability, mechanical strength, and failure mechanisms in a way that no standalone powder flow instrument can match.
Powder problem diagnostic guide
| Problem | What you observe | Powder Flow Analyser tests | Texture Analyser tests |
| Arching / Bridging | Flow stops suddenly; stable 'roof' forms over the outlet; may restart after tapping or vibration. |
Cohesion (1 speed) Bridging Factor indicates arch-forming tendency; Cohesion Index shows inter-particle bonding strength. Cohesion (4 speeds) Reveals whether cohesive bonding is speed-dependent. Powder Consolidation and Caking If arching worsens after rest or storage. |
Unconfined Yield Stress Rig Quantifies the stress at which a formed powder structure will collapse. Cake Strength/Break Test Measures force required to fracture cohesive structures at the outlet |
| Ratholing | A central channel flows freely but powder near the walls stays stuck; risk of sudden collapse and surge. |
Cohesion (1 speed) High Bridging Factor and consolidation often aligns with ratholing risk. Compressibility High compressibility supports 'will consolidate and become self-supporting'. Powder Consolidation and Caking Quantifies how much the powder sets up during rest |
Unconfined Yield Stress Rig Measures the unconfined strength of the stagnant wall mass. Powder Vertical Shear Rig Determines shear strength indicating whether sidewall material will remain stable or collapse. |
| Flooding / Flushing | Uncontrolled, liquid-like discharge; surges; dust clouds – powder won't meter or dose reliably. |
Cohesion (1 speed) Characterises the low resistance and weak internal structure typical of flooding-prone powders. Powder Flow Speed Dependence (PFSD) Identifies speed-dependent behaviour revealing aeration effects and uncontrolled discharge risk. |
Powder Penetration / Hardness Low penetration resistance confirms weak internal structure. Uniaxial Powder Compression Low yield point and work of compression indicate poor structural resistance. |
| Won't start flowing | Powder doesn't initiate flow when the gate opens; fails after a pause or overnight hold; needs agitation to restart. |
Powder Consolidation and Caking Directly replicates restart conditions; quantifies energy required to re-initiate flow. Caking Trending cake strength indicates increased restart failure risk. Cohesion (4 speeds) Captures low-speed start/stop sensitivity. Compressibility High relaxation indicates powder forms a stable consolidated structure during rest. |
Powder Caking and Consolidation Rig Measures the force and work required to break a consolidated bed and restart flow. Unconfined Yield Stress Rig Quantifies the strength of the consolidated structure that resists flow initiation. |
| Erratic discharge / Surging | Output rate oscillates; intermittent avalanches or pulses; weight control struggles. |
Cohesion (1 speed) Bridging Factor detects instability and event-like collapse behaviour. Powder Flow Speed Dependence (PFSD) Flow Stability captures progressive changes and discharge irregularity. |
Powder Vertical Shear Rig Characterises the shear behaviour contributing to intermittent arch collapse events. Powder Shear / Failure Energy to failure indicates how abruptly a powder structure releases. |
| Fill weight varies with line speed | Fill weight changes as dosing or conveying speed increases or decreases; higher speed starves or overfeeds. |
Powder Flow Speed Dependence (PFSD) Speed Sensitivity Ratio directly indicates whether fill rate changes with speed. Cohesion (4 speeds) Separates low-speed from high-speed behaviour for scale-up insight. |
Uniaxial Powder Compression Assesses compressive response under increasing load – relevant to volumetric filling consistency. Powder Penetration / Hardness Surface hardness variation indicates inconsistent powder state entering the fill cycle. |
| Dosing drifts during a run | Feed rate changes over time at constant settings – either running in or progressively running away. |
Powder Flow Speed Dependence (PFSD) Flow Stability reveals whether drift is progressive or stabilises over time. Cohesion (4 speeds) Helps separate speed-driven from structure-driven drift. |
Granule Compaction Rig Detects whether granule breakdown during handling is driving progressive change in feed rate. Powder Shear / Failure Monitors whether inter-particle strength changes as the powder is worked. |
| Caking / Hard lumps | Hard lumps after storage or transport; product needs hammering or sieving before use; poor restart from silos. |
Caking Measures how much cake forms and how strong it is after dynamic compaction cycling. Powder Consolidation and Caking Realistic 'after load + time' cake strength under defined conditions. Compressibility Determines whether the powder densifies strongly under stress. |
Cake Strength / Break Test Measures peak force and work to fracture – directly relevant to discharge and reprocessing specifications. Powder Vertical Shear Rig Quantifies how easily a formed cake shears and breaks under handling. Powder Penetration / Hardness Surface hardness of the cake indicates severity of bonding after storage. |
| Density changes after handling | Powder becomes noticeably denser after vibration or transport; bulk behaviour changes with head load. |
Compressibility Best indicator – profile shows how much the powder densifies under increasing load and how much recovers. Caking Column height ratio is a secondary sign of strong consolidation. |
Uniaxial Powder Compression Measures yield behaviour, stiffness and stress–strain response under static load. Powder Penetration / Hardness Tracks hardness changes as a function of consolidation state. |
| Sticking / Build-up on equipment | Powder adheres to walls, feeders, or hoppers; residues form; high cleaning burden. |
Cohesion (1 speed) High Cohesion Index indicates strong inter-particle and particle-surface bonding. Cohesion (4 speeds) Reveals whether adhesion is speed-dependent – relevant to feeder and conveyor design. |
Powder Vertical Shear Rig Measures wall friction and adhesive forces between powder and equipment surface. Powder Shear / Failure Quantifies inter-particle cohesive strength under static conditions. |
| Segregation / Blend non-uniformity | Composition changes over time or through a run; inconsistent potency, flavour, or particle size. |
Powder Flow Speed Dependence (PFSD) Identifies speed sensitivity and instability – two primary drivers of segregation in handling. Cohesion (1 speed) Detects cohesive behaviour and ratholing tendency that cause collapse-driven redistribution. |
Granule Compaction Rig Assesses mechanical strength differences between blend components. Powder Shear / Failure Weaker components that fail at lower force generate fines that segregate preferentially. |
| Attrition / Increasing fines | Product becomes dustier with handling; filters load faster; flow behaviour changes over time. |
Powder Flow Speed Dependence (PFSD) Flow Stability detects whether resistance changes progressively – indicating breakdown or hardening during the test. Cohesion (1 speed) Monitors whether increasing fines are raising cohesive resistance over time. |
Granule Compaction Rig Measures individual granule hardness and fracture resistance. Powder Penetration / Hardness Quantifies surface strength and susceptibility to mechanical damage. |
Not sure where to start?
The Cohesion test is the most widely used starting point for characterising powder flowability. If you're seeing problems that only appear after storage or at certain process speeds, the Caking or Powder Flow Speed Dependence tests are likely to give you the most insight. For a fuller picture of the mechanical side of the same problem, the Texture Analyser tests listed above are available on the same instrument – remove the PFA attachment and your Texture Analyser is ready to use.
For a fuller description of each problem including what causes it and how the Texture Analyser can also help, see Typical powder flow problems.
What parameters answer which questions?
| Customer question | Key parameters to read |
| Why won't it discharge? | Cohesion Index, Bridging Factor, Work to Break (after dwell) |
| Why does it fail after storage? | Cake Height Ratio, Mean Cake Strength, Column Height Ratio, Work to Break |
| Why does it change at higher speed? | Speed Sensitivity Ratio, Compaction Coefficient across speeds |
| Why is filling inconsistent? | Conditioned Bulk Density, Compressibility, Elastic Recovery |
| Why does restart get worse over time? | Work to Break (consolidation rig), Relaxation, Cake Strength |
| Why does it vary batch to batch? | Cohesion Index, PFSD, Conditioned Bulk Density |
