Which powder flow test do I actually need?

A plain-language guide to PFA test selection

The wrong way to choose a test

Most people approach powder flow testing the wrong way around. They look at the list of available tests, pick the one that sounds most relevant to their material, and run it. Then they look at the numbers and wonder what to do next.

The right way is simpler: start with the complaint. What is actually going wrong? What is the production problem, the customer complaint, or the scale-up risk you are trying to predict? The answer tells you which test to run.

No single Powder Flow Analyser (PFA) test can characterise a powder completely - because powders fail in different ways. A test that perfectly predicts hopper discharge behaviour tells you nothing about what happens after three days in a silo. A test that reveals speed sensitivity may say nothing about caking tendency. Good powder characterisation means asking the right questions, not collecting the most numbers.

Start here: the baseline fingerprint

Whatever your specific concern, always begin with the same two measurements. They take under ten minutes and immediately classify the powder into a behavioural family.

Step 1: Standard Cohesion test

Run the single-speed cohesion test and read two parameters together - never in isolation:

• Cohesion Index (CI) - measures how strongly particles resist separation. High CI means a sticky, cohesive powder that clings to itself and to equipment.
• Bridging Factor - measures whether resistance is smooth (cohesive) or irregular and event-driven (structural arching, force-chain formation). High Bridging Factor at low CI means the powder is geometry-driven, not sticky - and is just as dangerous.

Step 2: Conditioned bulk density

Using the split vessel, the PFA automatically calculates the conditioned bulk density of the powder after reproducible preparation. This helps you understand the packing state of the material and provides context for all subsequent measurements. Two powders with identical cohesion profiles but very different bulk densities will behave differently during volumetric filling - even if every other parameter matches.

After these two steps, you should be able to write one line: "This powder is mainly cohesive (high CI) / mainly arching-prone (high Bridging Factor) / moderate - and packs loosely/densely after conditioning." That one line guides every test decision that follows.

Before you start: choosing the right vessel material

Vessel material affects every measurement you take, so it should be decided before the first test - not treated as an afterthought.

The standard vessel is stainless steel, and it is the right default for most powders. It gives consistent, reproducible results and is suitable for the full range of cohesion, PFSD, compressibility, and caking tests.

Switch to a different material when your process uses one. If the powder contacts HDPE in your hopper, PTFE in your feeder, or glass in your equipment, the friction and adhesion properties at that interface directly affect flow behaviour. Testing in stainless steel when the real surface is PTFE may understate wall friction and give you an optimistic result.

Three situations where vessel material matters most:

• Sticky or electrostatically active powders - these are highly sensitive to surface energy differences between materials
• Powders being characterised for hopper or silo design - wall friction should be measured against the actual liner or wall material
• Supplier comparison or incoming QC programmes - fixing the vessel material is essential for reproducibility across batches and sites

When in doubt, run the baseline cohesion test in both materials. A significant difference in Cohesion Index or Bridging Factor between stainless and your process material tells you the interface is a controlling variable - and that your process vessel material should be used for all subsequent testing.

Now choose your path based on the problem

Problem: Inconsistent fill weights or dosing drift

The powder seems to flow, but fill weights vary across a production run, or performance changes when you increase line speed.

Run next:

• PFSD (Powder Flow Speed Dependence) - tests the powder at five increasing speeds and measures how resistance changes
• Or Cohesion at 4 speeds - if you want to separate cohesive effects from compaction effects specifically

Read these three things:

• Speed Sensitivity Ratio (Comp100/Comp10): close to 1.0 = speed-robust; above 1.0 = more resistant at speed = under-fill risk; below 1.0 = easier at speed = surge/over-fill risk
• Flow Stability: close to 1.0 = behaviour unchanged during handling; significantly above or below = the powder drifts during a production run
• Compaction Coefficient at low speed: the baseline effort required to move the powder at all

Add if needed:

• Compressibility - if fill weight varies because of changing consolidation under head load in the hopper
• Consolidation and Caking - if drift gets worse after downtime or restart

Problem: Hopper stoppages, bridging, or ratholing

The powder discharges intermittently, blocks the hopper outlet, forms arches that collapse suddenly, or simply won't start flowing after a pause.

Run next:

• You have already run cohesion - now focus on the Bridging Factor result
• Add PFSD to read the Compaction Coefficient at low speed: this is the baseline resistance your feeder must overcome

Interpret:

• High Bridging Factor = geometry-driven arching risk - hopper outlet size, wall angle, and surface finish become critical
• High CI = cohesive stickiness - build-up on walls, poor flow initiation, sensitivity to humidity
• High Compaction Coefficient at low speed = high baseline resistance - feeder and drive sizing matters

Add if problems worsen after pauses or storage:

• Consolidation and Caking Rig - answers 'it flowed yesterday but won't restart today'
• Compressibility - quantifies how much the powder densifies under the static head load in the hopper

Problem: Caking, lumps, or failure after storage or transport

The powder flows during production but forms hard lumps, solid zones, or resistant masses after being stored in hoppers, silos, big bags, or during transit.

Run next:

• Caking test - five compaction cycles measuring cake fraction (how much cakes) and cake strength (how hard it is to break)
• Consolidation and Caking Rig - applies a defined static load for a controlled dwell time, then measures the work required to restart flow
• Compressibility - quantifies how readily the powder consolidates under the applied load

Interpret:

• High cake fraction + high cake strength = severe restart risk, likely needs agitation or flow aids
• High work-to-break after dwell = time-dependent set-up - the problem gets worse the longer the dwell
• High compressibility = consolidation-prone - even vibration during transport may drive significant densification

Problem: Supplier or batch-to-batch variation

The powder specification says they are the same, but they don't behave the same on your line. You need an objective way to compare them.

Run a Minimum Viable Fingerprint for each material:

• Standard cohesion (CI + Bridging Factor)
• PFSD or 4-speed cohesion (speed sensitivity + flow stability)
• Conditioned bulk density
• One storage metric: either cyclic caking test or Consolidation and Caking Rig

This combination spans the major risk domains - dynamic flow, speed dependence, and storage behaviour - in a consistent, reproducible test set that can be used for supplier approval or incoming QC.

The triangulation principle - never read one number alone

The most common mistake in powder flow analysis is drawing a conclusion from a single parameter. Here are the six rules to memorise:

What a good testing journey looks like

Not every powder needs every test. Here is a practical four-level approach:

The one-line summary

Powder behaviour depends on how it moves, how it fails, and how it changes with time. The correct test depends on the complaint, not the material name.