Device D50 is the size class with 50% recovery to a stated discharge stream on a partition curve; it is not the sample PSD median or 50% of all feed solids. API screen D100 is determined by a separate prescribed laboratory test. Identify the method and stream basis before comparing values.
Separation isn’t a clean cut
No real separator removes everything above a size and nothing below it. Plot the probability that a particle reports to the removed stream against its size and you get an S-shaped grade-efficiency (partition) curve, not a vertical step. Small particles mostly stay; large particles mostly go; in between is a band where it’s a coin toss. A single “cut point” is just one chosen point on that curve.
D50 — the median (equiprobable) size
For a device partition curve, D50 is the size at 50% recovery to the selected stream. Specify whether recovery is measured or corrected and how the curve was constructed. Its value depends on particles, fluid properties, throughput and equipment settings; a quoted micron range is not a guaranteed field cut.
D100 — the size nothing passes
API screen D100 describes separation potential under the specified screen-cloth test. It is not a guarantee that all larger particles are rejected on an operating shaker. Hydrocyclone and centrifuge partition curves need not attain exactly 100% recovery at a finite size; do not assign them a universal D100.
Sharpness — the part a single number hides
Two separators can share a D50 while their partition curves have different slopes. Sharpness can be expressed as d75/d25, with values closer to one indicating a steeper curve. Use values from the same stated recovery basis; this ratio does not determine an API screen D100.
Why it matters for shaker screens
API RP 13C screen designation uses D100 from a prescribed physical test with graded aluminium oxide and reference sieves. The older RP 13E used an aperture-distribution method. These are different procedures; the old D50 and current D100 are not interchangeable measurements. An API number identifies a D100 range, while field performance also depends on fluid, flow, solids load and shaker settings.
Side by side
| D50 | D100 | |
|---|---|---|
| Definition | 50% of a size class reports to the stated stream | Screen separation potential in the prescribed laboratory test |
| Question it answers | Where is 50% size-class recovery on this curve? | What’s the largest that gets through? |
| Micron value | Method-dependent | Different test basis; no universal conversion |
| Typically quoted for | Cyclones, centrifuges; old RP 13E screens | API RP 13C shaker screens |
Key takeaways
Keep sample PSD percentiles, device partition-curve D50 and API screen D100 distinct. D50 describes size-class recovery, not overall feed removal. Compare screen labels on the same laboratory test basis and device performance on the same stream and recovery basis; verify actual field separation by representative sampling and solids balances.
