A gradation written as "0.5 – 1.0 mm" tells you almost nothing about how the bed will behave. It describes the extremes of the particle distribution, not its shape. Two batches can share that range and perform completely differently in service.
What the two numbers mean
Effective size (D₁₀) is the mesh size through which 10% of the material by weight will pass. Uniformity coefficient is D₆₀ divided by D₁₀ — the ratio between the size that passes 60% and the size that passes 10%. A UC of 1.0 would mean every grain is identical. Real filter sand sits between 1.3 and 1.6.
Why a high UC costs you twice
A wide distribution means small grains fill the voids between large ones. The bed packs tighter than designed, so head loss develops faster and filter runs shorten. That is the first cost.
The second appears at backwash. Fluidisation velocity depends on grain size, so a wide distribution has no single correct backwash rate. Set it for the coarse fraction and the fines wash out of the vessel. Set it for the fines and the coarse fraction never lifts, so the bottom of the bed is never cleaned. Over a few years the bed loses depth from the top and blinds from the bottom simultaneously.
What to specify
- Effective size, not just the size range — this is the number that sets filtration performance.
- Uniformity coefficient with a ceiling, typically ≤ 1.5 for potable duty.
- That the sieve analysis reported is from the lot being supplied, not a product average.
A supplier who cannot report the uniformity coefficient for the specific lot is not measuring it. That is worth knowing before the material is in the vessel.