Engineering Glossary

Absolute rating

The size of the largest particle that passes through a filter element under specified test conditions. NOT the same as nominal rating. Absolute ratings are derived from multi-pass test data (ISO 16889) and relate to specific Beta values at the rated size.

Beta ratio (β)

The ratio of upstream to downstream particle count at a specified size, per ISO 16889. β₁₀(c) = 200 means 200 particles >10 µm enter for every 1 that exits — 99.5% efficiency. The "(c)" suffix indicates ISO 11171 calibrated counting.

Cleanliness code (ISO 4406)

Three-number code representing particle count ranges per mL at ≥4 µm, ≥6 µm, and ≥14 µm. Code 17/15/12 means: 640–1,280 particles/mL at ≥4 µm; 160–320 at ≥6 µm; 20–40 at ≥14 µm. Each unit increase doubles the count.

Dirt holding capacity (DHC)

Total mass of standardized test contaminant (grams) captured by a filter from initial to terminal differential pressure, per ISO 16889 (fluid) or ISO 5011 (air). Determines service life at a given ingress rate.

Differential pressure (ΔP)

Pressure difference between the upstream and downstream sides of the filter element. Measured in mbar, Pa, or inH₂O. Increases as element loads with contaminant. Service limit is the maximum allowable ΔP before element replacement.

Face velocity

Fluid or air velocity through the filter face area (m/s or m³/h per m²). Higher face velocity increases pressure drop and can decrease efficiency. Target face velocity for air filters: 0.05–0.15 m/s per ISO 5011 design guidelines.

Fractional efficiency

Filter efficiency measured at each of multiple particle size intervals, characterizing the complete efficiency curve across all relevant particle sizes (per ISO 5011 at 0.5, 1, 2, 3, 5, 7, 10, 20, 40, 80 µm).

ISO 4406 code

See "Cleanliness code (ISO 4406)" above.

Most Penetrating Particle Size (MPPS)

The particle size with minimum filtration efficiency, typically 0.1–0.3 µm, where neither inertial/interception nor diffusion mechanisms dominate. ISO 29463 HEPA/ULPA filters are tested and rated at MPPS.

Nominal rating

PENDING ENGINEERING DOCUMENTATION — The industry has no standardized definition of "nominal" rating; values are manufacturer-defined. Engineers should always request and compare β(c) data rather than nominal ratings when specifying filters.

Bypass valve

Spring-loaded valve opening at a specified ΔP (typically 0.8–1.0 bar) to protect the system from flow starvation when the filter element is severely restricted. At bypass, unfiltered fluid enters the system — bypass is a protection-of-last-resort, not an operating mode.

Condition-based service (CBS)

Maintenance strategy replacing filter elements when measurable indicators (restriction threshold, oil analysis limits, particle count exceedance) signal that service is needed — not on a fixed calendar or mileage schedule.

β = 200 efficiency

99.5%

ISO 4406 code increment

×2 particle count per unit

MPPS range[ISO 29463]

0.1–0.3 µm

Bypass valve typical cracking pressure

0.8–1.0 bar

Standard DHC test contaminant[ISO 16889]

ISO A2 fine dust