Cut Point Test
Filter beta ratio and cut points from upstream/downstream counts.
Calculator
Upstream / downstream particle counts by size
Theory & method
The cut point of a filter or membrane is commonly rated by challenging it with fluidized microspheres of known size and counting particles upstream and downstream, per ASTM F795. At each size x, the beta ratio β(x) = N_upstream / N_downstream expresses how many particles the filter lets through for every one it captures; filtration efficiency follows as η(x) = (N_upstream − N_downstream) / N_upstream × 100%.
Two conventional beta ratios define the reported cut points: β = 1000 (99.9% removal efficiency at that size) and β = 10000 (99.99%). Because real test data rarely land exactly on those ratios, this calculator finds the crossing size by linear interpolation between the two adjacent measured points that bracket the target β.
The filter rating compares the β = 1000 cut point with the manufacturer's nominal pore size: a ratio below 0.5 is rated absolute (the filter reliably removes particles well below its nominal rating), below 1.5 is rated nominal (removal is centered near the nominal size), and above that is rated coarse (removal lags the nominal size significantly).
The steepness factor — the steepest local slope of the efficiency curve — indicates how sharply the filter transitions from letting particles through to capturing them; a steep curve means a narrow band of sizes separates "mostly passes" from "mostly captured". Test validity flags results built from too few points (fewer than 3), or a maximum efficiency below 90% (test conditions likely compromised).
How to use
- 01For each particle size tested, enter the upstream particle count (before the filter) and the downstream particle count (after the filter) — one row per size, at least 3 rows for a valid test.
- 02Enter the manufacturer's nominal pore size for the filter under test.
- 03Read the β = 1000 and β = 10000 cut points, the maximum efficiency, the steepness factor and the resulting filter rating.
- 04Check the test validity note: with fewer than 3 matched sizes or a maximum efficiency below 90%, treat the result as inconclusive and re-test.
Frequently asked questions
What do β = 1000 and β = 10000 mean?
They are conventional beta-ratio thresholds from multi-pass filtration testing: β = 1000 corresponds to 99.9% removal efficiency at that particle size, and β = 10000 to 99.99%. The cut point is the particle size at which the filter reaches that ratio.
Why is the filter rating sometimes "not determined"?
The β = 1000 cut point can only be interpolated if your measured beta ratios bracket 1000 — i.e. one point is below and the next above (or vice versa). If every measured size stays far from β = 1000, the crossing point (and therefore the rating) cannot be determined from the data provided.
Absolute, nominal or coarse — what changes in practice?
Absolute filters (cut point well below the nominal rating) give the most reliable, tightly bounded particle removal. Nominal filters center their removal near the stated pore size but allow more size-by-size variability. Coarse filters remove particles significantly larger than their nominal rating, offering less precise control.
How is this different from the ISO 3310-1 sieve calibration tool?
Sieve calibration verifies the physical geometry of a wire-cloth sieve's openings. This tool instead characterizes a filter's functional removal performance — how it actually behaves when challenged with particles — which is the relevant metric for depth filters, membranes and cartridges without simple geometric openings.
Normative references
- ASTM F795 — Standard Test Method for Determining the Performance of a Filter Medium Employing a Single-Pass, Constant-Rate, Non-Recirculating Fluidized Bed.
- ISO 16889 — Hydraulic fluid power filters — Multi-pass method for evaluating filtration performance.
- ASTM F838 — Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilized for Liquid Filtration (related multi-pass rating concepts).
- ISO 4003 — Porous metal materials — Determination of pore size distribution.