Quick definition: This tool calculates the maximum equivalent capillary diameter inferred from a wet bubble-point pressure, or estimates the ideal pressure for a specified equivalent pore size. It is intended for preliminary interpretation of sintered-metal quality-control measurements, not for predicting particle retention or filtration rating.
Bubble-point equivalent pore size calculator
Equation and measurement conditions
Under the ideal circular-capillary, completely wetted model used for interpreting ISO 4003 bubble tests: d = 4γ / Δp. Here d is in metres, γ is the surface tension in N/m and Δp is the gas-to-liquid differential pressure in Pa. In this tool Δp = measured gas gauge pressure − ρgh, where h is the liquid head above the relevant pore level in metres, ρ is wetting-liquid density in kg/m³, and g = 9.81 m/s².
Reverse mode estimates the gas pressure needed for the requested equivalent capillary diameter: pgas = 4γ/d + ρgh. The calculation assumes complete wetting and a reliable first-continuous-bubble reading. If liquid contact angle, wetting, leakage or other test conditions differ, the equation may not be applicable.
Example and engineering limits
With γ = 30 mN/m, gas pressure 12 kPa and no liquid head, the equivalent diameter is 10 µm. With 30 mm liquid head at 1,000 kg/m³, measured gas pressure is approximately 12.2943 kPa for the same capillary result.
Bubble-test diameter is not a pore-size distribution, a nominal or absolute micron rating, a particle-retention guarantee or standalone evidence of compliance. Review the test procedure, saturation conditions and acceptance criteria before approving a finished filter.
Frequently asked questions
Does bubble point equal filtration rating?
No. It indicates an equivalent largest interconnected pore pathway under the stated test model. Particle retention and efficiency require separate test conditions and evidence.
What if the surface tension is unknown?
Measure or document the actual wetting liquid and temperature before calculation; using a generic default cannot validate a real measurement.
Reference: ISO 4003:1977 — Bubble test pore size for permeable sintered metals (the current edition has been reconfirmed by ISO). To discuss test records and porous-metal elements, see sintered metal fiber felt or Contact Our Engineers.