KEVOS® MAKEOS · Quality
Filter Membrane Bubble Point Check (Integrity Test, Largest Pore, Other Wetting Liquid)
Does the bubble point I measured on this wetted filter membrane meet the maker’s minimum, corrected for my wetting liquid, and what largest pore size does it point to?
Bubble point integrity check of a wetted membrane filter by the Young–Laplace capillary relation: the maker’s minimum bubble point converted to the wetting liquid actually used in proportion to surface tension (both liquids fully wetting), the margin of the measured bubble point over it, and the equivalent largest capillary pore diameter 4 k γ ÷ Δp at the measured and at the minimum bubble point, against the converted minimum.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Measured bubble point | bar | Gas pressure difference across the wetted membrane when bulk gas flow begins. |
| Maker’s minimum bubble point | bar | Lowest bubble point the filter maker specifies for an intact membrane. |
| Surface tension of the maker’s test liquid | mN/m | Surface tension of the liquid the maker’s minimum refers to, at the test temperature. |
| Surface tension of your wetting liquid | mN/m | Surface tension of the liquid you wetted the membrane with, at the test temperature. |
| Pore shape correction factor | Correction factor for non-cylindrical pores; 1 for the ideal cylindrical capillary. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Measured bubble point
- What it is
- The gas pressure difference across the wetted membrane at which a steady stream of bubbles first appears downstream, showing the largest pores have cleared.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.001 bar to 100 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3.45 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher bubble point means smaller equivalent pores and a larger margin.
- Get it right
- Raise the pressure slowly and record the point of steady bulk flow, not the first few stray bubbles.
What this means: Maker’s minimum bubble point
- What it is
- The lowest bubble point the filter maker specifies for an intact membrane of this type and rating, in the maker’s own test liquid.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.001 bar to 100 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3.2 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Moves the pass line in proportion; the measured pore size does not change.
- Get it right
- Take it from the filter’s validation guide or certificate, noting which liquid it refers to.
What this means: Surface tension of the maker’s test liquid
- What it is
- The surface tension of the liquid the maker’s minimum bubble point refers to, at the test temperature.
- Units and format
- Enter a number in mN/m: mN/m. The form accepts 1 mN/m to 1000 mN/m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 72 mN/m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher value lowers the minimum converted to your liquid.
- Get it right
- Water is about 72 mN/m near 25 °C; use the value at your test temperature.
What this means: Surface tension of your wetting liquid
- What it is
- The surface tension of the liquid you actually wetted the membrane with, at the test temperature.
- Units and format
- Enter a number in mN/m: mN/m. The form accepts 1 mN/m to 1000 mN/m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 72 mN/m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The converted minimum and the equivalent pore sizes rise in proportion to it.
- Get it right
- Alcohol and alcohol–water mixtures are far below water; use data for the exact mixture used.
What this means: Pore shape correction factor
- What it is
- A correction for pores that are not straight round capillaries, applied to the equivalent pore diameter.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 to 1.5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The equivalent pore sizes scale in proportion; the pass or fail result does not change.
- Get it right
- Use 1 for the ideal cylindrical capillary; use another value only if your test standard or the maker gives one.
Method and formulas
- Across a curved liquid surface the pressure difference is γ (1/r₁ + 1/r₂) (Guggenheim §1.60–1.61); a fully wetting liquid in a cylindrical pore of diameter d holds gas back up to Δp = 4 γ ÷ d.
- The largest pore empties first, so the bubble point gives the equivalent largest pore d = 4 k γ ÷ Δp.
- For the same pores the bubble point scales with surface tension: minimum in your liquid = maker’s minimum × γ_test ÷ γ_maker.
- Example: 3.45 bar measured against a 3.2 bar minimum, both with water at 72 mN/m, passes with 7.8125 % margin; the equivalent largest pore is 0.83478 µm.
Limitations
- Both liquids must fully wet the membrane (zero contact angle); a partly wetting liquid gives a falsely low bubble point.
- The capillary pore diameter is an equivalent value, not the filter’s retention rating.
- The maker’s specified test method, temperature and liquid govern acceptance; this is a screening aid where sterile or regulated filtration is involved.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Accept a filter cartridge or disc after a bubble point integrity test
- Convert the maker’s water bubble point to an alcohol-wetted test
- Estimate the largest equivalent pore from a measured bubble point
Who it is for
- Quality and validation staff
- Process technicians
- Laboratory technicians
What you get
- Minimum bubble point in your wetting liquid
- Margin over the minimum
- Equivalent largest pore at the measured and minimum bubble points
Interpreting the result
- A bubble point well below the minimum points to a damaged membrane, a bad seal or O-ring, or incomplete wetting.
- Lower-surface-tension liquids such as alcohols give proportionally lower bubble points for the same pores.
References
- E. A. Guggenheim, Thermodynamics: An Advanced Treatment for Chemists and Physicists, 5th revised edition (North-Holland, 1967), §1.60 (interfacial tension of a curved interface), §1.61 (pressure within a bubble). Method reference; no text, tables or examples are reproduced.
- ASTM F316, Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test. Scope reference only; no values are reproduced.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.