Bubble Point Pore Size Calculator
From the Young–Laplace capillary model, the largest pore diameter is d = 4γ cos θ / ΔP: a bubble point of 30 psi with isopropyl alcohol (21.7 mN/m) corresponds to about 0.42 µm.
0.4196 µm
- γ = 21.7 mN/m, θ = 0°
- d = 4 γ cos θ / ΔP = 4 × 0.0217 × 1 ÷ 206843 Pa = 4.196 × 10⁻⁷ m
Bubble Point Formula
d = 4 γ cos θ / ΔP
d pore diameter (m), γ surface tension (N/m), θ contact angle, ΔP bubble-point pressure (Pa)
Capillary pressure holds the wetting liquid in a cylindrical pore until the gas pressure exceeds 4γ cos θ / d. The largest pore opens first, so the first steady bubbling identifies the largest pore. This relation underlies ASTM F316 bubble-point testing.
Worked Examples
- IPA, bubble point 30 psi (206.8 kPa): d = 4 × 0.0217 ÷ 206843 Pa = 0.42 µm.
- Water-wet 0.2 µm membrane: ΔP = 4 × 0.0728 ÷ 0.2 × 10⁻⁶ = 211.2 psi (14.56 bar).
Bubble Point by Pore Size
| Pore size | Water (psi) | IPA (psi) | IPA (bar) |
|---|---|---|---|
| 0.1 µm | 422 | 126 | 8.68 |
| 0.2 µm | 211 | 62.9 | 4.34 |
| 0.45 µm | 93.9 | 28 | 1.93 |
| 0.8 µm | 52.8 | 15.7 | 1.09 |
| 1 µm | 42.2 | 12.6 | 0.868 |
| 3 µm | 14.1 | 4.2 | 0.289 |
| 5 µm | 8.45 | 2.52 | 0.174 |
| 10 µm | 4.22 | 1.26 | 0.0868 |
| 20 µm | 2.11 | 0.629 | 0.0434 |
Commercial membrane ratings include empirical correction factors, so manufacturers' bubble-point specifications differ from these theoretical values.
Frequently asked questions
What is the bubble point of a membrane?
The lowest gas pressure that pushes the wetting liquid out of the largest pores of a fully wetted membrane, producing a steady stream of bubbles. It is used for pore-size estimation and filter integrity testing.
Why does the liquid matter?
Bubble point is proportional to surface tension. The same membrane shows a bubble point about 3.35 times higher with water than with isopropyl alcohol.
Is the calculated pore size exact?
It is a capillary-model estimate of the largest pore. Real pores are not cylinders, so manufacturers often apply an empirical shape factor and validate against challenge tests; compare like with like.
What contact angle should I use?
For fully wetting liquids such as IPA, Galwick or Porewick on most membranes, θ ≈ 0° (cos θ = 1). Water on hydrophobic membranes does not wet and should not be used for bubble point.
Sources
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology .
- The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .
Last reviewed: . Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.
Cite this page
APA
Scientific Converters. (2026, October 15). Bubble Point Pore Size Calculator. Retrieved October 17, 2026, from https://scientificconverters.com/calculators/bubble-point-pore-size/
IEEE
Scientific Converters, "Bubble Point Pore Size Calculator," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/calculators/bubble-point-pore-size/