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Bubble Point Test
Procedure

The bubble point test is the standard method for determining the largest pore size in a sintered metal filter element. This procedure page covers the test principle, step-by-step method, equipment requirements, and how to interpret results.

ASTM F316 Reference 5-Step Method PDF Download Available
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Procedure Document

Bubble Point Test Procedure

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Test Principle

What Is the Bubble Point Test?

The bubble point test measures the minimum gas pressure required to force a continuous stream of bubbles through a fully wetted filter element. This pressure corresponds directly to the largest pore size present in the filter media — the so-called "bubble point."

A wetted membrane retains liquid in its pores by capillary force. As upstream pressure increases, gas first breaks through at the largest pore because less capillary force must be overcome. The relationship between bubble point pressure and pore diameter follows the Washburn equation:

d = 4γ·cos θ / P
  • d — largest pore diameter (µm)
  • γ — surface tension of test liquid (mN/m)
  • θ — contact angle of liquid on filter material
  • P — bubble point pressure (Pa)

Higher bubble point pressure = smaller maximum pore size = tighter filtration grade.

Key Facts

  • Standard reference

    ASTM F316 / ISO 4003

  • Non-destructive

    Filter is not damaged during the test

  • Test liquids

    Isopropanol (IPA) or distilled water

  • Detectable pore range

    Typically 0.1 µm – 200 µm

  • At FILTURE

    Performed on 100% of sintered metal filter products before shipment

Method

Step-by-Step Procedure

The following procedure follows ASTM F316. Ensure the test environment is clean and the filter element is free of damage or contamination before starting.

01

Prepare the Filter Element

Inspect the filter element visually. It must be clean, undamaged, and free of particulate contamination. If the filter has been in service, clean and dry it thoroughly before testing.

Do not touch the filter media with bare hands — use clean gloves to avoid contaminating the pore structure.
02

Wet the Filter Completely

Submerge the filter element in the test liquid — isopropanol (IPA) is preferred for most sintered metal filters due to its low surface tension and good wetting of metal surfaces. Distilled water may be used for hydrophilic applications.

Ensure the liquid penetrates all pores completely. Allow to soak for a minimum of 2 minutes, or until no air bubbles emerge from the filter surface.

Use fresh, reagent-grade IPA or deionised water. Contaminated test liquid will give inaccurate results.
03

Assemble the Test Rig

Mount the wetted filter in the test fixture. Connect the regulated gas supply (dry nitrogen or clean compressed air) to the upstream side. Submerge the downstream outlet face in a shallow tray of test liquid, or use a pressure-decay measurement system.

  • Gas supply: dry N₂ or clean compressed air (oil-free)
  • Pressure regulator: 0–10 bar range, fine-control
  • Pressure gauge: 0.1 bar graduation (or better)
  • Seals: ensure all connections are leak-free before pressurising
04

Increase Pressure Slowly & Observe

Starting from zero, increase upstream gas pressure slowly and steadily — at a rate of approximately 0.1–0.2 bar per 10 seconds. Observe the downstream face or the submerged outlet tray continuously.

As pressure rises you may see individual, slow bubbles — this is normal diffusion flow. Continue increasing pressure until you observe a first sustained stream of bubbles at a consistent location. Record this pressure immediately.

Do not rush pressure increase — a rapid ramp causes liquid to be pushed from pores prematurely, giving a falsely low (optimistic) bubble point reading.
05

Record & Calculate

The pressure at which the first continuous bubble stream appears is the bubble point pressure (Pbp). Apply the Washburn equation to calculate the maximum pore diameter, or compare directly against the reference values in FILTURE's calibration chart for the relevant alloy and test liquid.

Document: test liquid used, test temperature, bubble point pressure, calculated pore size, filter element serial number, and test date. Retain results as part of the quality record.

Reference Data

Interpreting the Results

Typical bubble point pressure ranges for common FILTURE sintered filter grades, tested with IPA at 20 °C.

Filter Grade Mean Pore Size (µm) Bubble Point Pressure (IPA) Bubble Point Pressure (Water)
Ultra-fine0.5 – 2 µm> 3.0 bar> 1.2 bar
Fine2 – 10 µm1.0 – 3.0 bar0.4 – 1.2 bar
Medium-fine10 – 30 µm0.35 – 1.0 bar0.14 – 0.4 bar
Medium30 – 60 µm0.18 – 0.35 bar0.07 – 0.14 bar
Coarse60 – 100 µm0.10 – 0.18 bar0.04 – 0.07 bar

* IPA surface tension: 21.7 mN/m at 20 °C · Water surface tension: 72.8 mN/m at 20 °C · Contact angle assumed 0°. Values are indicative; actual bubble point depends on alloy, sintering conditions, and element geometry.

These grades correspond to our standard sintered filter cartridge range — see the SS sintered powder filter element and titanium sintered powder filter element pages for material-specific pore size data.

Download the Full Procedure

The complete Bubble Point Test Procedure document — including test setup diagram, data recording form, and calculation worksheet — is available as a PDF.

PDF format 336 KB Chinese (中文) + Diagrams
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Need a bubble point test report for your order?

Bubble point test reports are available on request for all sintered metal filter elements. Request a test certificate with your order.

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