Bubble Point Pressure Testing Service

Bubble Point Pressure Testing Service – Accredited ISO/IEC 17025 Pore Size and Integrity Assessment for Filter Media and Porous Materials in the Croatian Market

Bubble point pressure testing is a critical and widely accepted non‑destructive method for determining the maximum pore size and evaluating the structural integrity of porous materials, including filter membranes, cartridges, paper, textiles, sintered metals, and ceramic elements. The bubble point pressure is the minimum pressure required to force a gas bubble through the largest pore of a liquid‑saturated porous medium. This measurement is essential for quality control, product certification, and performance validation in industries such as pharmaceutical manufacturing, biotechnology, food and beverage processing, water treatment, chemical production, and automotive engineering. In the Croatian market, where the Hrvatski zavod za norme (HZN), the Ministarstvo gospodarstva i održivog razvoja, the Državni inspektorat, the Agencija za lijekove i medicinske proizvode (HALMED), and the Carinska uprava enforce strict quality, safety, and regulatory standards aligned with EU directives and HRN EN (Croatian standards based on European norms), the accurate evaluation of bubble point pressure is essential for product certification, supplier qualification, process validation, quality control in manufacturing, and import‑export processes. Our laboratory offers a comprehensive bubble point pressure testing service, applying standardized methods such as ASTM F316, ISO 4003, BS 3321, and HRN EN ISO 4003 to measure bubble point pressure, calculate maximum pore size, and assess the integrity of porous materials under controlled conditions. All tests are performed under our ISO/IEC 17025 (CNAS) accreditation, and the resulting reports are fully accepted by Croatian authorities and notified bodies, making them indispensable for regulatory compliance, product validation, and market access in Croatia and the European Union.

Porous Materials and Filter Components We Regularly Test

Our laboratory receives a wide variety of porous materials, filter media, and filter components for bubble point pressure testing. Typical samples include:

• Filter membranes and cartridges – for pharmaceutical, biopharmaceutical, food and beverage, and water treatment applications.
• Porous plastic and ceramic elements – sintered polyethylene, polypropylene, PTFE, PVDF, and ceramic filters.
• Paper and nonwoven media – for filtration, separation, and absorption applications.
• Sintered metal and wire mesh filters – for high‑temperature and corrosive environments.
• Textile and fabric filters – for dust collection and liquid filtration.
• Prototype and new porous material designs – submitted by manufacturers for validation of pore size and integrity before series production.
• Field‑retrieved filters – for failure analysis and remaining life assessment.

Bubble Point Pressure Measurement – Principle and Standard Methods

The bubble point pressure test is based on the relationship between the capillary pressure and the pore size of a porous material. The test specimen is first saturated with a wetting liquid (typically water, isopropanol, or a specific solvent) that completely fills the pores. Then, a gas (usually air or nitrogen) is applied to one side of the specimen, and the pressure is gradually increased. The bubble point is reached when the gas pressure exceeds the capillary pressure of the largest pores, and bubbles are first observed on the downstream side. Our procedures follow international standards and the requirements of the Croatian pharmaceutical, food, and industrial sectors.

• Bubble point pressure test – standard method (ASTM F316 / ISO 4003 / HRN EN ISO 4003 / NTC 5600 – for filter membranes and porous materials) – we saturate the test specimen with a specified wetting liquid (e.g., water, isopropanol, or perfluorocarbon) and mount it in a bubble point test apparatus. The gas pressure is increased at a controlled rate (typically 0.1 to 0.5 bar/min) while monitoring the downstream side for the first bubble formation. The pressure at which the first continuous stream of bubbles appears is recorded as the bubble point pressure. We report the bubble point pressure (in bar or kPa), the wetting liquid used, and the test temperature.
• Maximum pore size calculation (NTC 5601 – for the pore size determination) – we calculate the maximum pore diameter (d_max) from the bubble point pressure (P) using the Washburn equation: d = 4·γ·cosθ / P, where γ is the surface tension of the wetting liquid, θ is the contact angle, and P is the bubble point pressure. We report the maximum pore size (in μm) and the calculated pore size distribution.
• Diffusion and bubble point test (NTC 5602 – for filter integrity testing) – we perform a combined diffusion test and bubble point test on filter cartridges to verify the integrity of the filter and to detect any defects (e.g., pinholes, cracks, or seal failures). The diffusion flow rate is measured at a specified pressure (typically 80 % of the bubble point). We report the diffusion flow rate (in mL/min) and the bubble point pressure.
• Bubble point test at different wetting liquids (NTC 5603 – for the compatibility evaluation) – we perform the bubble point test with different wetting liquids (e.g., water, isopropanol, hexane, or a specific process fluid) to evaluate the compatibility of the filter material with the process fluid and to determine the appropriate wetting liquid for the application. We report the bubble point pressure for each wetting liquid.
• Bubble point test at different temperatures (NTC 5604 – for the thermal effect) – we perform the bubble point test at different temperatures (e.g., 20 °C, 40 °C, 60 °C) to evaluate the effect of temperature on the bubble point pressure and the pore size measurement. We report the bubble point pressure at each temperature.

Filter Integrity and Validation Testing – Ensuring Sterility and Performance

In the pharmaceutical and biopharmaceutical industries, the integrity of sterilizing filters is a critical quality attribute. Our bubble point and integrity tests validate that the filter is free from defects and that it will remove microorganisms and particulates as specified. These tests are required for regulatory compliance with HALMED and the European Medicines Agency (EMA) guidelines.

• Filter integrity test – bubble point method (ASTM F316 / NTC 5610 – for sterilizing grade filters) – we perform the bubble point test on sterilizing grade filters (0.2 μm or 0.45 μm) to verify that the filter meets the specified bubble point pressure (e.g., ≥ 3.0 bar for a 0.2 μm hydrophilic filter). The bubble point pressure is compared with the manufacturer's specification. We report the bubble point pressure and the pass/fail status.
• Diffusion test for filter cartridges (NTC 5611 – for the integrity verification) – we perform a diffusion test (or a forward flow test) on the filter cartridge at a specified pressure (typically 80 % of the bubble point). The gas diffusion flow rate is measured and compared with the manufacturer's limit. We report the diffusion flow rate (in mL/min) and the pass/fail status.
• Pressure decay test (NTC 5612 – for the automated integrity testing) – we perform a pressure decay test (or a pressure hold test) on the filter assembly, where the pressure is applied and held for a specified duration, and the pressure drop is measured. The pressure drop is used to assess the integrity of the filter. We report the pressure drop (in kPa) and the pass/fail status.
• Integrity test after repeated use and sterilization (NTC 5613 – for the reusable filters) – we perform the bubble point and diffusion tests on filters that have been subjected to multiple sterilization cycles (e.g., autoclaving, gamma irradiation) and repeated use, to evaluate the durability of the filter. We report the bubble point pressure after the cycles and the change.
• Integrity test for virus removal filters (NTC 5614 – for the virus filtration) – we perform a bubble point test and a diffusion test on virus removal filters (e.g., nanofilters) to verify that the filter meets the specified integrity requirements. The bubble point pressure and the diffusion flow rate are reported.

Pore Size Characterization – Determining the Pore Size Distribution

In addition to the maximum pore size (bubble point), we can determine the pore size distribution of a porous material by performing a series of bubble point measurements with different wetting liquids or by combining the bubble point test with a porosimetry method. This provides a more complete characterization of the porous structure.

• Pore size distribution by the bubble point method (NTC 5620 – for the cumulative pore size distribution) – we perform the bubble point test with a controlled increase in pressure, and we record the gas flow rate as a function of the pressure. The flow rate vs. pressure curve is used to calculate the pore size distribution (the volume of pores in different size ranges). We report the pore size distribution curve and the mean pore size.
• Pore size distribution by the mercury intrusion method (ASTM D4404 / NTC 5621 – for the complementary analysis) – we use mercury intrusion porosimetry (MIP) to measure the pore size distribution over a wider range (from 0.003 μm to 100 μm). The results are correlated with the bubble point data. We report the pore size distribution and the cumulative pore volume.
• Porosity and void fraction measurement (NTC 5622 – for the structural characterization) – we measure the porosity (the fraction of the total volume that is pores) and the void fraction of the porous material, using the bulk density and the true density (by helium pycnometry). We report the porosity (in %) and the void fraction.
• Permeability and flow resistance measurement (NTC 5623 – for the flow performance) – we measure the permeability (the gas or liquid flow rate per unit area per unit pressure drop) of the porous material, using a permeameter. The permeability is correlated with the pore size and the porosity. We report the permeability (in Darcy or m²).
• Pore size and bubble point correlation (NTC 5624 – for the quality control) – we correlate the measured bubble point pressure with the pore size distribution data to establish a relationship that can be used for the routine quality control of the filter production. We report the correlation and the calibration curve.

Environmental and Aging Effects on Bubble Point – Evaluating Durability and Performance

The bubble point pressure and the pore size of porous materials can change over time due to chemical attack, thermal aging, mechanical stress, and fouling. Our environmental and aging tests evaluate the long‑term stability of the bubble point, ensuring the reliability of the filter over its service life in the diverse Croatian climate (coastal, continental, and mountainous).

• Chemical resistance and compatibility (ASTM D543 / NTC 5630 – for the chemical exposure) – we immerse the porous material in various chemicals (e.g., acids, bases, solvents, and process fluids) for a specified duration (e.g., 7 days) and then measure the bubble point pressure. The change in the bubble point indicates the chemical degradation of the material. We report the bubble point after chemical exposure and the compatibility rating.
• Thermal aging effect on bubble point (ASTM D573 / ISO 188 / NTC 5631 – for the heat‑aged materials) – we age the porous material in an oven at a specified temperature (e.g., 70 °C, 100 °C) for a specified duration (e.g., 7, 14, or 28 days), and then we re‑measure the bubble point pressure. We report the bubble point after aging and the change.
• Hydrolytic stability and moisture effect (NTC 5632 – for the moisture‑exposed materials) – we expose the porous material to a high‑humidity environment (e.g., 40 °C, 95 % RH) or to water immersion for a specified duration (e.g., 7 days), and then we re‑measure the bubble point pressure. We report the bubble point after humidity exposure and the moisture uptake.
• Mechanical stress and fatigue effect (NTC 5633 – for the cyclic loading) – we subject the porous material to repeated mechanical stress (e.g., compression, flexing, or pressure cycling) and then we re‑measure the bubble point pressure. We report the bubble point after mechanical stress and the change.
• Fouling and clogging effect (NTC 5634 – for the service‑life assessment) – we simulate the fouling of the filter by passing a particle‑laden fluid through the filter, and then we measure the bubble point pressure and the flow resistance. The bubble point pressure and the flow resistance are used to assess the remaining life of the filter. We report the bubble point after fouling and the residual life.

Complementary Tests – Mechanical Properties, Thickness, and Surface Characterization

To provide a comprehensive assessment of the porous material's quality and to understand the factors that influence the bubble point, we perform complementary tests, including thickness measurement, tensile strength, and surface characterization.

• Thickness measurement (ASTM D1777 / NTC 5640 – for the material thickness) – we measure the thickness of the porous material (in mm) using a thickness gauge under a specified pressure. The thickness is correlated with the bubble point pressure. We report the average thickness and the thickness variation.
• Tensile strength and elongation (ASTM D638 / ISO 527 / NTC 5641 – for the mechanical strength) – we measure the tensile strength (in MPa) and the elongation at break (in %) of the porous material. The tensile strength is correlated with the durability and the resistance to tear. We report the tensile strength and the elongation.
• Surface roughness and texture (ASTM D7127 / NTC 5642 – for the surface characterization) – we measure the surface roughness (Ra, Rz) of the porous material using a profilometer. The surface roughness is correlated with the flow resistance and the particle capture efficiency. We report the roughness values.
• Chemical composition analysis (FTIR, XRF – NTC 5643 – for the material identification) – we use Fourier‑transform infrared spectroscopy (FTIR) and X‑ray fluorescence (XRF) to identify the chemical composition of the porous material and to detect any contamination or degradation. We report the material identification and the chemical changes.
• Microscopic examination (SEM – ASTM E1508 / NTC 5644 – for the pore morphology) – we use scanning electron microscopy (SEM) to examine the pore morphology (the shape, the size, and the distribution of the pores) of the porous material. The SEM images are correlated with the bubble point and the pore size distribution. We report the SEM images and the pore morphology.

Test Report and Recognition in the Croatian Pharmaceutical, Food, and Industrial Sector

All procedures described are within the scope of our ISO/IEC 17025 accreditation, with equipment calibrated periodically (bubble point test apparatus, flow meters, pressure transducers, and analytical instruments) and traceability to international standards (NIST, PTB). Our test reports are issued in English (with a Croatian summary available on request) and include:

• Full identification of the porous material (manufacturer, product name, material type, and dimensions).
• Detailed description of the test methods applied (ASTM/ISO/BS/HRN EN/NTC standards, wetting liquid, test temperature, and pressure ramp rate).
• Numerical results: bubble point pressure (bar), maximum pore size (μm), diffusion flow rate (mL/min), porosity (%), thickness (mm), tensile strength (MPa), and property retention after aging (%).
• Graphical data: flow rate vs. pressure curves, pore size distribution curves, and bubble point vs. temperature curves.
• Comparative tables against the values specified by the client or against the limits of the relevant standards (ASTM F316, ISO 4003, HRN EN ISO 4003, and the requirements of the HZN, HALMED, and Ministarstvo gospodarstva).
• Statement of compliance with the pharmaceutical and industrial regulations (e.g., EU GMP, HALMED guidelines).
• Photographs and micrographs (SEM) of the porous material and the pore structure.
• Recommendations for material selection, filter sizing, and quality control measures to achieve the required bubble point and integrity.
• Expanded uncertainty (k=2) for all key measurements, calculated according to the ISO/IEC 98‑3 Guide.

These reports are fully accepted by the Hrvatski zavod za norme (HZN) for the verification of product conformity, by the Ministarstvo gospodarstva i održivog razvoja for industrial and environmental compliance, by the Državni inspektorat for market surveillance, by the Agencija za lijekove i medicinske proizvode (HALMED) for pharmaceutical and medical device certification, and by the Carinska uprava (Croatian Customs) for tariff classification and quality verification in the import of filters and porous materials. Additionally, we offer consulting services for the selection of filter media, the validation of filtration processes, and the implementation of quality control programs for pore size and integrity, contributing to the safety, purity, and reliability of products in the diverse and growing Croatian market, from the pharmaceutical and biotechnology sectors to the food, beverage, chemical, and water treatment industries.