Overview
ISO 4793:1980 defines a standardized system for grading, classifying and designating laboratory sintered (fritted) filters by their porosity. Applicable to filters made of glass, vitreous silica, ceramics, metals and plastics, the standard divides filters into eight porosity classes (designated P1.6 through P250) based on a measured pore size index (expressed in micrometres). It also includes test procedures for assessing filter uniformity and measuring air permeability (Annexes A, B and C).
Key topics and requirements
- Porosity grading and designation: Filters are assigned to one of eight classes. The grade designation uses the letter P followed by the upper limit of the pore size index in micrometres (for example, P16).
- Pore size index (Annex A): A bubble-pressure method measures the air pressure at which the first air bubble breaks free from a wetted filter; this pressure is converted to an equivalent pore size index. The method specifies choice of test liquids (e.g., water, isopropanol, lower surface‑tension liquids) and precautions for wettability and chemical reactivity.
- Uniformity assessment (Annex B): After the initial bubble-pressure reading, the filter can be checked for uniform pore distribution across the working area by increasing pressure and observing bubble emergence across the surface.
- Air permeability (Annex C): A flow measurement at a specified pressure differential (e.g., 1 mbar) determines permeability. The standard provides methods to calculate air flow per unit area and requires minimum permeability values by grade (see Annex C / Table 2).
- Quality expectations: Filters must not shed particles under normal use and should withstand a working pressure differential of 1.013 bar. Manufacturers are recommended to aim for pore size indexes near the centre of each grade range.
Applications
ISO 4793:1980 is used to:
- Select appropriate fritted laboratory filters for sample preparation, filtration and analytical workflows.
- Define acceptance criteria and quality control tests for production of sintered filters (glass, ceramic, metal, plastic).
- Specify filter performance (pore size index, uniformity, permeability) in procurement documents and technical datasheets.
- Support laboratory equipment manufacturers and filtration accessory suppliers in consistent product classification.
Who should use this standard
- Laboratory managers and analytical chemists specifying filtration media
- Manufacturers of laboratory glassware and sintered/fritted filters
- Quality control and R&D teams developing porous filtration products
- Purchasers and procurement specialists preparing technical specifications
Related standards
Refer to other ISO standards covering laboratory glassware, porous materials and test methods for fluid flow and filtration for complementary requirements and more detailed test procedures.