ISO 8791-5:2020

Overview

ISO 8791-5:2020 specifies the Oken method for determining the roughness/smoothness of paper and board using air-leak techniques. Applicable to all printing papers and boards, this part of the ISO 8791 series defines the Oken smoothness measurement (expressed in seconds per 100 ml (s/100 ml)), the required apparatus, sampling, conditioning, calibration and test procedures for reproducible smoothness testing.

Key topics and technical requirements

• Measurement principle: Time for a fixed air volume to leak between measuring lands and the test surface is converted to Oken smoothness (see Annex A for principle).
• Apparatus types: Two permitted Oken tester configurations:
  • Water column type - uses water-column manometer, capillary (≈0.3 mm ID, ≈50 mm length) and scale plate; constant pressure ≈4.90 kPa (500 mmH2O).
  • Electronic sensor type - uses an electronic pressure sensor and digital display to report pressure and Oken smoothness.
• Measuring head: Loading weight and rubber plate (≥48 mm diameter) apply a clamping pressure of (98.1 ± 1.0) kPa. Rubber plate hardness: (50 ± 5) IRHD; rebound resilience ≥62%.
• Scale and calibration: Capillary length adjusted so constant K = 100; scale covers 0 s to ∞ s/100 ml. Calibration checks: zero reading 0 ± 0.01 kPa and leakage test pressure > 4.88 kPa (water-type adjustments described).
• Sampling & test pieces: Sample according to ISO 186; condition as per ISO 187. Prepare ≥10 test pieces per side (recommended 100 mm × 100 mm). Readings stabilized on scale/digital display; report mean smoothness to two significant figures.
• Precision & reporting: Annex B contains precision data. Standard deviation and coefficient of variation can be calculated when required.

Practical applications

• Quality control of paper and board surface properties for printing performance and coating processes.
• Supplier and incoming inspection of printing papers and boards where surface smoothness affects ink laydown, print uniformity and gloss.
• Research and development of paper coatings, calendering processes, and surface treatments.
• Acceptance testing in manufacturing, conversion and print-house environments.

Who should use this standard

• Paper and board manufacturers and mill laboratories
• Independent testing laboratories and certification bodies
• Printers, converters and packaging producers concerned with print quality
• Instrument manufacturers producing Oken testers and testing equipment

Related standards

• ISO 8791 series (other air-leak methods)
• ISO 186 (Sampling)
• ISO 187 (Conditioning)
• ISO 48 and ISO 4662 (rubber specimen properties used in measuring head)

Keywords: ISO 8791-5:2020, Oken method, paper smoothness, Oken smoothness, roughness testing, Oken tester, air leak methods, printing papers and boards.