ISO 8791-2:2013

Overview - ISO 8791-2:2013 (Bendtsen method)

ISO 8791-2:2013 specifies the Bendtsen air‑leak method for determining the roughness (smoothness) of paper and board. The test measures the rate of air flow between a flat circular land and the sample surface at a nominal differential pressure (1.47 kPa). Results are reported as Bendtsen roughness in millilitres per minute (ml/min). The method covers both variable‑area and electronic flowmeter Bendtsen testers and includes requirements for sampling, conditioning, apparatus, calibration and test procedure.

Key topics and technical requirements

• Principle: Clamp the test piece under a circular land and measure air flow at a nominal pressure of 1.47 kPa.
• Applicability ranges:
  • Variable‑area testers: approx. 5–3 000 ml/min.
  • Electronic testers: approx. 50–5 000 ml/min.
• Limitations: Not suitable for very soft papers (land indentation), high‑air‑permeance papers (air passes through sheet) or samples that won’t lie flat.
• Measuring head specification: circular land internal diameter 31.5 mm ± 0.2 mm, land width 0.150 mm ± 0.002 mm, mass 267 g ± 2 g.
• Apparatus components: compressor (clean, oil‑free), pressure stabilizing reservoir, pressure controlling device/manostat, flowmeter (variable‑area or electronic), measuring head, polished flat plate and weight/annulus.
• Flowmeter performance: variable‑area flowmeter ranges and readability defined; electronic flowmeter range 50–5 000 ml/min with error ≤ ±5 ml/min or ±5%. Avoid variable‑area readings above 1 200 ml/min due to pressure drop effects.
• Sampling and conditioning: follow ISO 186 (sampling) and ISO 187 (conditioning & atmosphere). Minimum test piece size 75 × 75 mm; cut at least 10 pieces per side.
• Calibration & verification: procedures and capillary‑tube calibration given (Annex B); care/maintenance guidance (Annex A); precision data (Annex C).

Applications - who uses ISO 8791-2 and why

• Paper and board manufacturers - routine quality control of surface roughness and process monitoring.
• Converters and printers - assessing surface smoothness related to printability, ink lay, coating performance and tactile properties.
• Materials testing laboratories - standardized roughness data for specifications, acceptance testing and R&D.
• Equipment suppliers - to design, calibrate and validate Bendtsen testers and accessories.

Practical benefits include consistent specification of surface properties, comparison across suppliers, and objective criteria for process adjustments and product development.

Related standards

• ISO 8791 series (Part 1 general method, Part 3 Sheffield method, Part 4 Print‑surf method)
• ISO 186 - Sampling to determine average quality
• ISO 187 - Standard atmosphere for conditioning and testing

Keywords: ISO 8791-2:2013, Bendtsen method, Bendtsen roughness, paper roughness, paper smoothness, air leak methods, Bendtsen tester, variable-area flowmeter, electronic flowmeter, paper testing.