ISO 15715:2025

Overview

ISO 15715:2025 - Binders for paints and varnishes - Determination of turbidity specifies an instrumental nephelometric method for measuring the turbidity (loss of transparency due to undissolved matter) of clear liquids used in coatings. It applies to resins and resin solutions, solvents, clear coating materials, monomers and any liquid where clarity is specified. The method produces objective turbidity values expressed in formazin nephelometric units (FNU).

Key topics and technical requirements

• Measurement principle: Turbidity is determined by measuring light scattering from a sample; the method uses a ratio (0°/90°) to derive turbidity insensitive to colour when measured correctly.
• Light source and optics:
  • Nominal wavelength: 860 ± 30 nm
  • Spectral bandwidth: ≤ 60 nm
  • Beam convergence: ≤ 1.5°
  • Measuring angle: 90° ± 2.5° (instrument-to-instrument consistency matters)
  • Aperture angle in the sample: 20°–30°
• Measurement cells:
  • Use manufacturer-specified optical cells or disposable cylindrical glass tubes
  • If disposable tubes are used: optical path length 11.0 mm to 25.4 mm and provisions for centring the tube are required
• Calibration:
  • Calibrate with formazin standards prepared per Annex A or with commercial stabilized formazin suspensions
  • Commercial standards: stable up to one year if stored at room temperature in the dark
  • Repeat calibration frequently; note that 0 FNU is not physically attainable
• Sample handling:
  • Condition to room temperature, avoid or remove air bubbles (ultrasonic, centrifuge, warming) and verify treatments do not alter turbidity
  • Clean and center cells/tubes before measurement
• Expression of results:
  • Report in FNU with defined rounding: