ISO 21501-2:2019

Overview

ISO 21501-2:2019 specifies calibration and verification methods for light scattering liquid-borne particle counters (LSLPCs) that measure particle size and number concentration of particles suspended in liquids. The standard covers single-particle light scattering measurements typically in the 0.1 µm to 10 µm size range and is intended to support reproducible, traceable measurements used to evaluate the cleanliness of pure water, chemicals and other liquids.

Key topics and technical requirements

The standard sets performance requirements and test procedures for LSLPCs, including:

• Measurement principle: single-particle detection by light scattering; pulse height relates to scattered light intensity and thus to particle size.
• Calibration particles: use of monodisperse, spherical calibration particles (e.g., PSL) with SI-traceable sizes and known refractive index (~1.59 at 589 nm).
• Size setting error: maximum permissible error (MPE) for size setting is 0.15 (15% of specified size).
• Counting efficiency: required ranges are 0.20–0.80 for particles near the minimum detectable size, and 0.70–1.30 for particles 1.5–3× the minimum detectable size.
• Size resolution: MPE ≤ 0.10 (10% of specified particle size) when evaluated with certified calibration particles.
• False count: determination of false counts per litre and 95% upper confidence limit (UCL); UCL must meet the manufacturer’s specified limit.
• Maximum particle number concentration: manufacturer-specified maximum; coincidence loss at this concentration ≤ 0.1 (10%).
• Sampling errors: evaluation of sampling flow rate, sampling time and sampling volume errors.
• Calibration interval and reporting: requirements for specifying calibration intervals and standardized reporting of test and calibration results.
• Uncertainty evaluation: guidance provided (Annex D) on estimating uncertainties of performance tests; additional informative annexes cover counting efficiency, size resolution and false count.

Note: measured optical particle size depends on the refractive index of particle and medium - results are equivalent to the calibration particles in pure water.

Applications and who uses this standard

ISO 21501-2:2019 is used by:

• Instrument manufacturers for design verification and pre-shipment calibration of LSLPCs.
• Calibration and testing laboratories establishing traceable, repeatable calibration procedures.
• Quality control and contamination-control engineers in industries such as pharmaceuticals, biotechnology, semiconductor manufacturing, precision machining and medical devices.
• Cleanroom and ultrapure-water monitoring programs where liquid-borne particle counts and size distributions are critical.

Practical benefits include improved measurement comparability between instruments, minimized inaccuracies in particle counting/size assignment, and standardized reporting for regulatory and quality systems.

Related standards

• Other parts of the ISO 21501 series (particle size distribution - single particle light interaction methods).
• Technical work by ISO/TC 24 (Particle characterization).