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ISO 27891:2015 — Aerosol Particle Number Concentration: CPC Calibration
Standard Methods for Accurate Calibration of Condensation Particle Counters
Introduction to CPC Calibration
ISO 27891:2015 establishes standardized procedures for calibrating condensation particle counters (CPCs) used to measure aerosol particle number concentration. CPCs are fundamental instruments in aerosol science, capable of detecting nanoparticles down to 1 nm by condensing a working fluid onto particles to grow them to optically detectable sizes. Accurate calibration is essential for atmospheric aerosol research, air quality monitoring, and nanoparticle toxicology studies.
CPCs achieve single-particle detection by supersaturating working fluid vapor. The critical supersaturation ratio determines D50 – the minimum detectable particle size, typically 3-10 nm for butanol CPCs and 2.5-5 nm for water-based CPCs.
The standard specifies calibration using reference aerosol electrometry — the most direct primary standard for number concentration. The reference method uses a Faraday cage aerosol detector measuring electrical current from charged particles, providing SI-traceable measurement.
Calibration Procedure and Performance Metrics
| Parameter | Method | Concentration Range | Uncertainty Target |
|---|---|---|---|
| Counting efficiency | Reference electrometer | 1E3-1E5 particles/cm3 | +/- 5% (k=2) |
| D50 detection size | Stepwise DMA scan | 1-100 nm | +/- 0.2 nm |
| Linear range | Dual CPC dilution | 1E0-1E7 particles/cm3 | +/- 10% upper limit |
| Coincidence correction | Pulse width analysis | >1E4 particles/cm3 | +/- 3% correction |
| Response time | Step concentration | T10-T90 | +/- 0.2 s |
The counting efficiency curve follows a sigmoidal function. D50 is defined as the particle diameter at 50% efficiency. For well-calibrated CPCs, efficiency plateaus at 95-100% for particles 2-3 times larger than D50. Calibration requires a minimum of five particle sizes spanning the D50 and plateau regions.
Working fluid purity is critical. Trace impurities in butanol can alter saturation vapor pressure by up to 5%, shifting D50 by 0.3-0.5 nm. Use HPLC-grade butanol (>= 99.9%) and replace filters every 6 months.
Engineering Applications
Atmospheric Aerosol Monitoring Networks
ISO 27891 protocols are referenced by global networks including GAW, ACTRIS, and IMPROVE. Network harmonization requires biennial recalibration with transfer standards ensuring inter-laboratory comparability within +/- 10%.
For field deployments, on-site calibration using a portable CPC as transfer standard is recommended. The transfer standard should be calibrated against a reference electrometer every 12 months.
Frequently Asked Questions
Q: How often should a CPC be recalibrated?
Every 12 months per standard. For critical applications, every 6 months. Factory recalibration is required after major maintenance.
Every 12 months per standard. For critical applications, every 6 months. Factory recalibration is required after major maintenance.
Q: Difference between butanol and water-based CPCs?
Butanol CPCs achieve lower D50 (down to 1 nm) but require careful handling. Water-based CPCs are more environmentally friendly with higher D50 (2.5-5 nm).
Butanol CPCs achieve lower D50 (down to 1 nm) but require careful handling. Water-based CPCs are more environmentally friendly with higher D50 (2.5-5 nm).
Q: Can ISO 27891 be applied to CPCs with different working fluids?
Yes, the methodology is working-fluid-independent. However, the reference method must account for differences in particle activation and growth kinetics.
Yes, the methodology is working-fluid-independent. However, the reference method must account for differences in particle activation and growth kinetics.
