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Improving Measurement Quality of Solid Particle Number Counters
Since the first particulate mass standards for heavy-duty engines, emissions have dropped dramatically. With modern aftertreatment, particulate filters push tailpipe concentrations to extremely low levels. The solid particle number (SPN) counter has become essential for both research and certification. However, measurement quality depends on proper setup, understanding particle physics, and correct instrumentation. This article summarizes key practices from SAE J3160™ (2020) to help engineers achieve reliable and consistent SPN measurements.
🔍 Understanding the Challenges in SPN Measurement
Accurate solid particle counting requires control of several physical phenomena. Two critical issues are particle coagulation and thermophoretic loss.
Particle Coagulation occurs when particles collide and merge, reducing number concentration. This effect becomes significant at high concentrations, typically above ~10⁴ particles/cm³ after dilution. Operating the counter above its linear range leads to underestimation of particle number. Proper dilution and knowledge of the counter’s linear range are essential.
Thermophoretic Loss happens when particles in a hot gas migrate toward cooler walls due to temperature gradients. Sample lines must be designed to minimize temperature differences. Heated lines and appropriate flow rates help keep losses below 10 % as recommended by J3160.
| Diluter Type | Key Characteristics |
|---|---|
| Ejector Pump | Simple design, uses compressed air; fixed dilution ratio (DR) range. |
| Rotating Disc | Wide DR control; robust for high concentrations; moderate particle loss. |
| Perforated Tube | Low particle loss; suitable for raw exhaust sampling. |
| Wide Range Continuous Diluter (WRCD) | Continuous variable DR; high precision; good for transient testing. |
🛠️ Design Insight: Proper diluter selection depends on the application. For raw measurement, a perforated tube or rotating disc diluter offers low particle loss. For dilute measurement, ejector pumps are common but careful calibration is needed to avoid altering the particle size distribution.
Recommended Practices for Raw and Dilute Measurement
J3160 distinguishes between two main sampling approaches:
- Raw Measurement: Sample taken directly from the tailpipe. Requires heated lines to prevent condensation and thermophoretic loss. Dilution is usually done in one or two stages. Avoid over-dilution to maintain enough particles for counting.
- Dilute Measurement: Sample taken from a dilution tunnel (e.g., CVS). The exhaust is already mixed with clean air, reducing concentration and temperature. However, care must be taken to avoid particle losses in the tunnel and transfer lines.
Altitude and high exhaust back-pressure also affect measurement. At high altitude, lower ambient pressure changes dilution ratios and counter performance—correction factors or specific protocols are needed. High back-pressure can alter exhaust flow and temperature, influencing particle formation and measurement.
⚠️ Common Mistake: Operating the particle counter above its linear range. Always verify that the concentration after dilution is within the instrument’s specified range to avoid coagulation errors.
Instrumentation and Validation
Key components of an SPN measurement system include the diluter, volatile particle remover (VPR), and condensation particle counter (CPC).
Diluters: As shown in the table above, each type has strengths. The Wide Range Continuous Diluter (WRCD) is recommended for transient cycles because it can adjust the dilution ratio in real time.
Volatile Particle Remover: To measure only solid particles, volatile components (hydrocarbons, sulfates) must be removed. The evaporation tube is the most common VPR: it heats the sample to ~300 °C to vaporize volatiles, which are then removed by dilution. Thermodenuders and catalytic strippers are alternatives, but the evaporation tube is preferred for its simplicity and low cost.
Validation Checks: Daily performance checks include system zeroing and a particle counter response check. Periodic gas dilution and aerosol particle penetration (PCRF) checks ensure the system maintains calibration. J3160 provides detailed procedures for these checks.
Frequently Asked Questions
What is the recommended linear range for a CPC in SPN measurements?
Most CPCs used for SPN have an upper limit around 10⁴ particles/cm³. Above that, coincidence and coagulation cause significant errors. Always check the manufacturer’s specification and, when in doubt, use a lower dilution ratio.
How can I mitigate thermophoretic losses in my sampling system?
Use heated sample lines kept within ±10 °C of the exhaust temperature. Minimize line length and avoid sharp bends. Flow rates should be high enough to reduce residence time but not so high that they affect dilution.
Why is volatile particle removal necessary?
Without removal, volatile particles can nucleate and form new particles or condense on solid particles, biasing the count. The VPR ensures that the measurement reflects only the solid (non‑volatile) particle number, which is what regulations target.
What altitude effects are most important for SPN counters?
Lower atmospheric pressure changes the compression ratio in diluters (ejector pumps) and the expansion in CPCs. This can alter dilution ratio and counting efficiency. J3160 recommends using pressure‑corrected dilution ratios or performing verification at the test altitude.
By following the practices in SAE J3160, engineers can improve the quality and repeatability of solid particle number measurements, supporting both development and compliance efforts.
