Calibration and Data Reporting Formats

For particle counter calibration, ISO 11171 refers to the NIST traceable procedure whereas ISO 4402 refers to the old ACFTD method. For example, ISO 11171 particle size data is expressed as “X pro(c)”, where ISO 4402 data was presented as “x pm” (x = particle size). What’s important to note is particle sizes acquired with ISO 11171 calibration are not the same as those obtained with ISO 4402.

Particle Size Measurements

ACFTD Sizes (µm) ISO 4402: 1991	NIST-Calibrated Sizes µm(c) ISO 11171
<1.0	4.0
1.0	4.2
2.0	4.6
2.7	5.1
3.0	5.1
4.3	6.0
5.0	6.4
7.0	7.7
10.0	9.8
12.0	11.3
15.0	13.6
15.5	14.0
20.0	17.5
25.0	21.2
30.0	24.9
40.0	31.7
50.0	38.2
100.0	70.0

The ISO technical committee adopted a revised procedure for reporting fluid-cleanliness measurements calibrated with the NIST-traceable method. ISO 4406:1999 uses three-code numbers corresponding to concentrations of particles larger than 4 µm(c), 6 µm(c)and 14 µm(c). The 1999 sizes matched up closely with the ISO4406 coding-system sizes (5 µm & 15 µm), once measured with ACFTD calibrated particle counters. However, the 4 µm(c) measurement equates to ~1 µm size if the ACFTD calibration procedure had been used.

Fluid cleanliness is now characterized from 0.01 - 2,500,000 particles/mL in three bin ranges with a series of numerical codes to indicate the count in each bin:

• First code indicates particle count above 4 microns
• Second code indicates particle count above 6 microns
• Third code indicates particle count above 14 microns

Primary particle-sizing calibration is currently performed using NIST SRM 2806b suspensions with particle size distribution certified by the US National Institute of Standards and Technology (NIST). A secondary calibration method with traceability to NIST uses ISO MTD suspensions which are independently analyzed with a liquid particle counter calibrated using the primary method. Concentration limits are determined by serial dilutions of a concentrated suspension.