A working reference on the international calibration standard for optical particle counters — the NIST primary-fluid history, the µm(b)/µm(c) reporting question, and what ISO 11171:2022 means for the commercial oil-analysis laboratory.
ISO 11171 — full title “Hydraulic fluid power — Calibration of automatic particle counters for liquids” — is the international procedure for sizing-calibrating optical particle counters used in lubricant and hydraulic-fluid cleanliness reporting. It defines two procedures: factory calibration (Annexes A through E, covering sensor noise determination, coincidence-error verification, flow-rate range, resolution at the 10µm peak, and accuracy verification against an Ultra Fine Test Dust table) and laboratory sizing calibration (Section 6, the procedure customer labs run annually using a NIST-traceable secondary standard).
The standard exists because optical particle counters need a defined particle-size reference. A counter’s sensor produces a voltage pulse when a particle blocks the laser. The amplitude of that pulse maps to a size only through a calibrated lookup table. That table has to be built against a reference material with known particle counts at known sizes — otherwise different labs reading the same sample on different instruments would produce different ISO 4406 codes. ISO 11171 defines the reference material (NIST-traceable Medium Test Dust) and the procedure for tying the instrument’s channel table to it.
The standard has been revised multiple times. Current is ISO 11171:2022, which replaced the 2020 and prior revisions. The major changes across revisions track the evolution of the NIST primary calibration fluid — specifically, the SRM 2806 family.
Why the calibration fluid changed, what it meant for your particle counts, and where things stand today.
| Year | NIST batch | What changed and why it mattered |
|---|---|---|
| 1997–2004 | SRM 2806 | The first NIST-traceable ISO MTD primary calibration fluid, replacing the discontinued ACFTD (Air Cleaner Fine Test Dust). Particle sizes redefined from 2/5/15µm (ACFTD) to 4/6/14µm(c) (MTD). This was the move from ISO 4402 to ISO 11171 calibration. |
| 2013 | SRM 2806a | New batch, certified by NIST using Filter Patch / ESM imaging. Statistically equivalent counts to original 2806 — labs saw no change in their reported codes. |
| 2014–2020 | SRM 2806b | New batch, certified by Filter Patch / SEM imaging. Certified counts ran 49% higher at >4µm, 45% higher at >6µm, and 86% higher at >14µm than 2806a. About 6% was explained by a slightly higher dust concentration in the new batch; the remaining 40–75% was attributed to a certification “error” in the prior 2806a, not to anything physical changing in customer fluids. Real customer ISO codes jumped 1–2 ranges industry-wide — not because oils got dirtier, but because the calibration reference shifted. |
| 2016 | ISO 11171:2016 | The standards committee’s fix for the 2806b jump. The 2016 revision allowed labs to report in either µm(b) (counts as measured against 2806b) or µm(c) (counts converted back to the equivalent 2806a curve using a fixed factor dc = 0.898 db). Dual reporting bridged the calibration gap. The conversion factor came from a round-robin across 15 laboratories in 4 countries. |
| 2020 | SRM 2806d | New batch, the first NIST particle-count primary certified by interlaboratory study (ILS) rather than single-lab imaging. Statistically aligned with the µm(c) reporting curve. Removed the structural need for µm(b)/µm(c) dual reporting. |
| 2022 | ISO 11171:2022 | Discontinued µm(b) reporting entirely. All compliant reporting is now in µm(c). ISO TC131/SC6 committee position: the dual-reporting era is over. |
| 2025–2026 | SRM 2806e & 2806f | Next two batches, certified by ILS. CINRG was one of 14 participating sensors across 4 countries. SRM 2806e is the next primary in current rotation; 2806f to follow when 2806e supply runs out. CINSTAN secondary fluids will be traceable to whichever NIST batch is current. |
The simple way to remember the arc: 2806 and 2806a are equivalent; 2806b jumped counts dramatically; ISO 11171:2016 patched the jump with dual-reporting; 2806d restored alignment with the µm(c) scale; ISO 11171:2022 retired µm(b). All current labs should be reporting in µm(c) only, with calibration traceable to NIST SRM 2806d (or successor 2806e/f).
Both µm(b) and µm(c) are ways of describing the same physical particle — what differs is the reference scale used to measure it.
The MTD test dust is irregular; every particle has multiple dimensions you could call its "size." NIST’s certification work measures the dust particles and assigns each size channel a count. The question is how those particle dimensions are measured. Different imaging methods (SEM with one analysis technique, filter-patch electron-microscopy with another) produced slightly different size assignments for the same physical particles — which is why the 2806a and 2806b certifications differed even though the dust itself was nominally the same.
µm(c) sizes are tied to the 2806a / 2806d certifications (where "c" historically referred to the cross-section-area-equivalent diameter convention). µm(b) sizes are tied to the 2806b certification. The fixed conversion factor dc = 0.898 db means — for example — that a particle reported as 4µm(b) corresponds to about 4.45µm(c). Same particle, different reference scale.
For a practical lab, this matters in one specific way: when you read an old report from 2014–2020 with µm(b) values, those numbers are not directly comparable to modern µm(c) reports without applying the conversion. ISO 11171:2022 simplified the going-forward picture — all reports are µm(c), full stop — but historical data still needs translation.
For a commercial oil-analysis laboratory, ISO 11171:2022 compliance comes down to four operational items:
ISO 11171 is "Hydraulic fluid power — Calibration of automatic particle counters for liquids" — the international procedure for sizing-calibrating optical particle counters using a NIST-traceable Medium Test Dust primary standard. It defines factory calibration (Annexes A through E covering sensor noise, coincidence error, flow rate, resolution, and accuracy verification) and the laboratory sizing-calibration in Section 6. The current revision is ISO 11171:2022.
NIST SRM 2806 is a series of NIST-issued Medium Test Dust primary calibration fluids dating to 1997. The series runs SRM 2806 (1997–2004), SRM 2806a (2013), SRM 2806b (2014–2020), SRM 2806d (current, certified 2020), with SRM 2806e and 2806f next in queue. Each revision reflects either a re-certification of existing material or a new batch certified through an interlaboratory study. CINRG participated in the ILS certifying SRM 2806e and 2806f.
µm(c) sizes are based on certified counts traceable to NIST SRM 2806a, 2806d, or the equivalent µm(c) reporting curve. µm(b) was a transitional reporting convention introduced when SRM 2806b certifications produced count data significantly higher than 2806a. ISO 11171:2016 allowed both forms (with a fixed conversion factor dc = 0.898 db); ISO 11171:2022 discontinued µm(b) reporting entirely. All current reporting is in µm(c).
SRM 2806b certified counts ran 49% higher than 2806a at >4µm, 45% higher at >6µm, and 86% higher at >14µm. About 6% of the change was explained by a higher test-dust concentration in the new batch; the remaining 40–75% was attributed to a certification "error" in the prior 2806a — not to anything physical changing in customer fluids. The fix was the dc = 0.898 db conversion in ISO 11171:2016 and then the full SRM 2806d re-certification in 2020. See the OilDoc 2017 paper for the underlying data.
Three things: (1) report particle sizes in µm(c) only — µm(b) reporting is no longer permitted; (2) use a calibration fluid traceable to NIST SRM 2806d (or its successor 2806e/f when issued); (3) confirm your particle counter has been calibrated against the current standard, with annual re-calibration documented per ISO 11171 Section 6. CINRG’s CINSTAN line ships traceable to NIST SRM 2806d with Certificates of Analysis on every shipment.
It needs to be re-calibrated to the current standard. Either re-calibrate using a CINSTAN Calibration Kit (CS-CINSTAN-CFK) traceable to NIST SRM 2806d, or use the dc = 0.898 db conversion to report µm(c) values during an interim period — though the conversion approach is no longer ISO 11171:2022 compliant on its own and is being phased out by accredited labs.
CINSTAN ships traceable to NIST SRM 2806d with a Certificate of Analysis. Calibration Kit, Process Control Standards, and Verification Fluids all on the current scale.
Tell us about your throughput, your test methods, and your facility. A CINRG engineer will help you scope the right configuration — and put you in touch with your nearest dealer.