The Calibration of Counters According to the ISO11171-2010 Standard

The official release of the ISO 11171 Standard in December 1999, offered the hydraulic particle counting sector clear methodology by which a suitable hydraulic particle counter could be characterized completely and calibrated with NIST traceable calibration suspensions and equipment.

The Table 1 in the ISO Standard document explained the elements needed for calibration or characterization. The term ‘characterized’ for this article will be defined as determining the performance of the instrument. The term ‘Calibrated’ will mean meeting pass/fail criteria or satisfying a specification.

  • Clause 6: Sizing calibration procedure (Calibration)
  • Annex A: Preliminary Instrument check (Characterization)
  • Annex B: Coincidence Error Limits (Characterization)
  • Annex C: Flow Rate Limits (Calibration and Characterization)
  • Annex D: Resolution (Calibration)
  • Annex E: Accuracy (Calibration)

The ISO 11171 standard is a common platform, which specifies a formal technique that ensures the counter is accurately calibrated. Manufacturers who perform this process must use the same yardstick for comparison and require spec-manship to take a rear seat to measured performance.

The certification details and resultant data package detail the functional attributes to the customer to ensure they do not use the instrument outside of its performance capabilities and anticipate a reliable and useful test result.

ISO 11171-2010 Modifications

The modifications provided in ISO 11171-2010 are:

  • The specific sequential steps to be followed during calibration are not mandated any longer
  • There are new guidelines to detect change in APC calibration
  • The new standard needs that minimum of 5000 particles are required to be measured to have statistically valid measurement for any particular channel setting
  • ISO11171-2010 does not any more require that a 0% concentration sample be analyzed.
  • The updated standard differentiates between adjustable and fixed flow rate instruments. Fixed flow rate instruments are needed to reliably offer a constant flow rate within ± 3%. For adjustable flow rate instruments, it is necessary that their working flow rate and flow rate limits be determined, in addition to their ability to consistently deliver a constant flow rate within ± 3%.
  • D.12 increases the acceptable 10µm resolution specification from 10% to 15%. With the help of an NIST traceable 10µm particle standard (with a COV less than 5%) the acceptance criteria was increased from < 10% @ 10µm to < 15% @ 10µm. Furthermore, the difference between RR and RL must now not exceed 5%.
  • According to section E.3, NIST RM 8632 dust can be used for verifying counting accuracy.
  • Annex H of the standard includes an example of the method of construction of a calibration curve.

Summary Table to show differences between 11171-1999 and 2010

The summary table 1 showing the differences between 11171-1999 and 2010 is given below:

Table 1. Differences between ISO 11171-1999 and 2010.

Standard ISO 11171:1999 ISO 11171:2010
Primary Calibration Suspension ISO MTD or ISO 12103-A3 NIST SRM 2806
Sequence of Calibration steps mandated not mandated
Minimum Count requirement No minimum requirement 5000 Count minimum per channel setting
Size larger than 50µm(c) Refer to ASTM F658-87 Refer to ISO 21501-3
Coincidence 0% concentration required (OK to use UFTD) 0% concentration sample is not required (must use RM 8632)
Flow Rate N/A ± 3%
Flow Rate Limit OK to use UFTD Must use RM 8632
Resolution ([email protected]µm) < 10% @ 10µm < 15% @ 10µm and RL-RR < 5%
Counting Accuracy Suspension OK to use UFTD Must use RM 8632


In this article the modifications made to the 11171-1999 standards have been enumerated. The changes may be very small, however are critical for enabling a meaningful and useful process to characterize and calibrate the hydraulic fluid Particle Counter.

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.

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