TEST METHOD

Gravimetric Analysis vs. Particle Counting

Two methods for measuring contamination levels in hydraulic and lubricating fluids

ENGINEERING OBJECTIVE

Select the appropriate contamination measurement method based on required data type (total mass, size distribution, or count), sensitivity, and whether ISO 4406 cleanliness codes or bulk contamination mass is the required output.

COMPARISON SCOPE

Covers gravimetric analysis (ISO 4405) and automatic particle counting (ISO 11171, ISO 4406) for hydraulic and lube oil fluid contamination measurement. Does not cover ferrographic analysis, spectrometric oil analysis (SOAP), or biological contamination testing.

GOVERNING STANDARDS

ISO 4405:1991ISO 11171:2010ISO 4406:2021ISO 16889:2022

OPTION DEFINITIONS

AGravimetric Analysis

Fluid is filtered through a membrane filter of known weight, dried, and reweighed. The contamination mass per unit volume is calculated as (final weight − initial weight) / fluid volume, expressed in mg/L or mg/100 mL. ISO 4405 defines the procedure for hydraulic fluids. Gravimetric analysis captures the total mass of particles above the membrane pore size (typically 0.45–1.2 µm) but provides no information about particle size distribution, particle count, or the ISO 4406 cleanliness code.

ADVANTAGES

+Simple, low-cost equipment — calibrated analytical balance and membrane filters; no particle counter required

+Measures total contamination mass including particle types that scatter light differently and may undercount in optical APCs

+Standard method for filter element dirt holding capacity measurement (ISO 16889 gravimetric weighing of element)

+Valid for fluids with high light absorbance or fluorescence that interfere with optical particle counting

+Historical measurement method — large legacy database of mg/L values for comparison

+Captures both hard particles and soft contamination (gel, sludge, varnish) that may not be counted accurately by APC

LIMITATIONS

−Does not produce particle size distribution data — cannot calculate ISO 4406 cleanliness codes

−Cannot distinguish between a few large particles and many fine particles of the same total mass — fundamentally different contamination profiles give identical gravimetric results

−Minimum detectable mass depends on balance sensitivity — not suitable for ultra-clean fluids at ISO 14/12/9 or cleaner

−Slow procedure — membrane preparation, filtration, drying, and weighing takes hours vs. minutes for APC

−Single-use membrane filter consumables — cost adds up in high-frequency monitoring programmes

−Cannot be used in-line or continuously — offline analysis only

TYPICAL APPLICATIONS

·Filter element dirt holding capacity measurement per ISO 16889 (element weighing before and after test)

·Bulk contamination measurement for oil analysis when ISO 4406 coding is not required

·Fluids with high optical opacity (some cutting fluids, dark oils) that defeat optical particle counters

·Total contamination audit where mass (not count) is the required metric

·Historical legacy systems where gravimetric data is the existing baseline

BAutomatic Particle Counting (APC)

An ISO 11171:2010-calibrated light-extinction optical particle counter (APC) counts and sizes individual particles in a known volume of fluid. Counts are reported per mL at specified size thresholds (≥4 µm(c), ≥6 µm(c), ≥14 µm(c) per ISO 4406:2021). The three-threshold counts are converted to ISO 4406 range codes. APCs can measure continuously at high flow rates, enabling on-line monitoring. ISO 11171 calibration with PSL spheres ensures inter-laboratory comparability. Particle size distribution data enables identification of contamination sources and degradation mechanisms.

ADVANTAGES

+Produces ISO 4406 cleanliness codes directly — required for component protection specifications

+Particle size distribution data reveals contamination source (abrasive silica, metallic wear, seal degradation particles have different size distributions)

+Fast results — modern APCs deliver results in minutes; on-line APCs provide continuous monitoring

+Detects contamination at concentrations below gravimetric detection limits — suitable for clean systems at ISO 14/12/9 and cleaner

+On-line monitoring capability — APC can be installed in-line to detect contamination events in real time

+ISO 11171 calibration ensures global inter-laboratory comparability — same fluid measured at any certified lab gives equivalent codes

+Required for ISO 16889 filter element performance testing (Beta ratio measurement)

LIMITATIONS

−Requires ISO 11171-calibrated instrument — capital cost significantly higher than gravimetric balance

−Optical extinction method is sensitive to fluid opacity — dark or heavily contaminated fluids require dilution before measurement, adding procedural steps

−Air bubbles and water droplets are counted as particles — sample handling to remove aeration is critical

−Counts particles (counts/mL) but not their mass — total contamination mass cannot be derived from APC data alone

−Soft particles (gel, varnish, elastomer fragments) may be counted inconsistently vs. hard particulate of the same size

−Calibration drift without regular PSL recalibration leads to erroneous cleanliness codes

TYPICAL APPLICATIONS

·ISO 4406 cleanliness code measurement for hydraulic system condition monitoring

·ISO 16889 filter element Beta ratio testing (mandatory APC method)

·On-line hydraulic system contamination monitoring with particle count alarms

·Fluid cleanliness acceptance testing for new systems and after flushing

·Condition monitoring for predictive maintenance programmes

·Component protection verification for proportional and servo valve systems

ENGINEERING COMPARISON MATRIX

DIMENSION	A — Gravimetric Analysis	B — Automatic Particle Counting (APC)
Output data type	Total mass [mg/L or mg/100 mL]	Particle count per mL by size threshold
ISO 4406 code output	Not possible — no particle size data	Direct output — count at ≥4, ≥6, ≥14 µm(c)
Particle size data	None — mass only	Full size distribution (size spectrum)
Calibration requirement	Calibrated analytical balance per ISO 4405	ISO 11171:2010 PSL calibration — mandatory
Minimum detection	Limited by balance sensitivity (~0.1 mg)	Single particle detection to 1 µm(c) and below
Speed	Hours — membrane prep, drying, weighing	Minutes; on-line = continuous real-time
Fluid opacity sensitivity	Not affected by optical properties	Dark/opaque fluids require dilution or alternate sensing
Soft particle detection	Captures gel, varnish, sludge by mass	Soft particles inconsistently counted by optical method
ISO 16889 requirement	Used for element DHC weighing only	Required for Beta ratio measurement

WHEN TO USE A

Gravimetric Analysis

✓Filter element dirt holding capacity measurement per ISO 16889 test procedure

✓Total contamination audit for fluids where optical particle counting is impractical (high opacity)

✓Historical comparison with legacy gravimetric database

✓Contamination mass measurement where ISO 4406 cleanliness codes are not the required output

WHEN NOT TO USE

✗When ISO 4406 cleanliness codes are the required output — gravimetric cannot produce cleanliness codes

✗ISO 16889 Beta ratio testing — APC is required by the standard

✗Ultra-clean fluid verification (ISO 14/12/9 or cleaner) where mass differences are below balance detection limit

WHEN TO USE B

Automatic Particle Counting (APC)

✓All ISO 4406 cleanliness code measurement and verification

✓ISO 16889 filter element Beta ratio performance testing

✓Hydraulic and lube oil system condition monitoring programmes

✓Component protection verification for servo and proportional valve systems

✓Any application requiring particle size distribution data for contamination source diagnosis

WHEN NOT TO USE

✗Heavily contaminated or opaque fluids without dilution capability — aeration and opacity cause counting errors

✗When total contamination mass (not count) is the required metric for comparison with historical gravimetric data

✗Environments where APC calibration maintenance cannot be sustained — uncalibrated APC data is not ISO 4406 compliant

ENGINEERING IMPLICATIONS

01ISO 4406 cleanliness codes can only be assigned by APC — gravimetric analysis does not produce cleanliness codes; systems specified by ISO 4406 targets must use APC for verification.

02For filter element performance testing per ISO 16889, APC is mandatory — gravimetric measurement is used only for dirt holding capacity (element weight) determination, not for Beta ratio.

03In dark or heavily aerated fluids where APC accuracy is questionable, gravimetric analysis may be the more reliable contamination mass indicator, but it cannot substitute for APC in cleanliness-code systems.

04When troubleshooting rapid cleanliness code degradation, APC particle size distribution data provides contamination source identification that gravimetric mg/L data cannot — a spike in 14 µm+ particles indicates a different source than a spike in 4 µm particles.

RELATED KNOWLEDGE

STANDARDS

ISO 4406 →ISO 11171 →ISO 16889 →ISO 4405 →

TECHNOLOGIES

NANOFORCE →SYNTRAX →

ARTICLES

testing and validation →fluid cleanliness →contamination control →

Gravimetric Analysis vs. Particle Counting

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