The particle count test (or fluid cleanliness test) is a critical component of oil analysis that measures the number and size distribution of solid particles suspended in a lubricating oil, hydraulic fluid, or other industrial fluid.
Here's a breakdown of what it is, why it's important, how it's done, and how results are interpreted:
Purpose & Importance:
Monitor Contamination: Identifies the level of solid contaminants (dirt, dust, sand, wear metals, soot, fibers, seal debris) in the fluid.
Predict Component Wear: Excessive particles accelerate abrasive wear of bearings, gears, pumps, valves, and cylinders. Monitoring trends helps predict potential failures.
Assess Filter Performance: Determines if filters are working effectively to remove contaminants.
Verify Cleanliness Levels: Ensures new oil meets required cleanliness standards before filling equipment or confirms that flushing procedures were successful.
Proactive Maintenance: Enables maintenance teams to schedule oil changes or filter replacements before significant damage occurs, saving costs and downtime. It's a cornerstone of predictive maintenance programs.
Ensure System Reliability: Clean oil is essential for the reliable operation of sensitive hydraulic and lubrication systems, especially in high-precision industries (aerospace, power generation, manufacturing).
How It's Done (Common Method - Automatic Optical Particle Counting):
Sample Collection: A representative oil sample is carefully drawn from a live system (under pressure and temperature) using a clean bottle and tubing via a dedicated sampling port to avoid external contamination.
Sample Preparation: The sample bottle is vigorously agitated to ensure particles are uniformly suspended. Highly opaque or dark samples might require dilution with clean solvent.
Passing Through Sensor: A precise volume of the prepared sample is pumped through a flow cell illuminated by a light source (laser or white light).
Detection & Sizing:
As particles pass through the light beam, they either block light (Obscuration) or scatter light (Light Scatter).
A detector measures the change in light intensity caused by each particle.
The magnitude of the change is proportional to the particle's size.
The number of changes detected gives the count.
Categorization: Particles are electronically counted and categorized into predefined size ranges (bins). Common standard size thresholds are ≥4µm, ≥6µm, and ≥14µm.
Reporting Standard (ISO 4406:2017):
The most common way to report results is using the ISO Cleanliness Code (ISO 4406:2017).
This code expresses the particle count per milliliter (mL) of fluid at three specific size thresholds:
≥4µm (c)
≥6µm (d)
≥14µm (e)
How the Code Works:
The actual particle count for each size threshold is measured.
This count is converted to a logarithmic scale using the formula: Code = log₂(Particle Count per mL).
The resulting number is rounded down to the nearest integer.
The code is reported as c/d/e (e.g., 18/16/13).
Example: A code of 18/16/13 means:
18: Between 1,300 and 2,500 particles per mL ≥4µm (2¹⁸ = 262,144? No! Remember: Code = log₂(count), so count = 2^Code. 2¹⁸ = 262,144 particles per 100mL? Correction: The ISO standard defines ranges. Code 18 corresponds to >1300 to ≤2500 particles per mL (it's based on the scale, not simply 2^Code). See an ISO 4406 table for exact ranges per code number. Code 18 means approximately 1301-2500 particles/mL ≥4µm.
16: Between 320 and 640 particles per mL ≥6µm.
13: Between 40 and 80 particles per mL ≥14µm.
Lower Codes = Cleaner Oil: A lower ISO code number indicates fewer particles (cleaner fluid). For example, 14/12/9 is significantly cleaner than 19/17/14.
Interpreting Results:
Trending is Key: A single result provides a snapshot. The real power comes from tracking the ISO code over time. A rising trend indicates increasing contamination or wear.
Comparing to Targets: Every machine or system has a target cleanliness level based on component sensitivity and manufacturer recommendations. Results are compared against these targets.
High Particle Counts Signal:
External Contamination: Ingress of dirt through breathers, seals, or during maintenance.
Internal Wear: Generation of wear particles from gears, bearings, pumps, etc.
Filter Bypass/Failure: Filter is not capturing particles effectively.
Oil Degradation: Soot (in engines), varnish precursors, additive precipitation.
Water Contamination: Can cause particle agglomeration (making small particles clump into larger ones).
Size Distribution: The pattern across the size bins can offer clues:
High counts only in small sizes (≥4µm, ≥6µm) often suggest external dirt ingress.
High counts in larger sizes (≥14µm) often suggest significant internal wear.
Action: Results exceeding targets or showing adverse trends trigger actions like: investigating contamination sources, changing filters, changing oil, inspecting components, or performing additional tests.
Key Factors Affecting Accuracy:
Sampling Technique: Critical to get a representative sample without introducing external dirt.
Sample Handling: Agitation before analysis is vital. Air bubbles can be counted as particles.
Calibration: Particle counters must be calibrated regularly using traceable standards.
Fluid Properties: Very dark fluids, high water content, or air entrainment can interfere with optical methods. Alternative methods (e.g., pore blockage) may be used in these cases.
Common Applications:
Hydraulic systems (especially servo-valves)
Turbine oil systems
Gearboxes
Circulating oil systems
Compressor lubrication systems
Engine oil analysis (often combined with wear metal analysis and soot measurement)
Assessing new oil cleanliness
Verifying system cleanliness after flushing or assembly
In essence, the particle count test is a vital diagnostic tool that quantifies fluid cleanliness, acting as an early warning system for contamination and wear, ultimately protecting machinery and ensuring reliability. It's usually performed alongside other oil analysis tests (viscosity, elemental spectroscopy, moisture, acid number) for a complete assessment.