Overview of Industrial Oil Cleanliness Testing

Industrial oil cleanliness testing is a crucial test for evaluating the content of solid particulate contaminants in industrial oils such as lubricating oil and hydraulic oil. It is essential for ensuring the normal operation of mechanical equipment and extending its service life.

Hazards of Unqualified Cleanliness

• Causes abnormal wear of mechanical parts

• Leads to decreased performance of hydraulic systems

• Causes oil line blockage

• Increases equipment failure rate

Detection Methods and Standard Systems

Oil cleanliness testing is mainly achieved through particle counting, gravimetric methods, and microscopy. Particle counting is currently the most mainstream and accurate method. Microscopic oil cleanliness testing has multiple levels of international, national, and industry standards, focusing on the operational specifications and result determination of particle filtration and microscopic observation and counting. International standards are represented by the ISO system and German DIN standards.

Common Testing Standards

• ISO 4406 (Solid Particle Counting)

• NAS 1638 (National Aerospace Standard)

• SAE AS4059: An upgraded design based on NAS 1638, with finer classification and covering more particle size ranges

• DIN 51455:2020 (German Institute for Standardization Automatic Light Microscope Testing Standard)

DIN 51455:2020 Core Requirements

This standard applies to fresh mineral oils, synthetic oils, and some used oils. The specific requirements are as follows:

• Equipment Parameters: The microscope pixel resolution must reach 1 μm/pixel, equipped with bright-field illumination and cross-polarizers, and supporting background brightness correction.

• Particle Size Detection: Focusing on particles ≥5 μm and ≥15 μm, with particle size defined by the maximum Feret diameter.

• Operating Procedures: Use a 0.8 μm nitrocellulose membrane filter, with sample volume selected according to the oil type (20-200 ml for fresh oil, 1-20 ml for used oil).

• Result Determination: Cleanliness level is expressed using ISO 4406 coding, and key parameters such as threshold (70%/85%) and blank value must be indicated.