Overview of common sampling techniques in the context of technical cleanliness testing

Figure 2: OLYMPUS CIX100 – Microscope-based system solution for technical cleanliness.

Figure 3: For the preparation of the subsequent analysis, a filter membrane is dried.

Figure 4: Round sample holders with white and black backgrounds for filter membranes with diameters of 25 mm (left), 47 mm (center), and 55 mm (right).

Figure 5: Positioning of the filter membrane for filtering the test filter sample (a); mounting of the filter membrane on the sample holder (b).

Figure 5: Positioning of the filter membrane for filtering the sample to be examined (a); mounting of the filter membrane on the sample holder (b).

Figure 6.a: Mounting of the adhesive tape with adhering particles on the special sample holder for further analysis

Figure 6.b: The software of the OLYMPUS CIX100 system enables particle analysis of a tape sample after capturing a rectangular inspection area.

Figure 7.a: Preparation of a particle trap

Figure 7.b: The software of the OLYMPUS CIX100 system analyzes a particle trap according to the VDA 19.2 standard.

Figure 1: Specially designed OLYMPUS CIX100 sample holders for various sample types: a and b) Filter holders with white and black backgrounds - available in three different sizes, c) Particle trap holder, and d) Tape holder.

Figure 1: Specially designed OLYMPUS CIX100 sample holders for various sample types: a and b) Filter holders with white and black backgrounds - available in three different sizes, c) Particle trap holder, and d) Tape holder.

Figure 1: Specially designed OLYMPUS CIX100 sample holders for different sample types: a and b) Filter holders with white and black backgrounds - available in three different sizes, c) Particle trap holder, and d) Tape holder.

Figure 1: Specially designed OLYMPUS CIX100 sample holders for various sample types: a and b) Filter holders with white and black backgrounds - available in three different sizes, c) Particle trap holder, and d) Tape holder.

Overview of Common Sampling Techniques in Technical Cleanliness Testing

Technical products in nearly all industries require a certain level of cleanliness. Contaminations with unwanted particles and residues in production facilities, laboratories, and especially on the surfaces of technical products often pose significant risks. These contaminations shorten product lifespan, often degrade product performance, and can also lead to risks during product use. Knowledge of these risks has led to the definition of stricter national and international cleanliness standards. Installing a technical cleanliness inspection system is a crucial step to regularly monitor the cleanliness of a production environment, as well as to avoid production downtime, material, and energy waste. Sample preparation plays an important role in this process.

Workflow of a Cleanliness Analysis

The technical cleanliness analysis begins with sample preparation and the selection of the technical parts to be inspected. Subsequently, sampling is carried out to collect micro-particle contamination. The following procedures are among those used in sampling:

– Filter membranes to capture contaminants after parts washing or during direct liquid filtration,
– Tape Lift (special adhesive tape) for collecting particles from delicate surfaces that cannot be washed, or
– Particle traps to collect sedimenting airborne contaminants in assembly processes or cleanrooms.

The samples obtained through these procedures are then mounted on special sample holders.

Modern microscopic systems for technical cleanliness, such as the OLYMPUS CIX100, make sample holder positioning a simple step before the automated cleanliness inspection takes place. An intuitive workflow combined with the automation of all steps after sample mounting helps perform inspections with minimal risk of human error and sample contamination. With a single scan, the system can detect contaminants up to 2.5 μm and distinguish between metallic particles, non-metallic particles, and fibers.

Overview of Common Sampling Techniques

Various methods are available for separating contaminants from the component. The choice of extraction/sample collection method depends heavily on the primary purpose of the technical cleanliness and the industry sector. Generally, there are three main extraction methods:

Washing Method

In the automotive industry, as well as in pharmaceutical and machine manufacturing, the extraction via liquid is most often the most suitable technique. Microparticulate contaminants are removed from components through washing, rinsing, or ultrasonic baths. The liquid used for extraction should be compatible with the component material, the filter device, and the membrane. After washing, the rinse liquid is filtered, and the filter membrane is dried. In most cases, the next step is weighing the dried filter membrane using an analytical balance. The gravimetric result provides an initial estimate of residual particles, but the size, shape, and other properties of the particles remain unknown and require subsequent visual analysis. Finally, the dried, weighed filter membrane is mounted on the filter holder.

Filter membranes come in different diameters. The size of the filter membrane used depends on the application and industry:

– Filter membranes with a diameter of 47 mm are commonly used in aerospace, automotive, and oil industries. This is the standard filter diameter used in most cases.
– For oil analysis, membranes with a diameter of 25 mm are also employed.
– Filter membranes with a diameter of 55 mm are used in machine maintenance and production with a high particle load.

In addition to filter size, filter membranes are selected with either a white or black background depending on the application.

– Black background: When an aggressive chemical is used to rinse particles, residues of the rinse fluid may remain on the filter membrane. The black background sample holder is mainly made of anodized aluminum and is largely inert to chemical substances (i.e., it does not react chemically).
– White background: A white background offers an advantage when using woven mesh filters. Mesh filters are often used to accelerate the filtration process, as the rinse fluid can flow much faster through the filter membrane. When examining a mesh filter, the microscope can look through the mesh onto the sample holder. The black background would show through the mesh and be misinterpreted as particles. Therefore, a white background sample holder is recommended for mesh filter examinations.

For the OLYMPUS CIX100, special sample holders for membranes with diameters of 47 mm, 25 mm, and 55 mm are available with both black and white backgrounds. The system software already includes presets tailored to different membrane sizes for the inspection area, allowing the user to automatically adjust the scan size with a single click. Predefined parameters for each sample type are also integrated, enabling even inexperienced operators to achieve compliant results easily.

Direct Liquid Filtration

This method is frequently used for inspecting the purity of oil. Oil loses its lubricating properties when exposed to microparticles, moisture, and salts. This leads to corrosion, additive breakdown, and the formation of resins and deposits. Mechanical parts, such as valves, begin to jam, seize, and wear out.

Repairing these parts is costly and time-consuming. Therefore, conducting a cleanliness analysis is important to assess the level of contamination in the oil. The benefits of clean fluids, especially oil, in machinery include:

– Minimized maintenance time and costs
– Maximum performance and productivity
– Improved longevity of components and machines
– Fewer plant shutdowns
– Fewer repairs and hardware replacements

All these advantages help save money, as fewer contaminants in fluids lead to energy savings and longer machine lifespans. For example, the cleaner the oil, the lower the oil temperature, the higher the viscosity, and the better the performance. Less maintenance and fewer repairs also save personnel and hardware costs.

The workflow for direct liquid filtration begins with sampling oil from the system under investigation. The fluid passes through a vacuum filtration machine, where suspended solids are filtered and collected on a filter membrane. As with the washing method, the filter membrane is mounted on a special filter holder and used for visual inspection and analysis.

Tape Lift Sampling

The Tape Lift sampling method is a quick and simple technique to remove particles from a surface and determine its cleanliness level. This method is used wherever surfaces of components need to be free of contamination, as contaminants can impair product performance and reliability. Industries such as aerospace, space technology, electronics manufacturing, and solar panel production utilize this method.

The tape lift method can be used whenever the surface can be sampled without damage by applying adhesive tape. Generally, metals, metal coatings, and oxide layers are not altered by this process. Before applying to painted, vapor-deposited, or optical coatings, a preliminary test is recommended to exclude potential damage.

For sampling, a special adhesive tape is applied to the surface to be examined. This results in a direct transfer of particles from the surface onto the tape. After removal, the tape with attached particles is mounted on a special tape lift sample holder. The OLYMPUS CIX100 not only offers suitable sample holders but also includes an integrated analysis process according to the ASTM E1216-11 standard. This standard statistically determines the size and location of the sampling area to accurately estimate the surface cleanliness over large areas. The user defines the sampling plan considering the surface geometry and orientation relative to gas flow, gravity, and obstructions, according to the relevant standard instructions. These factors can influence particle fallout and the capture of particles on the surface.

Particle Trap

Particle traps are frequently used as a sampling method to monitor environmental cleanliness in assembly and logistics processes in production and cleanrooms. A particle trap, consisting of an adhesive pad roughly the size of a filter membrane, is placed at locations with potential particulate contamination for a defined period to collect airborne particles. The active period is called the sedimentation time. Once sampling is complete, the trap with attached particles is mounted on a special sample holder for analysis. This analysis determines the number and size distribution of particles and calculates the sedimentation value (also called Illig value). The sedimentation value is a single numeric value derived from the count of particles detected during sedimentation across different size classes. During calculation, detected particles are weighted according to their size, as larger particles pose a much greater potential for damage than smaller ones. Based on the sedimentation value, facilities can compare environmental cleanliness at different locations over a specific period. This helps identify areas with higher contamination levels and optimize these areas to prevent particle ingress that could damage components and assembled systems. The sedimentation value is included in the final analysis report. The inspector must document not only the overall cleanliness results but also the sedimentation time and the location of the particle trap in the report.

Conclusion

With increasing quality requirements, technical cleanliness of components, fluids, or environments has become increasingly central to the manufacturing process. International and national guidelines describe the methods and documentation requirements for determining contamination and demand more detailed information about the type of contamination, such as particle count, particle size distribution, and particle properties. In a contamination control system, parts are randomly sampled from the production line and examined. Depending on the application, different sampling methods and special sample holders are used. The evaluation is then carried out according to relevant standards. The following table provides an overview of the available filter holders for the described procedures for the OLYMPUS CIX100, their application areas, and the supported standards.