Compare particulate measuring systems with each other

Clean rooms are becoming increasingly important for industrial environments: the VDA 19 and ISO 16232 guidelines have introduced binding requirements for the automotive industry and mechanical engineering to control and maintain technical cleanliness. Component cleanliness serves as an important quality criterion here, since it is decisive for the reliability and function of machines as well as production parts: "It is a matter of detecting possible contamination and quantifying the harmful particles," explains Ralf Nerling, General Manager of the Nerling Group and member of the Board of Directors at CEC. "With this know-how, employees can produce more efficiently and also save time and money."

Complete plant with process chain for quality analysis
In order to be able to guarantee component cleanliness, it is usually necessary to install a cleanroom or cleanroom system and to regularly check the room conditions and products there. "The cleanliness analysis can either be contracted out by component manufacturers to accredited laboratories or carried out in a company's own laboratory," says Nerling. When operating a cleanliness laboratory, it is crucial that it is implemented directly in the production environment - and in such a way that the distances between the production steps and the laboratory are short. This reduces the risk of contamination and ensures an uninterrupted flow of materials.

However, an individual overall solution that takes this into account and optimally responds to the conditions on site was previously associated with very high costs and extensive logistical planning. Since the manufacturers of the individual cleanroom components rarely had an insight into the process chain before and after the use of their own equipment, a comprehensive discussion of the process sequence and extensive consultation were only possible to a limited extent. At parts2clean 2017 in Stuttgart, the Cleaning Excellence Center therefore presented a complete system with a complete process chain from cleaning to quality analysis in the clean room according to VDA 19.1, in which a total of more than 20 companies were involved: In detail, the deburring (Benseler) and cleaning of components (Dürr Ecoclean) in the cleanroom as well as the transport through the gray zone by means of a cleanroom-compatible sluice trolley (Kögel) into the cleanroom of the test laboratory (Ner-ling) were demonstrated to prove the cleanliness of the components. In the test laboratory of the clean room class ISO7 or also ISO8 (Nerling) the process sequence rinsing (glasses) - filter drying (Binder) - gravimetry (Sartorius) - micro copy (Jomesa) and protocol production (CleanControlling) was shown.

Ensuring measurement quality
Various standardized methods can be used to determine cleanliness: "First, the particles are extracted from the test object using ultrasound, spraying, rinsing, shaking or via a test rig close to the application," explains Nerling. "They are filtered off and subsequently analyzed." Depending on the type of particle, various microscopy systems and flatbed scanners are used for this purpose.

For metallic particles, for example, a linearly polarized reflected-light microscope is used. For particle sizes > 3 µm, material microscopy is best suited, for particles > 25 µm, stereo microscopy is best suited.

In order to ensure correct measurement and precise results in the long term, in-house laboratories are recommended to check their particulate measurement systems as part of their quality management in accordance with ISO 9001 and to include the results of the interlaboratory comparison in their quality management manual. Laboratory interlaboratory tests carried out by an independent provider, in which identical reference samples are tested using the same procedures but different measuring systems, are particularly suitable for this purpose. They not only provide the opportunity to obtain a precise analysis of one's own measurement quality, but also to compare the measurement results of one's own system anonymously with those of other laboratories. For accredited testing laboratories, it is even obligatory to regularly participate in laboratory interlaboratory comparisons, such as those offered by the CEC three times a year: "According to ISO/IEC 17025, a self-assessment of the procedures is first necessary in order to obtain accreditation," explains Nerling. "In addition, the standard requires annual participation in an interlaboratory comparison in order to ensure the quality of the results in the long term.

Uniform comparative testing according to VDA 19.1
The CEC interlaboratory comparisons always follow a standardized procedure: First, a laboratory registers its various measuring systems for the laboratory interlaboratory comparison, regardless of the manufacturer of the system. "Various light-optical measuring instruments are permitted: stereo, zoom and material microscopy systems as well as flatbed scanners," says Nerling. Subsequently, the laboratory is provided with one test filter per registered system, which is tested by the participant himself according to the standard analysis of VDA19.1.

The data resulting from this examination are transferred into a test report. "The required contents are the measured number of particles of the different typifications (differentiation into metallic shiny and non-shiny particles as well as fibers), in the respective size classes from 50 μm to > 3,000 μm as well as the percentage filter occupancy with particles and fibers, measured against the total area of the filter membrane," says Steffen Haberzettl, office manager at the CEC. "In addition, the largest and second largest particles and fibers are specified for the individual typings, as well as their measurement of length and width." Once the test reports of all measuring systems have been received by the CEC, the confidential evaluation is carried out in accordance with DIN 38402-45/3/.

"In this process, the z-score is calculated for each measured value of the respective particle sizes in the individual size classes in order to obtain a statement about the classification of the respective measurement results," Haberzettl explains. "The identification of the largest and second largest particles as well as the fibers is carried out via an image-based comparison.

In addition, the deviation of length and width from the average is calculated." Once this is complete, all participants receive an anonymized result report for each registered measuring system, which allows them to compare their results with those of the other laboratories. In this way, potential weaknesses can be identified and eliminated immediately.

The solution
In order to ensure a watertight anonymity of the participants, a number key was assigned to each measuring system, which makes the respective instrument and its results identifiable only for the associated laboratory.

Finally, each system that has successfully participated receives a CEC test badge that allows auditors to immediately identify when the instrument was last involved in a test.