Technical Feature I: Body-Box Test: Investigation of a test method
04/01/2005
Examining a practice-oriented test method with respect to the reproducibility of results
Carsten Moschner, Dastex Reinraumzubehör GmbH
Thomas von Kahlden, CCI-von Kahlden GmbH
It is indisputable that cleanroom garments are still one of the most important components of contamination control in the cleanroom. Apart from textile-related parameters, other factors, including fit of the garments, behavior of the user, clothing components and the undergarments, all affect particle delivery. The technical documentation for cleanroom garments, however, often just contains data referring to the selected textile. Since the test methods for important material properties such as particle retention capacity, water vapor permeability, electrostatic behavior, etc., are not standardized, the end user does not have the ability to compare the technical data of the respective suppliers with each other. Only results that are obtained under the same test conditions and methods are comparable with each other.
Comparability of data
Apart from the problems of comparability shown above, data usually refers only to the technical properties of materials that are not yet processed, i.e., to the fabric of the roll. At best, aging is simulated by cleaning the fabrics several times before the tests (e.g., fifty washing and drying cycles). Solid information as to the product properties after a complete piece of clothing and/or a clothing system has been manufactured from the materials is the exception and obvious effects like “positive pressure” and “thermionics” (under the cleanroom garments) are completely ignored.
Given the above, it is important to focus the mind of the users on the implications of positive pressure and thermioncs for cleanroom garments. Design of an approved, informative test method, and the resulting test expenditure, can prevent easy dissemination of information, but with this in mind we concentrated on an international recommended practice (IES-RP-CC003.2) for a comparative test method-the so-called Body-Box Test. Here, a practice-oriented approach forms the basis of its experimental setup, a short summary of which is shown below:
• A defined cleanroom size with a grill-plate floor contains only the test subject wearing the cleanroom garments to be examined.
• The test subject accomplishes a defined movement program in a given time frame.
• From the exhaust air (extracted from underneath the grill floor) air samples are taken, which are supplied to a particle counter.
Putting the method to the test
The particles delivered by the test subject need to be recorded and this is the “weak point” of the test method given in the recommendation, as the exact position of the particle counter is not defined. With undefined flow conditions in the exhaust air it is very probable that the results of measurement can vary greatly depending on the positionof the particle counter. Reproducibility is highly unlikely and the comparison between different cleanroom garment systems (with same test person and same test run) is therefore severely compromised.
Elimination of this weakness would give this test method the possibility of accomplishing meaningful user-oriented investigations. With given reproducibility, details of the clothing can be compared with each other, as can different cleanroom fabrics, different production methods, etc. Comparisons between disposable clothing and cleanroom garments become more informative and possible consequences of the “pump effect” with fabrics more impermeable to air compared to air-permeable materials may become apparent.
On this basis, the two enterprises CCI-von Kahlden GmbH and Dastex Reinraumzubehör GmbH&Co.KG together modified the Body-Box Test described above and installed an appropriate test stand on the area of Dastex Reinraumzubehör GmbH&Co.KG (see Fig. 1 and Fig. 2). Important considerations for the execution of the project were:
• constant incident-flow velocity on the subject in the test chamber
• production of a variable flow rate up to 0.5 m/s with conformity over the surface of +/- 5 percent
• arrangement of the form of the air baffles and their surfaces in such a way that these can be cleaned at any time and do not form “dead water” areas
 Figure 3. Another important consideration was that air circulation be arranged in such a way as to guarantee representative sampling. |
• arrangement of the air circulation in such a way that representative sampling is guaranteed (see Fig. 3).
• additional sampling places for particle measurements in the box, making it possible to accomplish further measurements in the direct field of the test person, e.g., at workstation height
• appropriate options for measurement and documentation of temperature, humidity and air speed in the test chamber
The objectives of the facility
It is the ambition of the operator (Dastex) to further deepen the investigations already accomplished on the “pump effect” and to optimize the existing cleanroom garment systems for different locations. These investigations should provide improvement in wearer comfort without accepting “losses” on other technical characteristics such as particle release.
At the same time, this test stand is also at the disposal of end users for their own investigations. Several cleanroom operators have already shown interest in the possibility of testing an existing clothing system against a new “optimized” system or to carry out efficiency testing on specific clothing components.
Contrary to the “classical tests” investigating the individual fabric parameters, this test-with its improved reproducibility-is more practical in so far as the particles being captured are those actually shed by each subject with their respective garment. Although objections could be raised that the particles captured in the exhaust air could include non-operator-related particles, the following points run counter to this:
• The floor construction selected with this test stand is not comparable with typical grill floors as airflow is much “simpler” here due to very large floor openings. The floor construction allows air to flow virtually unhindered and thus all particles are transported into the measuring range.
• The air circulation system allows almost all particles to be seized. Particle production occurs here in exactly the same way as in actual cleanrooms; only the movement and locations of deposition can be more complex in a normal cleanroom environment.
• Many users do not have smooth, level floors. Particles that fall to the floor do not necessarily remain there and cannot be guaranteed to land in the exhaust air in high concentrations. Movements of personnel and turbulent airflows prevailing within the floor range of a cleanroom can transport these particles back into higher regions such as workstations, machines, etc.
 Figure 4. This method allows the placing of individual points of sampling within the measuring area. |
Since this method allows the placing of individual points of sampling within the measuring area, it is the intention of the operator to also take measurements in front of the test subject at workstation height (see Fig. 4). These measurements at workstation height and the simultaneous measurements in the exhaust air should ideally point out a relationship and allow us to be able to draw inferences in the future merely from the measurements of the exhaust air.
The first concrete results of these tests are expected this year, particularly since there are already several end users who have expressed an interest in examining cleanroom garments in this test chamber. The commissioning of this test chamber has made it possible to put at the user’s disposal practical data on the efficiency of the cleanroom garments used in each case and/or comparisons with newly introduced garments. III
Carsten Moschner is managing director of Dastex Reinraumzubehör GmbH&Co.KG. He is engaged in further development of textile cleanroom garments with regard to technical aspects like particle retention capacity, antistatic properties, etc., as well as with regard to wearer comfort. For many years, Mr. Moschner has contributed to according national specialist boards. He can be reached at [email protected].
Thomas von Kahlden is managing director of CCI-von Kahlden GmbH and among other things, he is involved with particle measurement, flow visualization and improvement of cleanroom processes. He is also engaged in different national specialist boards.