PARTICLE MEASUREMENT IN CLEAN ROOM TECHNOLOGY

Transcription

1 WHITEPAPER ENGLISH PARTICLE MEASUREMENT IN CLEAN ROOM TECHNOLOGY PARTICLE MEASUREMENT Particle measurement in cleanrooms. WP EN, V1R0,

2 PARTICLE MEASUREMENT IN CLEAN ROOM TECHNOLOGY WHITEPAPER Content Content... 2 History Particle measurement in clean room technology What do particle counters measure actually? How do particle counters work? Can I use any particle counter with any monitoring system? Where to place the sampling probe? With or without integrated vacuum pump? Setting the measurement time What else should be taken into account?...6 History Document number: wp en Version / Revision: V1R0, Status: Initial version, released History Version Date Comment / Note V1R Initial Version Table 1: History PAGE 2 OF 6 WP EN, V1R0,

3 WHITEPAPER PARTICLE MEASUREMENT IN CLEAN ROOM TECHNOLOGY 1 Particle measurement in clean room technology The central task of cleanroom technology is the protection of products against particulate contamination. While there are millions of tiny particles in normal rooms, which are for the most part below the threshold of perception, the ISO defines various cleanroom classes by permissible limits on the number of particles in size from 0.1μm to 5,0μm. The table below shows the maximum admissible number of particles per cubic meter of air for each cleanroom class. Cleanroom classes according to ISO (particles per m³) Class 0,1µm 0,2µm 0,3µm 0,5µm 1,0m 5,0µm ISO ISO ISO ISO ISO ISO ISO ISO ISO Table 2: Cleanroom classes according to ISO What do particle counters measure actually? In order to capture such small particles with reasonable reliability in real-time, we need a powerful measurement principle which can detect both the size of the individual particles and the number within a determined volume of air. While counting particles generally causes fewer problems, as long as the quantity is not too large, the correct classification of the particle size is actually only possible with idealized spherical particles. Since in practice, particles can, however, take any shape, a unique assignment to a certain size class is de facto impossible. For this type of particle, we therefore use the term "equivalent diameter", which states that the optical properties correspond to a spherical particle having the same diameter. 1.2 How do particle counters work? The measurement procedure practically all particle counters used in clean room technology are based on is the optical scattering of light NIOTRONIC GMBH WP EN, V1R0, PAGE 3 OF 6

4 PARTICLE MEASUREMENT IN CLEAN ROOM TECHNOLOGY WHITEPAPER In this method of measurement the light reflected by the particles in a stream of air that is passed through a tightly shaped laser beam inside the measuring cell is used to both count the particles and measure their size. When the particles carried by the airflow pass the laser beam, they cause extremely short flashes of light which are focused by corresponding optics to a single point and are detected by a sensitive photodetector. The intensity of the light flash allows conclusions about the particle size, although this is not always unambiguously possible due to the non-monotonicity of the relationship between particle size and scattered light intensity in some size ranges (Mie theory). In addition, it cannot always be guaranteed that all the particles pass through the "active volume" strictly sequentially, i.e. at higher particle concentrations, a certain amount of so-called coincidences is also to be expected - in other words, when two or more particles are simultaneously in the active volume, the particle counter cannot distinguish whether this is one larger, or several smaller particles. Due to the problems described, the count and size classification of real occurring particles is not comparable with the precision of a temperature measurement. Only a small part of the air in the room can be measured, due to the low flow rate (usually 28,3l / min), and neither a statement about the total particle number in the room is made, nor information is obtained about the current particle concentration at the other end of the room. Nevertheless, scattered light particle counters have been established as a reliable measuring device for local recording and detection of particle contamination in clean room technology. 1.3 Can I use any particle counter with any monitoring system? Nowadays, laser optical particle counters are available from different manufacturers with different properties in respect to their counting efficiency, number of evaluable size classes, as well as different equipment and interfaces. Often, the manufacturer also provides associated data acquisition and recording software, which is usually tailored to their particle counter. This allows most of the parameterization of the particle counter and offers various automation functions such as starting and stopping the measurement, etc., which are not realized in connecting the particle counter via analog interfaces (4. 20mA etc.). There are only a few manufacturers of particle counters, which keep the data transfer protocol publicly available to enable third-party monitoring systems to connect to the particle counter. These solutions, however, are to be treated with caution, as third-party suppliers of monitoring systems often do not have all the information necessary to implement the communication protocol fully and correctly. For this reason it is advisable to use the monitoring software of the particle counter manufacturer, since in case of problems it can be ensured that the necessary expertise is available to resolve them. In general, a connection via analog interfaces is discouraged because the measuring cycles of the particle counter run asynchronously to the scan cycle of the monitoring system, i.e. there may be a delay between the occurrence of particulate contamination and the display and alarm in the monitoring system. 1.4 Where to place the sampling probe? In the interest of the shortest possible reaction time, the sampling probe of the particle counter should be placed as directly as possible at the location of critical processes, e.g. in filling, bearing in mind that the length of tubing between the sample probe and the particle counter should also be kept as short as possible. Experiments have shown that deposition of large particles can occur at hose lengths of less than 5 m. Depending on the location of the particle counter with respect to the PAGE 4 OF 6 WP EN, V1R0,

5 WHITEPAPER PARTICLE MEASUREMENT IN CLEAN ROOM TECHNOLOGY sampling probe, a more or less large height difference has to be overcome. Particularly large particles are not carried by the air stream through large differences in height (e.g. a vertically rising tubing), but instead follow the gravitational field of the earth, they fall down and therefore never arrive in the measuring cell of the particle counter and, consequently, cannot be measured. Similar problems arise with sagging, horizontally guided hoses. This may in turn lead to a deposition of large particles, as these tend to stick to the lowest point of the hose line. This effect is not as pronounced as the one with vertically guided hoses, however, particles adhering to the tube's inner wall can be removed again at a later time, which may then lead to a false reading or an unwanted alarm. Considering this requirement, it is ideal to plan for the construction of a utility tunnel, as this allows the installation of the particle counter directly on the rear side of the respective cleanroom wall, which facilitates easy access for maintenance and calibration purposes and also reduces hose lengths to a minimum. Unfortunately, this is rarely possible in practice for structural reasons. 1.5 With or without integrated vacuum pump? Moreover, it is necessary to decide whether particle counters are to be used with an integrated vacuum pump, or just a pure counter, which needs an external central vacuum system to operate, usually dimensioned for several counters. The former is offered for very small systems with few or only one particle counter, as this will be significantly cheaper due to the significantly reduced installation effort and the omission of the central vacuum system. Also, existing clean room facilities can usually be upgraded with particle counters with integrated vacuum pumps, since a power supply and a network connection to connect to the monitoring system will be available almost everywhere. The only downside is the increased noise due to the built-in vacuum pump. For larger clean room facilities with more than 2-3 particle counters, cost advantages are significant for counters without vacuum supply in conjunction with an external, usually a central vacuum system accommodated in a separate technical area, despite the increased installation expenses. This option is also much more pleasant for the staff present in the clean room, since the particle counters in-use do not generate any noise. 1.6 Setting the measurement time The particle limits defined in ISO relate to one cubic meter of room air. The typical flow rates of common particle counters, however, amount to 28.3, 50 or 100 l/min. In order to be able to represent the measured value in particles / m³, the measurement result of the particle counter must be converted. The latter in turn depends on the set measuring time. In order to measure one cubic meter with a particle counter whose nominal flow measures 28,3l / min, the measuring time should be set to 35,318min, which in practice, however, would be completely unacceptable, since the first measurement result would be available only after the expiration of 35,318min. A temporal relation to events in the clean room is almost impossible in this case. The situation is not much better for counters with a higher nominal flow rate either. In order to shorten the response time, it is therefore strived for to set the measurement time as short as possible, and to convert the result in particles per cubic meter. In order to convert the measurement result of the particle counter in particles per cubic meter at a measurement time of one minute, it must be multiplied by , since times 28.3l (= 1cf) gives exactly one cubic meter. With increasing shortening of the measurement time and increasing of the conversion factor, however, a problem of a statistical nature appears, as in shorter 2016 NIOTRONIC GMBH WP EN, V1R0, PAGE 5 OF 6

6 PARTICLE MEASUREMENT IN CLEAN ROOM TECHNOLOGY WHITEPAPER measurement intervals with the same particle concentration, fewer particles per measuring cycle are counted, thus increasing the statistical uncertainty. In addition, multiplication with a large conversion factor means that when particles are measured, their numbers are vastly exaggerated. At too short measurement times, a single particle may in extreme cases cause a threshold exceedance, although far fewer particles are present in the cubic meters that is to be measured. For this reason, the reduction of the measurement time of the particle counter is severely limited. One way out of this situation is the accumulation of measured values in conjunction with a short measurement time. The last N measured results of the particle counter are always stored and added. Thus, a measurement result can be obtained related to one cubic meter in spite of a short measurement time and without a conversion factor. This process is only supported by a few particle counters on the market. 1.7 What else should be taken into account? Since all measuring devices must be recurrently maintained and calibrated, it is of considerable importance that the manufacturer of the particle counter or its partners should be able to offer support in this regard quickly and easily. For smaller installations with few particle counters, shipping for calibration is an option, while for larger systems, onsite calibration can also be performed. Another useful feature is a replacement service that guarantees always fully functional and currently calibrated particle counters. In this case, the manufacturer sends up-to-date calibrated replacement equipment and in return receives the particle counters that are to be calibrated. Often this presents itself as an excellent solution because it de facto does not come to any downtime of the plant due to the annual calibration. Niotronic Hard- & Software GmbH office@niotronic.com, PAGE 6 OF 6 WP EN, V1R0,