At 18:51 14-07-98 +0000, you wrote: the A HEPA filter is: High Efficiancy Particulate Air filter. A HEPA filter is built so that 9.97% of particles 0.3 micron and larger are removed from the air passing through. You see HEPA filters more and more in consumer related things (at least I have in the States). Example: Air filters in many modern automobiles, possibly all autos, use a HEPA filter for engine air intake filtering. Clearances between engine parts are closer nowadays which improves engine performance and efficiency plus smaller engines are stressed harder in relatively large modern cars, both thus requiring really clean air intake. Also, I think I've seen an advertisement for a fancy grade household vacuum cleaner advertising it has a HEPA filter on its exhaust. Less allergens blown about. We use Nilfisk-brand cleanroom vacuum cleaners at work. Made in Denmark I believe. Cost around US$1000 each but they're meant for use *in* cleanrooms and *must* not throw junk about from their exhaust. Yes, you are correct re having something to do with size and number of particles. Cleanroom grading is based on Federal Standard 209E. I have the rev. "D" document around the office buried somewhere in some papers. This standard seems to have been adopted outside the USA too. Cleanroom technology has been around a long time in pharmaceutical manufacturing and medical fields and adapted to semiconductor manufacturing in the last 20-30 years. Anyway, here's a short classification table from 209 (hope the formating comes out okay w/text-only displays!): Max. number of particles Class: Max. no. of part. per cubic foot(per liter) English system per cu. ft.(per l.) 0.5 micron and larger (metric system) > 5.0 microns 100 (3.5) 100 (3.5) * 10,000 (350) 10,000 (350) 65 (2.3) 100,000 (3500) 100,000 (3500) 700 (25) How is this measured? A particle counter is used. These machines typically use light-scattering to count the particles passing through a special optical chamber with specially ground curved mirrors. The class number is obtained by pumping sampled air through the chamber at a rate of one cu. ft. per minute. A low power He-Ne laser is used in our particular unit as a stable, coherent light source. Its beam passes through the chamber at a right angle to the air flow. To the side of the path of both the airflow and laser beam a detector array is mounted. When an air particle passes across the beam path, light is reflected off from it to all of the mirrors and bounces to the detector. Our machine has a linear CCD array. When many particles pass across the beam this chamber acts as an opticle integrator and the light from all the particles is 'seen' by the array. The higher the output from the array, the larger the quantity of particles. Also, a relative size of the particle is also established. A group of cells on the array may have light hitting them dependent upon the particle size to yield an integrated value equivalent to a particular size. The analog output is run through an A/D converter and processed by one of the venerable Z80 CPUs and sent to a CRT display and printer. (I told you in the previous post that the concept was simple yet the device was a little complicated!) To sumarize the determination of the count: Take exactly one cubic foot of air and pass it through the counter during exactly one minute and count the particles. Therefore, a class 10,000 cleanroom has less than or equal to 10,000 particles in one cu. ft. of air that you can count during one minute which are 0.5 microns and larger. Same for class 1000, etc. Typical modern cleanrooms at chipmakers are class 100 or class 10. There's even a few *Class 1* cleanrooms around (Dept. of Defense is believed to have had one of the first built several years ago.) Some drive makers have advertised they build their drives in class 100 or class 1000 cleanrooms. That yields excellent quality control in that failure from physical defects is held much lower. Imagine this: I recall once reading that the normal air we breathe, when it is extremely clean and quite clear, has a billion or more particles per cu. ft. larger than 0.5 micron. The number skyrockets when plant pollen, smoke have allergies to some airborne things and I feel great when I'm working in the cleanroom. Ours tests to less than class 100 though it is designed for 10,000. No wonder I feel better! During a dry winter day, as we walk along, there are literally billions of skin and dust particles shed from our person nearly all of which are too small to see. Imagine such stuff falling in our modern hard drives where the flying height of the heads is in the tens of microns range. This is only one example of the sources of contamination we'd have to watch for in our homemade cleanrooms/cleanbenches. Just so the spit from yer chaw ain't blown all over! <g> I guess this turned out to be part of a cleanroom primer after all. Regards, Chris -- --