Hi, as I understand it - the difference could be whether the chamber is humidity controlled. Much of the RAM on the market is a sponge hanging on a metal wall in a semi sealed environment. When the temperature drops in the main building (as it does when the site facilities turn off the heating during winter holidays) the moisture in the trapped air within the chamber condenses and is absorbed by the RAM. The increased weight places excessive stress on the glue and the block falls off the wall or ceiling. In longer RAM - I have experience of 72" - the RAM starts to droop with the weight of absorbed moisture - looking rather like the nose of Concord on landing. I have no other explanation - this one was provided to me by the people who sold me the RAM in the first place! Regards Tim P Please consider the environment before printing this email.
_____ From: [email protected] [mailto:[email protected]] On Behalf Of Price, Ed Sent: Monday, November 19, 2007 2:01 PM To: IEEE EMC Discussion Group Subject: RE: Semi-Anechoic Chamber Question *** WARNING *** This mail has originated outside your organization, either from an external partner or the Global Internet. Keep this in mind if you answer this message. _____ From: [email protected] [mailto:[email protected]] On Behalf Of Philo Beddo Sent: Saturday, November 17, 2007 10:22 AM To: [email protected] Subject: Semi-Anechoic Chamber Question Regards All, I was speaking with a co-worker in North America this past week, he mentioned, at times the absorber material (cones) may seperate from the walls of the chamber proper. I was curious about this as my employer is considering construction of a semi-anechoic facility in the near future. My understanding is that in some cases the absorber material will remain fixed on the walls for many years, and in other cases only two or three years before sagging and seperation become an issue. When we begin to specifiy and construct our facility, what performance should I expect over a 5-10 year period with respect to the bonding of the cones to the chamber proper? Is this an ongoing maintence issue with various contruction materials, or are there known defects in the adhesion process? Is there a process to check the existing absorbers after they are in place year over year? Finally, how will the chamber performance (EME and EMI) be compromised if the cones sag or become seperated from the chamber proper? Phil: In my current chamber, I use 24" cones on the floor and walls. I have had no sagging or separation problems in the 12 years since the initial installation. There was once a worry that oil-based adhesives might dissolve the urethane foam, so people were using a special foam-rated adhesive. I tried that adhesive, and I didn't think it was sticky enough. I have used simple panel adhesive; one brand widely available in the USA is "Liquid Nails." I use a rather large amount, about a half-tube on each 24" x 24" block of foam. Also, you should "butter" the foam face evenly but thoroughly. Stick the foam block in place, wiggle it a little bit, and then pull it off the wall for a couple of minutes to let the adhesive surface "tack." Then stick it back on, bracing for several hours as needed. It's a tedious and dirty process, but once done, you shouldn't have anything peeling off the walls. I imagine that the urethane foam degrades from ultraviolet, oxygen and ozone exposure. UV shouldn't be a factor in a chamber. I have seen urethane foam that has become either friable or gooey after perhaps 20 years. Ed Price <mailto:[email protected]> [email protected] WB6WSN NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (FAX) Military & Avionics EMC Is Our Specialty - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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