Are you considering refurbishing a Semi Anechoic Chamber (SAC)? If so, this short sidebar article should help you. In 2007, IBM refurbished an 18 year old SAC. The overhaul was a challenging project as it had to convert the facility designed for an abandoned mission to a far more complex one testing Enterprise Servers (very large main frame computer systems). The Normalized Site Attenuation (NSA) volume doubled. The electrical power increased from 36 KW to 400 KW. The turntable diameter expanded to 6.0 meters while its weight capacity went from 3,000 pounds to 25,000 pounds. Finally, HVAC capacity tripled.
For IBM, this project was a significant capital investment. The chamber, at some 80’ long x 45’ wide x 29’ high, still performed well; however, the absorber did not fare so well over time. In addition, considerable advances in absorber performance since the chamber was installed 18 years ago made refurbishment a practical consideration. Investing in new absorber and refurbishing our existing chamber made sense in our case. Given the multi-million dollar amount of this refurbishment, however, we had to be thorough in our process to select the optimal contractor. Hopefully, you can benefit from our experience!
Building a new SAC or refurbishing an old one is a detailed technical construction project. First and foremost, it is very important to understand what the refurbished chamber will be used for. The project manager must understand what is going in it and how it will be used. You need to understand basic physical characteristics such as the EUT’s dimensions, weight, electrical requirements and HVAC. You should also forecast future requirements as it is very expensive to upgrade later. At this point, you have to decide if the existing SAC can be upgraded for the new mission. There is no hard and fast answer here; you have to use your technical knowledge, experience and judgment if it can be upgraded. Among other factors, Normalized Site Attenuation (NSA) volume and structural integrity are major considerations.
If your engineering judgment is “yes,” then it is time to call in potential contractors. Have your test requirements firm and crisp. Dig out the original blueprints; these are most helpful for the bidders. Better yet, have copies available.
After all the contractors reviewed the existing SAC, IBM had extensive discussions with them over an extended period since this was a technically challenging project that pushed the limits of the current facility. We wanted to insure that the contractors understood what IBM wanted and, in fact, could do the transformation. IBM had two major concerns: EUT volume and turntable structural integrity. The diameter of the turntable would increase to 6.0 meters and come close to the potential edge of anechoic material. In addition, the floor to ceiling height would decrease by 12 inches. The larger turntable and smaller height affects Normalized Site Attenuation. The other major concern was the new turntable’s larger diameter and increased weight capacity.
In addition, we had extensive discussions on how Enterprise Servers would be tested with their numerous signal and power cables. Also, some products are cooled with chilled water; we had to contend with fairly inflexible hoses. These detailed discussions ensured the EMC Lab personnel could test with ease.
As you are having these discussions, it is important to understand that each contractor has proprietary techniques, information and insight. We went to extensive lengths to ensure there was no “cross contamination” between the bidders. Each one had unique insights about the project, but we did not share such information with others.
Once these discussions are finished and you are confident that all potential contractors understand what is required, you can develop a uniform bid document. Such a bid document will contain both a written description and drawings. Remember, the bid becomes a legal contract upon acceptance so make sure all details are included and accurate.
The following discussion is pertinent to both new construction and refurbishment. However, these items were of particular interest to the refurbishment project.
NSA performance is most critical. If the refurbished chamber does not pass the Normalized Site Attenuation requirement, it can not be legally used for product certification work. It is that important. In IBM’s case, the new test volume was so large that it came close to the anechoic foam tips. IBM’s Distinguished Engineer, Dr. Bruce Archambeault, modeled the SAC (Dr. Archambeault’s modeling report showing predicted versus actual measured performance of the new absorber material installed in IBM’s chamber can be seen here). Most buyers cannot do this. Because of the considerable investment in the new absorber, Dr. Archambeault modeled the predicted performance of the new absorber and required that all bidders provide modeled data with documentation of actual measurements to verify the accuracy of the modeling. If you do not have someone on staff that is skilled in modeling, you should ask your bidders for a comparison report showing the predicted AND measured results of the proposed absorber. Also, it is important that the bidders know what antennas and instrumentation you use. In the contract, you should specify an NSA margin and decide how the final performance test should be conducted.
Important to refurbishments is a shielding effectiveness test (SE) with 100 dB being standard performance. It is important to patch up leaks. Chances are that the facility has many “untreated” penetrations. In IBM’s case, we saw light through one! Usually the SE test is performed just before the anechoic material is applied. There will be some areas where you will not achieve the 100 dB performance level. You need to understand when penetrations for electrical, light, HVAC and signals are performed. A second SE test may be necessary.
Be sure to understand the fire protection service your insurance carrier requires and incorporate this in the bid package. There is a good chance that the requirements are more stringent then what was originally required. Understand that fire heads impact NSA performance.
It is a good idea to be mindful of the local construction codes and the building inspectors who enforce them. If you are not, these folks can stop you cold!
I could go on but space limitations force me to stop. The most important advice that I can offer is that a chamber overhaul is a detailed technical construction project. You need to stay on top of budgets, schedules, the contract, the prime bid winner and all their subcontractors – not to mention your own company’s requirements. You have to play well with others. There is no magic bullet; just plain hard work to ensure success. But if you do, you will have a successful project. The original chamber lasted 18 years; we hope the refurbished one lasts as long.
For further information, please contact the author at ericrs@us.ibm.com or at phone: 919-543-5397. 
