SATURATIONThe flux density should be less than 7,500 Gauss forCO-NETIC and less than 21,000 Gauss for NETIC. If the flux density is greater, thicker material or multiple layers will be required. For high attenuation of weak and moderate strength fields, a single layer of CO- NETIC AAAlloy is normally adequate. In high strength fields, light gauge CO-NETIC may saturate and become less effective. If layers of CO-NETIC and NETIC are used in combination to increase the flux capacity, the NETIC material should always be placed nearest the source of interference. ATTENUATIONThe attenuation or shielding efficiency of a magneticshield is specified as the attenuation ratio. This is the ratio of measured field before shielding to that meas- ured after shielding. Attenuation in decibels is 20 times the logarithm (base 10) of the shielding ratio. A properly designed single layer CO-NETIC shield of small size will easily provide 30 to 40 db attenuation. Attenuation of 60 db or more is attained by utilizing multiple layers of shielding material. Attenuation decreases in shields of large volume, or having large openings or unusual configurations.µ• tA(attenuation ratio) = D The need for de-rating the attenuation highlights theimportance of selecting the optimum size shield. Attenuation should be calculated for two sizes; a cryogenic atmosphere shield and a room tempera- ture shield.For operation at cryogenic temperatures, the per-formance factor must be applied, reducing the atten-uation achieved. The advantage is that the shieldvolume is smaller. If the shield is at room temperature, no de-rating of permeability is necessary, but the shield volume is likely to be larger in order to surround the apparatus.Because shield size enters the formulas for inductionand attenuation, thicker materials may be needed in either case. The optimum choice can be made by examining both possibilities.Approximate performance factors at typical cryo-genic temperatures are shown in the graph below. To use the factors, multiply the room temperature attenuation by the performance factor shown for the operating temperature (T) of the shield:Performance factor at TA
T =A• 100A
T =Attenuation ratio at temperature T.

VI Low Temperature Operation

CO-NETIC AAAlloy is affected by operation at lowtemperatures. The saturation induction ( which deter- mines maximum field) remains unaffected, but the permeability decreases as the temperature drops, because of reduced molecular mobility. MAGNETIC SHIELD CORP. Perfection Mica Co.740 N. Thomas Drive Bensenville, IL60106 U.S.A. Fax 630-766-2813 Phone 630-766-7800