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In page Centrifuge:

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To avoid having to perform a mathematical calculation every time, one can find nomograms for converting RCF to rpm for a rotor of a given radius. A ruler or other straight edge lined up with the radius on one scale, and the desired RCF on another scale, will point at the correct rpm on the third scale.[1] Based on automatic rotor recognition, modern centrifuges have a button for automatic conversion from RCF to rpm and vice versa.[citation needed]