In centrifugation the clearing factor or k factor represents the relative pelleting efficiency of a given centrifuge rotor at maximum rotation speed. It can be used to estimate the time (in hours) required for sedimentation of a fraction with a known sedimentation coefficient (in svedbergs):
The value of the clearing factor depends on the maximum angular velocity of a centrifuge (in rad/s) and the minimum and maximum radius of the rotor:
As the rotational speed of a centrifuge is usually specified in RPM, the following formula is often used for convenience:
Centrifuge manufacturers usually specify the minimum, maximum and average radius of a rotor, as well as the factor of a centrifuge-rotor combination.
For runs with a rotational speed lower than the maximum rotor-speed, the factor has to be adjusted:
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The K-factor is related to the sedimentation coefficient by the formula:
Where is the time to pellet a certain particle in hours. Since is a constant for a certain particle, this relationship can be used to interconvert between different rotors.
Where is the time to pellet in one rotor, and is the K-factor of that rotor. is the K-factor of the other rotor, and, the time to pellet in the other rotor, can be calculated. In this manner, one does not need access to the exact rotor cited in a protocol, as long as the K-factor can be calculated. Many online calculators are available to perform the calculations for common rotors.
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