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 t (in hours) required for sedimentation of a fraction with a known sedimentation coefficient s (in svedbergs):
t = k s The value of the clearing factor depends on the maximum angular velocity ω of a centrifuge (in rad/s) and the minimum and maximum radius r of the rotor:
k = ln ( r m a x / r m i n ) ω 2 × 10 13 3600 As the rotational speed of a centrifuge is usually specified in RPM, the following formula is often used for convenience:
k = 2.53 ⋅ 10 5 × ln ( r m a x / r m i n ) ( R P M / 1000 ) 2 Centrifuge manufacturers usually specify the minimum, maximum and average radius of a rotor, as well as the k factor of a centrifuge-rotor combination.
For runs with a rotational speed lower than the maximum rotor-speed, the k factor has to be adjusted:
k a d j = k ( maximum rotor-speed actual rotor-speed ) 2The K-factor is related to the sedimentation coefficient S by the formula:
T = K S
Where T is the time to pellet a certain particle in hours. Since S is a constant for a certain particle, this relationship can be used to interconvert between different rotors.
T 1 K 1 = T 2 K 2
Where T 1 is the time to pellet in one rotor, and K 1 is the K-factor of that rotor. K 2 is the K-factor of the other rotor, and T 2 , 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.
