Question 1

What is the difference between RPM and RCF?

Accepted Answer

RPM (revolutions per minute) measures how fast a rotor spins, while RCF (relative centrifugal force), measured in x g, quantifies the actual force applied to the sample. RCF depends on both RPM and rotor radius: RCF = 1.118 x 10^-5 x r x RPM^2. Two centrifuges at the same RPM but with different rotor radii will produce different g-forces, which is why protocols should specify RCF rather than RPM for reproducibility.

Question 2

How do I find my rotor radius?

Accepted Answer

The rotor radius (r) is the distance from the center of the rotor (axis of rotation) to the bottom of the sample tube when in the rotor. This measurement is typically found in the rotor's technical specifications or user manual. It is NOT the radius of the rotor body itself. For fixed-angle rotors, measure to the bottom of the tube at its angled position. For swinging-bucket rotors, measure to the bottom of the bucket when fully extended horizontally.

Question 3

Why should I use RCF instead of RPM in protocols?

Accepted Answer

Using RCF ensures reproducibility across different centrifuges and rotors. A protocol calling for 300 x g will pellet mammalian cells regardless of which centrifuge you use, but the same RPM on different rotors produces different forces. Scientific journals and regulatory agencies require RCF values. This is especially critical when scaling between microcentrifuges and floor-model centrifuges where rotor radii differ significantly.

Question 4

What is the formula to convert RPM to g-force?

Accepted Answer

The formula is: RCF (x g) = 1.118 x 10^-5 x r x RPM^2, where r is the rotor radius in millimeters and RPM is revolutions per minute. To convert the other way: RPM = sqrt(RCF / (1.118 x 10^-5 x r)). The constant 1.118 x 10^-5 is derived from the physics of circular motion (4 x pi^2 / 3600^2 / 9.81) and converts angular velocity to relative centrifugal force.

Question 5

Can I exceed the maximum RPM of my rotor?

Accepted Answer

Never exceed the maximum rated RPM of your rotor. Rotor speed limits are determined by the rotor's material strength, design, and the maximum centrifugal force it can safely withstand. Exceeding the limit risks catastrophic rotor failure, which can destroy the centrifuge and cause serious injury. Always account for sample density — when using solutions denser than water (e.g. CsCl gradients), the maximum RPM must be de-rated according to the manufacturer's guidelines.

Question 6

What RCF do I need to pellet different cell types?

Accepted Answer

Common centrifugation forces: mammalian cells (adherent or suspension) pellet at 200-300 x g for 5 minutes; bacteria (E. coli) require 3,000-4,000 x g for 10 minutes; yeast cells need 3,000 x g for 5 minutes; cell debris/lysate clarification uses 12,000-16,000 x g for 20 minutes; mitochondria isolation requires 10,000 x g; and virus/exosome pelleting needs 100,000+ x g in an ultracentrifuge. Always use the gentlest force that achieves separation to maintain cell viability.

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