If my old centrifuge was set for 5,000 RPM and my new and larger centrifuge is set for the same speed, then why are my results different?
It has to do with the radius of the spin. If you spin a sample at 5000 RPM at 100 millimeters from the center of spin, you will imply a force of 2,795 RCF (g’s) onto the sample. Now, spin that same sample in a larger centrifuge at the same speed (let’s say 130 mm from center), and suddenly you’re at 3,634 RCF (g’s). You raised the force by about 33% simply by using a larger centrifuge! So when you talk about protocols and spinning at a set speed, you need to know the radius of the spin to be sure you are still following your laboratory’s protocol. To calculate RPM or RCF for your new centrifuge, try the RCF / RPM calculator.