rpm conversion to g force
Students in my lab assume the rpm displayed on our Sorvall Fresco microcentrifuge is the same as the g force. Do you have a chart with the conversion for this?
1. Determine the Radius (r) in mm of the rotor. This is found in your rotor manual and usually gives the minimum, avg and maximum. Use whichever comes closest to the center of your sample.
2. RCF=1.12 r (RPM/1000)^2
A chart (called a nomogram) is a useful reference to show the relationship between Speed Radius & RCF ('g' force) but it is so imprecise that you should never use one for accurate results.
The formula given is roughly correct (and given by Beckman) but more precise and given my most others is:
RCF (x g) = 1.118 x radius (mm) x (rpm/1000)²
You can see the importance of the radius (often forgotten). Everyone expects to get more 'g' if the rotor goes faster, but forget that a 20% higher radius gives a 20% higher RCF AT THE SAME SPEED.
The most frequently quoted radius, is the point farthest from the center of the rotor (which is not the bottom of the tube in an angle rotor). Other radii of interest are Minimum Radius (surface of the liquid closest to the center of the rotor) and Boundary Radius (where separated components meet, if not pelleted).
I can provide you with an Excel-based Centrifugation calculator that calculates
- RCF, Radius, rpm (calculating one from the other two parameters),
- shows you what RCF you'd get in a different rotor or centrifuge using the same speed,
- Shows what speed to use to get the same RCF (even if you don't know what its is and/or don't have RCF control!) in a different rotor or centrifuge.
If you would like to receive a copy, please contact me.
A professional version is also available including integral (for highest reproducibility) and k-factor (for optimized protocol transfer) for establishments taking my interactive centrifugation training course.
Hope this helps,