What you are given
- Speed (rpm): 10000
- Rotor radius (cm): 10
Formula: RCF = 1.118·10⁻⁵ · r · rpm²
Physical
How do you convert rpm to g-force (rcf)?
rcf = 1.118 x 10^-5 x r x rpm^2, with the radius r in cm. Because rpm is squared, the same speed in two different rotors gives two very different forces, which is why protocols should quote g and not rpm.
Worked example
Every number below is computed by the calculator, not typed in
The working
- Formula:
RCF = 1.118 × 10⁻⁵ · r · rpm² - Substitute:
RCF = 1.118 × 10⁻⁵ · 10 · 10000²=11180 ×g
At r = 10 cm, that is 11180 ×g.
What each symbol means
- RCF: Relative centrifugal force (×g)
- k: Lumped constant (1.118 × 10^-5) (1/cm)
- r: Rotor radius (cm)
- rpm: Rotor speed (rpm)
A printable one-page version is on the formula sheet.
Where this usually goes wrong
- Copying rpm from a protocol written for a different rotor. Always convert through g.
- Using the rotor radius at the top of the tube rather than at the pellet.
- Mixing radius units. The constant above expects centimetres.
Try it yourself
Two problems, with the working revealed
Problem 1 RPM ⇄ RCF
Convert between rpm and rcf for this rotor.
- Convert: ×g → rpm
- Force (×g): 20000
- Rotor radius (cm): 9
Show the answer and the working
At r = 9 cm, that is 14100 rpm.
- Rearrange:
rpm = √( RCF / (1.118 × 10⁻⁵ · r) ) - Substitute:
rpm = √( 20000 / (1.118 × 10⁻⁵ · 9) )=14100 rpm
Open this in the calculator to change the numbers.
Problem 2 RPM ⇄ RCF
Convert between rpm and rcf for this rotor.
- Speed (rpm): 10000
- Rotor radius (cm): 10
Show the answer and the working
At r = 10 cm, that is 11180 ×g.
- Formula:
RCF = 1.118 × 10⁻⁵ · r · rpm² - Substitute:
RCF = 1.118 × 10⁻⁵ · 10 · 10000²=11180 ×g
Open this in the calculator to change the numbers.
Do it with your own numbers.
The RPM ⇄ RCF calculator is free, shows every step, flags the hazards, and works offline once loaded. No account, no signup.
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