RPM / RCF Converter

Convert between RPM and Relative Centrifugal Force (RCF, x g). Formula: RCF = 1.118 x 10-5 x r x N2

Rotor Radius (cm) ?
RPM
RCF (x g)
Stokes Settling Velocity

Calculate terminal settling velocity under gravity or centrifugal force. v = d2(rho_p - rho_f) x a / (18 x mu)

Particle Presets
Particle Diameter (um)
Particle Density (g/mL)
Fluid Density (g/mL)
Fluid Viscosity (cP)
Centrifugal RCF (x g) ?
Sigma Factor Calculator (Equivalent Settling Area)

Calculate the Sigma factor for different centrifuge geometries. Sigma represents the equivalent settling area in m2.

Lab Batch
Tubular Bowl
Disc Stack
Centrifuge Presets
Bowl Volume (mL)
Inner Radius r1 (m) ?
Outer Radius r2 (m)
RPM
Centrifugation Scale-Up (Q/Sigma Equivalence)

Scale from lab batch centrifugation to continuous production by maintaining the same Q/Sigma ratio.

SOURCE

Lab Centrifuge

Process Volume (mL)
Centrifugation Time (min)
Source Sigma (m2) ?
TARGET

Production Centrifuge

Target Sigma (m2)
Production Volume (L)
Separation Efficiency vs. Particle Size

Shows theoretical separation efficiency for different particle sizes given the centrifuge parameters and flow rate.

RCF (x g)
Sigma (m2)
Flow Rate (L/h)
Density Diff (g/mL)

Frequently Asked Questions

What is the Sigma factor in centrifugation?
The Sigma factor (equivalent settling area) represents the cross-sectional area of a gravity settler that would have the same clarification capacity as the centrifuge. It normalizes centrifuge performance regardless of geometry, making it possible to compare tubular bowl, disc stack, and batch centrifuges. The unit is m2, and higher Sigma means greater separation capacity.
How do you scale up centrifugation using the Sigma factor?
Maintain the same Q/Sigma ratio between lab and production. In the lab, Q = V/t (volume processed per unit time). For the production centrifuge, calculate Q_target = (Q_lab / Sigma_lab) x Sigma_production. This ensures equivalent clarification performance. A safety factor of 0.5-0.7 is commonly applied to account for non-ideal flow.
Disc stack vs. tubular bowl: which should I use?
Tubular bowl centrifuges generate very high g-forces (up to 20,000 x g) and are excellent for separating small particles or clarifying low-solids feeds. However, they have limited solids-holding capacity. Disc stack centrifuges offer continuous solids discharge and much higher throughput, making them the standard for large-scale cell harvesting in bioprocessing (e.g., monoclonal antibody production).
How do I convert RPM to RCF (g-force)?
Use the formula RCF = 1.118 x 10-5 x r x N2, where r is the rotor radius in centimeters and N is the speed in RPM. This gives the centrifugal force as a multiple of gravitational acceleration. Always report centrifugation conditions in RCF (not RPM) for reproducibility, since the same RPM gives different g-force on different rotors.
What is Stokes' Law and how does it apply to centrifugation?
Stokes' Law gives the terminal settling velocity of a spherical particle: v = d2(rho_p - rho_f) x a / (18 x mu). Under gravity, a = g (9.81 m/s2). In a centrifuge, a = omega2 x r, which can be thousands of times larger. The key insight is that velocity scales with d2, so doubling particle size increases settling speed 4-fold. This is why cell debris (0.5 um) is much harder to remove than whole cells (5-15 um).
Why is separation efficiency dependent on particle size?
Settling velocity scales with d2 per Stokes' Law. In a continuous centrifuge, particles must settle to the bowl wall before being carried out with the supernatant. Larger particles settle faster and are captured more efficiently. At a given flow rate, there exists a critical particle size below which particles escape with the centrate. Reducing flow rate or increasing Sigma (more discs, higher RPM) improves capture of smaller particles.

How to scale up centrifugation in bioprocessing

Centrifugation is a critical unit operation in downstream bioprocessing, used for cell harvesting, clarification, and inclusion body recovery. The challenge of scale-up lies in translating batch lab conditions (where you spin a fixed volume for a set time) to continuous production centrifuges (which process feed at a constant flow rate).

The Sigma factor (equivalent settling area) provides the bridge between scales. By maintaining the same Q/Sigma ratio, you ensure equivalent clarification at any scale. The Sigma factor accounts for differences in geometry, RPM, and centrifuge type (batch, tubular bowl, or disc stack).

Stokes' Law governs particle settling: v = d2(rho_p - rho_f) x g / (18 x mu). In a centrifuge, g is replaced by the centrifugal acceleration omega2 x r, enabling separation of particles too small or too close in density for gravity settling. Understanding the d2 dependence is crucial for predicting which particle populations will be separated at a given centrifuge setting.