About This Tool

This Centrifuge RCF Calculator is a laboratory support tool designed for researchers, students, and technicians to accurately interconvert between rotational speed (RPM), rotor radius, and Relative Centrifugal Force (RCF or g-force).

What This Calculator Does

Centrifugation protocols often specify a required RCF (g-force) rather than a specific RPM. This is because RCF is a standardized measure of the force applied, independent of the specific centrifuge and rotor used. This calculator allows you to:

• Calculate RCF: Determine the g-force generated when you know the rotor radius and the desired speed (RPM).
• Calculate RPM: Find the correct speed (RPM) to set on your centrifuge to achieve a specific RCF required by a protocol, given your rotor’s radius.
• Calculate Radius: Determine the required rotor radius to achieve a specific RCF at a known RPM, which can be useful for verifying rotor specifications.

When to Use It

This calculator is essential in various laboratory scenarios:

• Adapting Protocols: When a published protocol was performed on a different centrifuge, you can use this tool to determine the correct RPM for your specific rotor to match the required RCF.
• Protocol Development: For designing new centrifugation steps and ensuring reproducibility.
• Instrument Calibration & Verification: To check if the RCF displayed by the centrifuge matches the calculated value based on its speed and rotor dimensions.
• Educational Purposes: For teaching students the relationship between speed, radius, and centrifugal force.

Inputs Explained

• Speed (RPM): Revolutions Per Minute. This is the rotational speed of the centrifuge rotor. It is a measure of frequency, not force.
• Rotor Radius (r): The distance from the center of the centrifuge spindle to the bottom of the sample tube when it is in the rotor. It is crucial to measure this distance accurately for your specific rotor and tube combination. The calculator accepts this value in millimeters (mm), centimeters (cm), or inches (in).
• RCF (x g): Relative Centrifugal Force, also known as g-force. It is the force exerted on the contents of the rotor as a result of rotation, expressed as a multiple of Earth’s gravitational force (g).

Results Explained

Based on the calculation mode you select, the tool provides one of the three key parameters:

• Calculated RCF (x g): The resulting g-force, typically rounded to the nearest whole number.
• Required Speed (RPM): The rotational speed needed to achieve the target RCF, typically rounded to the nearest whole number.
• Required Radius: The rotor radius that would be needed for the given parameters, calculated to two decimal places. This result is provided in the unit you selected for the input (mm, cm, or in).

Formula / Method

The calculations are based on the standard formula that relates RCF, rotor radius, and rotational speed. The radius (r) must be in millimeters (mm) for this formula.

Step-by-Step Example

Let’s say a protocol requires you to spin a sample at 15,000 x g. You measure your rotor radius and find it is 85 mm. You need to find the correct RPM to set on the centrifuge.

1. Select Calculation Mode: Choose “Calculate Speed (RPM)”.
2. Enter RCF: Input 15000 into the RCF field.
3. Enter Radius: Input 85 into the Rotor Radius field and ensure the unit is set to mm.
4. Calculate: The calculator will use the formula to compute the required speed.
5. Result: The tool will output approximately 12,568 RPM. You would set your centrifuge to the closest possible value.

Tips + Common Errors

• Measure Radius Correctly: The most common error is incorrect radius measurement. Always measure from the center of the centrifuge’s spindle to the point at the bottom of the tube where your sample will be most compacted. Do not use the radius to the top of the tube or the edge of the rotor.
• Check Units: Double-check that you have selected the correct units (mm, cm, in) for your radius measurement. An incorrect unit will lead to a drastically wrong result.
• RCF vs. RPM: Always remember that RCF is the standardized force, while RPM is the speed. Protocols should ideally specify RCF for reproducibility. If a protocol only gives RPM, it is only valid for the exact rotor model used in that protocol.
• Do Not Use Diameter: Ensure you are inputting the radius (half the diameter), not the full diameter of the rotor path.

Frequently Asked Questions (FAQs)

What is the difference between RCF and RPM?
RPM (Revolutions Per Minute) is the speed at which the rotor is spinning. RCF (Relative Centrifugal Force) is the force exerted on the sample due to that rotation. RCF depends on both the RPM and the radius of the rotor. Two different centrifuges spinning at the same RPM but with different rotor sizes will produce different RCF values.

How do I measure my rotor radius accurately?
Use a ruler or calipers. Measure the distance from the center of the drive shaft (spindle) to the bottom of the sample tube while it is in its bucket or slot in the rotor. If the tube is at an angle, measure to the point furthest from the center. Refer to your rotor’s manual for the official radius measurement.

Why is it better to use RCF in protocols?
RCF is a standardized and reproducible unit of force. By specifying RCF, a protocol can be replicated on any centrifuge, regardless of its rotor size, by simply calculating the corresponding RPM for that specific machine.

What does ‘x g’ mean?
‘x g’ means “times gravity.” It’s a way to express the RCF as a multiple of the Earth’s standard gravitational force. An RCF of 500 x g means the force applied is 500 times stronger than gravity.

Can I use this calculator for any type of centrifuge?
Yes, this calculator can be used for microcentrifuges, benchtop centrifuges, ultracentrifuges, etc. The physics principle is the same. The key is to have an accurate radius measurement for your specific rotor.

The calculator result is slightly different from my centrifuge’s display. Why?
This can happen for a few reasons. Your centrifuge might be using a pre-programmed radius for the rotor that is slightly different from your measurement. There might also be minor rounding differences in the calculation constants used by the manufacturer’s software.

What is the constant 1.118 x 10⁻⁵ used in the formula?
This is a conversion constant derived from physical principles to reconcile the units involved (revolutions per minute, millimeters, and g-force) into a single, simple equation.

What if my sample is in a swinging-bucket rotor?
The principle is the same. Measure the radius from the center of the spindle to the bottom of the tube when the bucket is in its fully horizontal (spun-out) position.

References

• Protocols and calculations for centrifugation – Thermo Fisher Scientific
• Centrifugation Formulas and Charts – Beckman Coulter
• Centrifugation Know-how – Eppendorf
• Nomogram for Computing Relative Centrifugal Force (PDF) – Thermo Fisher Scientific

Disclaimer

This tool is for informational and educational purposes only. It is not a substitute for professional laboratory guidance, manufacturer’s instructions, or established institutional protocols. The user assumes all risk for the application of information generated by this calculator. Always verify calculations and adhere to all safety procedures recommended for your specific equipment and experimental context.