Laboratory centrifuges: selection, rotors and RPM versus RCF

Start with the required separation

Before comparing centrifuges, describe what must happen to the sample. Are you clarifying an extract while retaining the supernatant, collecting a precipitate or separating fractions? Identify which fraction will proceed to analysis and which losses, contamination or resuspension could affect the result. An apparently clear supernatant alone does not show that the analyte remains in the intended fraction.

Record the matrix, solvents, volume range, vessels, samples per session and acceptable waiting times. Useful capacity depends on the configuration actually used, including controls and positions required for balancing. It does not necessarily equal the advertised maximum number of positions.

Connect these needs to the documented preparation workflow. Under EU GMP, equipment suitability is the starting point: a catalogue feature must become a verifiable requirement justified by the method.

Compare rotor geometries

In a fixed-angle rotor, tubes retain the inclination built into the design. In a swing-bucket rotor, buckets change orientation during rotation. This changes the sedimentation path and the position of the deposit relative to the vessel, affecting how you recover the pellet, supernatant or fractions.

Do not assign one geometry to every method out of habit. Compare useful capacity, separation path, access to the sample and the risk of disturbing the separated material. Required time cannot be inferred from maximum speed alone: geometry, matrix characteristics and the separation objective all contribute to the outcome.

For a transfer, identify the exact rotor, bucket, adapter and tube. “Rotor for 15 mL tubes” is insufficient if two configurations have different radii or sample positions. Request drawings and data for the proposed assembly; visual similarity does not establish compatibility.

Convert RPM and RCF using the right radius

RPM means revolutions per minute. Relative centrifugal force, RCF, is the ratio of centrifugal acceleration to standard gravity. It is dimensionless and commonly expressed as a multiple of g; it is not a mass in grams. The circular-motion relationship gives:

RCF ≈ 1.118 × 10⁻⁵ × r × n², where r is in centimetres and n in revolutions/min. The rounded coefficient uses g₀ = 9.80665 m/s² and converts minutes to seconds. Do not enter a radius in millimetres while retaining this coefficient.

Simulated example: assume a maximum radius r_max, from the axis to the inside bottom of the tube in its operating orientation, of 10.0 cm for A and 15.0 cm for B. At 3,000 RPM, the rounded formula gives 1,006.2 × g and 1,509.3 × g, respectively: B is 50% higher. These are calculated values, not measurements of actual speed.

To obtain A’s calculated RCF on B, n_B = 3,000 × √(10/15) ≈ 2,449 RPM. This is a mathematical comparison, not an authorised setting. Check the radius definition used by the method and the instrument: minimum, average and maximum are not interchangeable. An adapter may change the tube position. Matching RCF still leaves sedimentation path and cycle conditions to compare.

Check compatibility and loading of the assembly

The centrifuge, rotor, buckets, adapters, caps and tubes form a system. A combination must be explicitly permitted for the intended speed, temperature, matrix and load. One component’s limit does not authorise the entire assembly to operate at that limit.

Request applicable mass, density and filling limits, permitted loading patterns and balancing criteria. Equal volumes need not mean equal masses when liquids, vessels or accessories differ. Do not introduce generic tolerances in grams: follow those for the documented configuration.

Also assess chemical resistance, closure and mechanical support of the vessel. A tube that fits into an adapter may still be unsuitable. Do not modify parts or compensate for an unapproved configuration with an intuitively chosen speed. Missing information means compatibility remains unproven before use.

Specify temperature, duration and ramps

Define the temperature the sample requires and the period over which it must be maintained. A refrigerated centrifuge does not automatically demonstrate that every sample reaches and remains at the displayed value. Load, starting conditions, rotor and duration must be included in the conditions assessed.

Clarify when timing starts: at run initiation or when the set speed is reached. Two programmes with identical nominal durations may therefore provide different exposures. Specify acceleration and braking compatible with the method; different braking may disturb a sensitive separation.

For critical analytes or matrices, connect the comparison to stability and sample-preparation recovery evidence. Selection should make the complete cycle controllable, including waiting before fraction recovery, rather than only the period at constant speed.

Plan installation and safe operation

Before ordering, check space, a stable support, power, ventilation, maintenance access and clearances required by the instructions. Consider sample chemical and biological risks, necessary containment and management of leaks or breakages. A closed lid alone is not evidence of aerosol containment.

IEC 61010-2-020:2026 addresses safety of electrically powered laboratory centrifuges and replaces the 2016 edition. The public record confirms its scope; it does not establish conformity of a particular model here. Assess the declared edition, local adoption and safety documentation in the purchasing context.

Define training, pre-use checks, maintenance and rotor tracking where required. Respect complete stopping and protective devices: no check justifies opening during rotation or defeating interlocks. Qualification and maintenance require specific plans beyond this initial selection.

Turn the comparison into a decision

This original matrix helps formulate precise requests before purchase. Complete it for each candidate configuration, recording the document received, gaps and the responsible person’s decision.

Simulated case: a laboratory transfers a procedure from rotor A to B in the example and proposes identical RPM. Review immediately identifies a different RCF. Correcting the calculation is necessary but does not complete the transfer: the responsible person compares geometry, tubes, load, time, temperature and fraction recovery, then approves targeted tests before routine use. The final criterion is documented suitability of the system for the method.

Explore further topics in the liquid handling and sample preparation area.

For the related steps, see: Centrifuge qualification: speed, temperature and time; Centrifuge maintenance: vibration, rotors and faults.

Sources and scope

• OpenStax, Rice University — Physics, circular motion: physical relationship between radius and acceleration.
• NBS Technical Note 491 — Gravity measurements: standard gravity; historical metrological reference.
• University of Tennessee EHS — Centrifuge Safety: geometry and safety; local instructions are not transferred as universal limits.
• IEC 61010-2-020:2026, edition 4: public record and scope consulted, not the full text.
• EU GMP Chapter 3: equipment suitability, installation and maintenance.