Key takeaway
Identical RPM can produce different relative centrifugal force in different rotors. The force also changes with the square of speed.
Read the sample position as well as the speed
A preparation log says that a tube was spun at 4,000 rpm for ten minutes. The receiving scientist asks whether that reproduces a protocol run. Without the rotor and sample-radius convention, the speed alone cannot answer. Time and temperature are additional conditions, not substitutes for the missing geometry.
Eppendorf’s protocol guidance retains sample, vessel, rotor, force and temperature, and describes maximum radius to the tube bottom. It also explains that an adapter can move the tube inward. Beckman’s RCF discussion likewise distinguishes radius and rotational speed, with force varying as speed squared. These are manufacturer explanations of parameters, not an approval to transfer a particular biological protocol.
Compare three fictional configurations
This hypothetical comparison assumes the same radius convention and ideal rigid rotation. Configuration A has an effective sample radius of 8 cm at 4,000 rpm. B has a 12 cm radius at the same speed. C returns to 8 cm but runs at 2,000 rpm. Define A’s relative force as 1.00; no absolute g value is asserted.
Because relative force is proportional to radius times speed squared, B is 12/8 = 1.50 times A. C is (2,000/4,000)² = 0.25 times A. The table completes the comparison without copying an unseen instrument force display. Matching speed would therefore fail to match this defined force between A and B.
| Configuration | Effective radius | Speed | Force relative to A |
|---|---|---|---|
| A | 8 cm | 4,000 rpm | 1.00 |
| B | 12 cm | 4,000 rpm | 1.50 |
| C | 8 cm | 2,000 rpm | 0.25 |
| Historical row | Unknown rotor/adapter | 4,000 rpm | Unresolved |
Do not infer the radius from a rotor nickname
Record rotor part/model, fixed-angle or swinging arrangement, bucket and adapter where relevant, vessel identity, radius convention and the source of that dimension. A maximum, average or minimum sample radius describes a different point. Eppendorf’s adapter warning is a reason to recover the actual configuration rather than use the largest radius listed in an unrelated brochure.
Keep requested and achieved speed/force separate when the instrument retains both. Save time-at-setting versus total elapsed run according to the actual protocol and instrument records; acceleration and braking information may matter to the scientific reviewer. A ten-minute timer entry should not be silently relabeled as ten minutes at target conditions.
If a later researcher reconstructs force from documented geometry and speed, label it as derived and retain the calculation version. Do not overwrite a historical force field just to make it agree. A disagreement can reveal different radius conventions, a changed adapter or an unresolved export mapping; the archive alone may not identify which explanation is right.
Make a bounded transfer decision
The completed review accepts the force ratios for A, B and C under their explicit assumptions. It holds the historical row. It does not predict pellet yield, recovery, purity or sample viability from the ratios. Equivalent force alone does not demonstrate that two preparation procedures are scientifically equivalent.
Ask the laboratory owner for the protocol version and rotor/adapter record before treating the history as a reference preparation set. Use Inventory to describe which periods retain those links and Due diligence to state the receiving experiment’s needs. An initial discussion can use the fictional comparison; real preparation logs, sample material and recipient access each retain their separate review boundaries.
This distinction is useful even when no protocol transfer is planned. A reader comparing historical batches can stratify known configurations and leave unresolved runs visible. Deleting the unknown rows would conceal the limitation, while assigning every row the current rotor would create a historical fact that was never recorded.