A centrifuge is balanced when the load keeps its center of mass in the middle of the rotor. Pairs across from each other do that, and a tube of water fixes the counts that won't.
At thousands of rpm, a small difference in weight around the rotor is magnified and the centrifuge wobbles or shakes. INTEGRA Biosciences notes this wears on the instrument over time.
Many centrifuges switch off when they sense an imbalance. Beckman Coulter points out that a milder imbalance can still spoil a separation without tripping that sensor.
Stand the centrifuge on a level, steady surface before you load it.
Place tubes in pairs, directly across the rotor from each other. Both tubes in a pair need the same mass, which for the same liquid means the same volume.
Close the lid and run at the speed the protocol gives. Protocols often list g-force, and the RCF calculator turns it into rpm for your rotor.
Add a balance tube. Fill a tube of the same type with water and set it across from the sample that has no partner.
INTEGRA stresses that the water tube needs the same mass as the sample, not always the same volume. A denser sample needs a little more water to match.
A scale settles it. The milligram scale in home lab starter equipment weighs a tube to 0.001 g.
Some odd counts balance without water when the tubes can be spaced evenly. Three tubes in a 6-place rotor, one in every other slot, are balanced.
Tubes of equal mass, spaced symmetrically. A count in the last column needs a water tube to reach a count that works.
| Rotor places | Counts that balance | Counts that need a water tube |
|---|---|---|
| 6 | 2, 3, 4, 6 | 1, 5 |
| 8 | 2, 4, 6, 8 | 1, 3, 5, 7 |
| 10 | 2, 4, 5, 6, 8, 10 | 1, 3, 7, 9 |
| 12 | 2 through 10, and 12 | 1, 11 |
| 24 | 2 through 22, and 24 | 1, 23 |
3 tubes in a 6 or 12-place rotor. Space them evenly, a third of the way around from each other. That is every other slot in a 6-place rotor and every fourth slot in a 12-place rotor.
3 tubes in an 8-place rotor. No spacing works. Add a water tube and load two pairs across from each other.
5 tubes in a 12-place rotor. Place 3 evenly, then add a pair directly across from each other in two empty slots.
5 tubes in a 6 or 8-place rotor. No arrangement balances. Add a water tube and load three pairs.
7 tubes in a 12-place rotor. Place 3 evenly, then add two pairs across from each other in the empty slots.
1 tube in any rotor. Always add a water tube directly across from it.
Rotors come in different place counts, so check yours before each run. Each mini centrifuge page lists its rotors, including the OHAUS Frontier 5306, SCILOGEX EZee and Thermo mySPIN 6.
The ONiLAB here holds six 15 mL tubes. By the table, 2, 3, 4 or 6 tubes balance on their own, and 1 or 5 need a water tube.
Its lid stays locked until the rotor stops, and the timer runs up to 99 minutes. For smaller tubes in a mini centrifuge, see mini centrifuge tube adapters.
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Holds six 15 mL tubes, with adapters for smaller ones. Runs 300 to 5000 rpm, up to 2600 × g, with a lid that locks until the rotor stops.
Check that every cap is closed, the tubes are all the same type and none is cracked. If the centrifuge starts to shake or creep across the bench, stop it and reload.
If it still shakes on a load you have checked, stop using it and contact the manufacturer. A rotor or a bearing that is failing does not announce itself any other way.
