Balance and run a centrifuge safely, and convert between rpm and relative centrifugal force (× g).
Awareness level only. This lesson explains the ideas so you recognise them. It does not qualify you to do this work. Before you use lab equipment or handle chemicals, biological materials or patient samples for real, you need the training your employer or institution requires, and you must work under supervision until they sign you off.
Why it matters
Centrifuges separate things by density: cells from culture medium, plasma from blood cells, a DNA pellet from ethanol, a precipitate from its solution. They are among the most common instruments in any biology, chemistry or clinical lab, and among the most dangerous when misused. A rotor spinning at thousands of revolutions per minute stores a lot of energy. An unbalanced load can make the centrifuge shake, "walk" across the bench, or in the worst case fail the rotor. Broken tubes can release aerosols of whatever was inside.
How it works
Spinning a sample creates a force that pushes denser particles outward, toward the bottom of the tube. They collect as a pellet. The liquid above is the supernatant. Separation depends on particle size and density, the liquid's viscosity, the force and the time.
RPM versus RCF
Protocols should state the relative centrifugal force (RCF) in multiples of gravity (× g), not the speed in revolutions per minute (rpm). The same rpm gives different forces in rotors of different radius:
RCF = 1.118e-5 x r x rpm^2
where r is the rotor radius in centimetres (to the middle or bottom of the tube, as your protocol specifies).
Worked example. A rotor with r = 10 cm at 3,000 rpm:
RCF = 1.118e-5 x 10 x 3000^2 = 1.118e-5 x 10 x 9,000,000 = about 1,006 x g
The same 3,000 rpm in a rotor with r = 15 cm gives about 1,509 × g. If a method says "1,000 × g for 10 minutes", you calculate the rpm for your rotor, or use the instrument's RCF setting.
Types of centrifuge
| Type | Typical maximum | Common uses |
|---|---|---|
| Microcentrifuge | about 15,000 to 21,000 × g | 1.5 and 2 mL tubes, DNA and protein preps |
| Benchtop centrifuge | a few thousand × g | Blood tubes, cell pellets, 15 and 50 mL tubes |
| High-speed floor centrifuge | tens of thousands × g | Larger volumes, subcellular fractions |
| Ultracentrifuge | hundreds of thousands × g | Viruses, ribosomes, density gradients |
Ultracentrifuges and high-speed rotors have strict rules: rotor logs, speed derating as rotors age, and specific training. Do not use one until you are trained on it.
Balancing a load
A centrifuge must be balanced: tubes of equal mass in positions symmetric about the centre.
- Use matching tubes, filled to matching volumes. For critical runs, check pairs on a balance.
- Place each tube directly opposite another tube of equal mass.
- With an odd number of samples, add a balance tube filled with water to the same mass, or use a symmetric pattern. In a 6-place rotor, three tubes at every other position (a triangle) are balanced.
- Swing-bucket rotors: all buckets must be in place, even empty ones, and paired buckets should match.
Worked example. You have 5 tubes and a 12-place rotor. Number the positions 0 to 11 like a clock. A triangle of positions 0, 4 and 8 is balanced, and so is the opposite pair 3 and 9. Together, positions 0, 3, 4, 8, 9 hold five equal tubes in balance. Without a pattern like this, add a sixth water tube opposite the odd one.
Running it safely
- Inspect tubes for cracks and use tubes rated for the speed.
- Close tube caps, and for biological material use sealed rotors or safety cups (aerosol-tight buckets). Open them in a biosafety cabinet if your SOP says so.
- Close the lid, set time, speed or RCF, and temperature, then start.
- Stay nearby until it reaches speed. If you hear unusual noise or feel vibration, stop it.
- Never open the lid until the rotor has completely stopped, and never slow it by hand.
- If a tube breaks, follow your lab's spill procedure. For biological material this usually means waiting before opening to let aerosols settle, then disinfecting.
- Log use where required.
Common mistakes
- Following an rpm value written for someone else's rotor.
- Balancing by tube count instead of mass (a full tube opposite a half-full one).
- Leaving out a swing bucket.
- Opening the lid during the run down.
- Using tubes past their rated speed.
Quick recap
- Protocols should specify RCF (× g); convert with RCF = 1.118e-5 x r x rpm^2.
- Balance by mass and symmetry; use a water balance tube when needed.
- Sealed rotors or safety cups for biological samples; never open until stopped.
- Higher-speed instruments need specific training and logs.
Try it
Open the Do tab to work out balanced loading patterns and RCF conversions on paper.