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Best Practices for Balancing Samples in a Lab Centrifuge

By Aimee O'Driscoll, 12 March 2019

Balancing tubes correctly is essential for the safe and efficient operation of a lab centrifuge. If tubes aren’t properly balanced, you risk damaging the unit or creating a safety hazard. Even if there’s a small imbalance that doesn't cause drastic issues, it could still negatively impact the separation process and affect the quality of your samples.

Balancing your centrifuge tubes isn’t difficult, but there are best practices you need to follow to make sure you get it right. We’ve discussed other aspects of safely using a centrifuge, including inspecting tubes before use and checking the maximum load tolerance, in a past post, but here, we’ll focus specifically on the following tips for balancing samples correctly:

  1. Balance by mass not volume
  2. Don’t leave buckets out
  3. Load tubes symmetrically
  4. Use “dummy” tubes if needed
  5. Consider a unit with an imbalance sensor

Let’s take a look at each of these in more detail.

1. Balance by Mass Not Volume

If all of your samples are of similar density, then both the mass and volume of samples will match. In this case, it’s possible to fill tubes based on sight. That being said, it’s better to weigh tubes to be sure.

If the density of some sample materials is different to that of others, mass needs to be considered and not volume. After all, the concept of balance that we’re discussing here hinges on mass.

Note that when it comes to volume, manufacturers will often specify how full a tube can be, for example, not more than two-thirds full.

2. Don’t Leave Buckets Out

If you’re using a centrifuge that has multiple buckets, it’s possible you only need to use some of those for your samples. It may be tempting to simply leave out empty opposing buckets. However, this is not a good idea as running without buckets can damage the rotor and even result in catastrophic failure.

3. Load Tubes Symmetrically

Loading tubes symmetrically sounds simple in theory but for some it can be a difficult concept to grasp, especially when you’re dealing with balancing multiple buckets, and particularly those that swing on a pivot.

The simplest centrifuges to balance are those that just have a rotor with no buckets and a small number of spaces, for example, the D1008 EZee Mini-Centrifuge which has space for just eight tubes.

A larger number of spaces can pose issues when loading by sight. For example, the Z207-M Compact Microcentrifuge (below left) holds 24 tubes and the Z216 Series High Speed Microcentrifuges (below right) holds 44 tubes.

 

Microcentrifuge examples.

 

If you’re loading three or more tubes in these centrifuges by sight alone, it’s easy to make a mistake. As such, it’s best to count spaces to make sure. If you frequently use a similar number of tubes, for example, three or a multiple of three, then you may want to add markings to your centrifuge to make things a bit quicker.

Things also get more complicated when you have multiple buckets to balance. Below is an example of a four-bucket centrifuge rotor with eight samples loaded correctly (left) and incorrectly (right).

 

Balanced and unbalanced centrifuge.

 

Here are the general rules to follow:

  • Opposing loads should be balanced.
  • Tubes should be balanced across the rotor’s centre of rotation.
  • For swing buckets, each bucket also needs to be balanced across its pivotal axis to ensure that tubes reach a horizontal position at operating speed.

Below is an example of a swing bucket rotor balanced correctly.

 

Balanced centrifuge.

 

Aside from following these rules, it’s important to consult the manufacturer's guide as it may offer specific advice regarding loading.

4. Use “Dummy” Tubes if Needed

There are bound to be times when the number of samples you need to centrifuge doesn’t provide you with a balanced load. In this case, you can use “dummy” tubes. These can be filled with water or another substance, but the material used should be of a similar density to that of the samples. That being said, tubes should still be balanced by mass, not by volume.

5. Consider a Unit With an Imbalance Sensor

Even if you think you’ve loaded your samples correctly, there is always the possibility of human error. This is where a unit that has a built-in imbalance sensor comes in handy. If it senses an imbalance, the unit will stop running immediately.

 

Units with imbalance sensors.

The LC-8 Centrifuge and the NU-C300 General Purpose 3 Liter Centrifuge  both come with an imbalance sensor.

The NU-C300 even lets you adjust the tolerance level of the imbalance sensor based on what you expect to see during your application.