When operating a centrifuge What happens if the centrifuge is not properly balanced?

August 6, 2021

Balancing your tubes correctly is essential for the safe and efficient operation of a lab centrifuge.

Balancing your tubes correctly is essential for the safe and efficient operation of a lab centrifuge. If tubes are not balanced properly, you do risk damaging the unit or creating a potential safety hazard. Even if there is a slight imbalance that does not cause serious issues, it could still negatively impact the separation process and affect your sample’s quality. Balancing the centrifuge tubes is not hard, but there are practices that you need to follow to make sure you get it all done right. In this blog, we will focus on what goes into balancing your samples correctly in a lab centrifuge. Read on to learn more!

Balance Samples by Mass Not Volume

If all of the samples are of similar density, then both the mass and volume of samples won’t match. In this case, it is possible to fill tubes based on sight. That being said, it is better to weigh tubes just to be sure. If the density of some materials is a bit different from that of others, mass has to be considered, and not the volume. After all, the concept of balance that we are discussing here all hinges on mass. Not that when it comes down to volume, manufacturers will typically specify how full a tube can be. A good example would be no more than two-thirds full.

Do Not Leave Buckets Out

If you are using a centrifuge that has various buckets, it is possible you only need to use some of those for your lab samples. It might be tempting to leave out your empty opposing buckets. However, this is not a great idea as running without buckets could damage the rotor and even result in some catastrophic failure. Keep this in mind when organizing and balancing your laboratory equipment.

Use “Dummy Tubes” when Needed

There are bound to be some times when the amount of samples you need to be centrifuge does not provide you with a balanced load. If this is the case, you could use “dummy” tubes. These could be filled up with water or another substance, but the material should be a similar density to that of your samples. That being said, tubes should still be balanced out by mass, not volume.

Load Your Tubes Symmetrically

Loading tubes symmetrically sounds easy in theory, but for a few, it could be a difficult concept to understand, especially if you’re dealing with balancing various buckets, and particularly those that swing on a pivot. The easiest centrifuges to balance are those that have a rotor without buckets and a small number of spaces. To find the right type of centrifuge for your laboratory, come by the Lab People today!

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This entry was posted on Friday, August 6th, 2021 at 3:26 pm. Both comments and pings are currently closed.

In order for a rotor to run smoothly and safely at its operating speed, the load it carries must be balanced. A rotor can be properly balanced by following some simple rules:

1. A rotor must never be run with buckets missing, although opposing buckets may be left empty.
2. All opposing loads must balance within a certain weight as specified by the centrifuge manufacturer’s instruction manual.
3. If opposing buckets are run with a partial load of tubes in their adapters, these tubes must be arranged symmetrically, both with respect to the pivotal axis of each bucket and across the center of rotation. With some partial loads, it may be difficult or impossible to achieve the correct symmetry in both sets of opposing buckets. The simplest solution is to fill one or more tubes of the same size with water, or a denser liquid if necessary, and use them to balance the load symmetrically.

Correct Rotor Symmetry

Most centrifuges are equipped with an imbalance detector which turns the centrifuge off before any eccentric rotation caused by a load imbalance can damage the drive shaft or bearings. However, the improper distribution of tubes in carriers or adapters can cause poor separations even if the imbalance isn’t severe enough to trigger this detector. In these situations, the buckets won’t pivot to the required horizontal position during the run, resulting in poor density separations or re-mixing of sedimented material during deceleration. Also, the possibility of tube breakage during the run is greatly increased when the buckets are not horizontal at operating speed.

You may notice that the centrifuge vibrates when the rotor is accelerating or decelerating at low speeds. This is normal, and occurs as the rotor passes through a so called critical speed range where any small vibrations are temporarily amplified. Your separations will not be disturbed during deceleration, because the centrifugal force is still high enough to stabilize them. However, you should not select an operating speed within the range where these exaggerated vibrations occur. Your instruction manual will tell you what speeds to avoid.

Correct Symmetry When Balancing a Partial Load – A Top View of a Horizontal Rotor

Example of a balanced load:

Assuming all tubes have been filled with an equal amount of liquid, this rotor load is balanced. The opposing bucket sets A-C and B-D are loaded with an equal number of tubes and are balanced across the center of rotation. Each bucket is also balanced with respect to its pivotal axis.

Example of an unbalanced load:

Even if all the tubes are filled equally, this rotor is improperly loaded. None of the bucket loads are balanced with respect to their pivotal axes. At operating speed, buckets A and C will not reach the horizontal position. Buckets B and D will pivot past the horizontal. Also note that the tube arrangement in the opposing buckets B and D is not symmetrical across the center of rotation.

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