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Centrifuges are indispensable tools in the laboratory. However, their use often presents various challenges; many users are unfamiliar with the concepts of "balancing" and "unit conversion between RPM and g," which frequently leads to operational errors. Therefore, I would like to provide a systematic overview of centrifuges, focusing specifically on these two key topics.
Centrifuge Principles
The operating principle of a centrifuge primarily relies on centrifugal sedimentation; by harnessing the centrifugal force generated through rotation, it facilitates the separation, concentration, and purification of substances with varying densities within a solution.
When a suspension is subjected to high-speed rotation, the immense centrifugal force causes the suspended microscopic particles—such as organelles or precipitates of biological macromolecules—to settle at a specific velocity. This process facilitates the separation, concentration, and purification of the solution; the settling velocity of the particles is determined by the centrifuge's rotational speed, as well as the mass, size, and density of the particles themselves.
When a suspension—defined as a mixture in which minute solid particles are suspended within a liquid medium—is allowed to stand motionless, the influence of the gravitational field causes the suspended particles to gradually settle. The rate of this sedimentation is contingent upon the size, morphology, and density of the particles, as well as the intensity of the gravitational field and the viscosity of the liquid medium. For instance, particles such as erythrocytes (red blood cells), which measure several micrometers in diameter, exhibit an observable sedimentation process under the influence of normal gravity. Furthermore, the sedimentation of substances within a medium is invariably accompanied by the phenomenon of diffusion.
Diffusion is a phenomenon of mass transfer resulting from the thermal motion of particles, driven primarily by density gradients. For particles smaller than a few micrometers—such as viruses or proteins—which exist in solution in a colloidal or semi-colloidal state, it is impossible to observe the sedimentation process using gravity alone.
This is because the smaller the particles, the slower their sedimentation rate, while the effects of diffusion become correspondingly more pronounced. Since diffusion hinders sample separation, increasing the gravitational force can help overcome these adverse effects, thereby enabling the separation of biological macromolecules.
The Function of Centrifuges and Relative Centrifugal Force
A centrifuge utilizes the powerful centrifugal force generated by the high-speed rotation of its rotor to compel particles suspended in a liquid to overcome diffusion and accelerate their rate of sedimentation, thereby separating substances within a sample that possess different sedimentation coefficients and buoyant densities. But what exactly is centrifugal force?
In the context of relativity, centrifugal force refers to the force experienced by a point mass undergoing curvilinear motion, which acts to pull it toward the center of its trajectory. When the external force acting upon an object is insufficient to provide the centripetal force required for its motion, the object will tend to move in a direction away from the center of the circle. This phenomenon—the object moving away from the center—is known as the centrifugal phenomenon; it is also referred to as centrifugal motion. Centrifugal motion occurs as a result of the absence or insufficiency of centripetal force.
Centrifugal action is based on the principle that any object undergoing circular motion at a specific angular velocity is subject to an outward centrifugal force. The magnitude of the centrifugal force (Fe) is equal to the product of the centrifugal acceleration (ω²r) and the mass of the particle (m):
Relative Centrifugal Force (RCF) refers to the magnitude of the centrifugal force acting on a particle within a centrifugal field, expressed as a multiple of Earth's gravity; its unit is the gravitational acceleration "g." Since the rotor radii—or the distances from the centrifuge tubes to the center of the axis of rotation—vary among different centrifuges, the resulting centrifugal forces also differ.
Consequently, scientific literature commonly employs the term "Relative Centrifugal Force" or the notation "number × g" to specify centrifugal force—for instance, "25,000 × g," which indicates a relative centrifugal force of 25,000. Provided that the RCF value remains constant, a sample will yield identical results regardless of which centrifuge is used. Relative Centrifugal Force is, in essence, the ratio of the centrifugal force to the force of gravity:
Generally, during low-speed centrifugation, relative centrifugal force is often expressed in terms of rotational speed (rpm), whereas during high-speed centrifugation, it is expressed as a numerical value followed by "xg."
Centrifuge Construction and Types
Here, centrifuges are classified into three categories based on speed: low-speed centrifuges, high-speed centrifuges, and ultra-high-speed centrifuges.
Low-speed centrifugestypically operate at speeds below 10,000 rpm and are suitable for the separation of biological samples such as cells and microorganisms. High-speed centrifuges operate within a range of 10,000 to 30,000 rpm and are frequently used for the separation of macromolecules, such as proteins and nucleic acids. Ultra-high-speed centrifuges operate at speeds exceeding 30,000 rpm and can be utilized for the separation of minute particles, such as viruses and subcellular organelles.
The structure of a centrifuge is relatively simple, consisting primarily of key components such as an electric motor, a centrifuge rotor, a speed controller, a timer, centrifuge tubes, and a base. The following section introduces three main types of centrifuges:
1. Low-Speed Centrifuge
Its structure is relatively simple, comprising key components such as an electric motor, a centrifuge rotor, a speed controller, a timer, centrifuge tubes, and a base.
(1) Electric Motor: The primary component of the centrifuge, typically of the series-excited type. It consists of two parts: the stator and the rotor.
(2) Centrifuge Disc (and Rotor Head): The centrifuge disc is commonly made of cast aluminum; it features a flat-topped conical shape with a central circular aperture. It is mounted onto the motor shaft at the upper end of the motor and then secured in place by tightening a nut. The disc is fitted with 6 to 12 symmetrically arranged holes, angled at 45 degrees, designed to accommodate test tubes.
(3) Speed Control Mechanism: The speed control mechanism (for the electric motor) involves connecting a multi-tapped choke coil or a disc-type variable resistor in series between the power supply and the motor.
(4) Centrifuge Tubes: Centrifuge tubes are primarily manufactured from plastic or stainless steel. Plastic centrifuge tubes are equipped with caps; prior to centrifugation, these caps must be securely fastened. Stainless steel centrifuge tubes possess high structural strength, resist deformation, and are resistant to heat, freezing temperatures, and chemical corrosion.
2. High-Speed (Refrigerated) Centrifuge
This apparatus comprises a rotating mechanism, a speed control system, a temperature control system, a vacuum system, a centrifugation chamber, a rotor head, and various safety protection devices. Due to its high rotational speed and the inclusion of a low-temperature control unit, the temperature within the centrifugation chamber can be regulated and maintained within a range of 0°C to 40°C. The rotational speed, temperature, and duration can all be controlled with strict precision, and their values are displayed via either analog pointers or digital readouts.
(1) Rotating Assembly: Motor, rotor shaft (and its connecting components);
(2) Speed Control System: Reference voltage source, speed regulator, current regulator, power amplifier, motor, speed sensor, etc.;
(3) Vacuum System: Designed to overcome air friction (which generates heat) and ensure the centrifuge attains the rotational speed required for normal operation;
(4) Temperature Control Unit: Composed of four main components: a compressor, a condenser, a capillary tube, and an evaporator;
(5) Safety Protection Devices: Main power supply overcurrent protection, drive circuit overspeed protection, refrigeration unit overload protection, and operational safety interlocks.
3. High-Speed (Refrigerated) Centrifuge: Consists of a drive assembly, a speed control system, a temperature control system, a vacuum system, and a rotor.
How to Balance a Centrifuge?
A centrifuge rotates thousands of times per minute; this means that even the slightest weight imbalance around the rotor will be amplified, potentially causing the instrument to wobble or vibrate. This compromises its performance and reliability, shortens its service life, and—in the worst-case scenario—could lead to the centrifuge exploding, thereby injuring laboratory personnel. Therefore, understanding how to properly balance a centrifuge is absolutely essential.
1. Balancing Fixed-Angle Rotors
For balancing fixed-angle rotors, one generally needs only to keep the "Center-Symmetry Method" in mind. Taking the 12-place fixed-angle rotor shown in the figure below as an example, samples can be positioned by adhering to this principle.
2. Horizontal Rotor Balancing
In practical applications, the most vexing challenge is balancing horizontal rotors. This is because one must not only ensure the symmetry of the centrifuge tubes within a single bucket but also verify that the tubes in the opposing bucket are properly balanced. When balancing under these circumstances, two key principles must be strictly observed:
(1) When placing centrifuge tubes within a single bucket, ensure that the bucket's center of gravity is positioned precisely at its geometric center;
(2) When placing centrifuge tubes in the opposing bucket, use the placement in the first bucket as a reference point; strictly adhere to the principle of symmetry relative to the rotor's central axis to determine the appropriate placement positions.
3. Balancing Odd Numbers of Tubes
Furthermore, situations involving the balancing of an odd number of tubes are frequently encountered. When faced with such a scenario, most students typically opt to insert a "blank" balancing tube (filled with water). While this approach is simple and straightforward, it can prove rather cumbersome in experiments involving frequent centrifugation or where sample masses and volumes vary significantly. So, how exactly should one proceed when encountering such a situation? Taking a 24-place fixed-angle rotor as an example, the procedure can be categorized into three distinct scenarios: If there is only one tube or exactly 23 tubes, the only option is to prepare a single tube of water to serve as the counterweight;
(1) The "Triple-Placement" Balancing Method: When dealing with an odd number of centrifuge tubes that constitute a multiple of three—such as 3, 6, 9, 15, or 21 tubes—they can be arranged in a clockwise pattern, as illustrated in the diagram below;
(2) The "2-Unit Plus 3-Unit" Balancing Method: When dealing with a total of 5, 7, 11, 13, 17, or 19 units, one may adhere to the principle of central symmetry by first balancing a group of three units, and then balancing the remaining two units; when combined, the overall system remains balanced.
Ultimately, the balancing of a centrifuge rotor is governed by the principle of symmetry: the relative masses of the two centrifuge tube cups situated on a single moment arm must remain identical. In practice, simply loading the rotor wells sequentially—following a pattern such as 2+2+2...+3—ensures that each set remains balanced, thereby guaranteeing a foolproof operation. The illustrative diagram below visualizes this principle with even greater clarity:
What is the relationship between rotational speed (rpm) and relative centrifugal force (g)?
For centrifuges, the units typically used to indicate speed are revolutions per minute (rpm) and centrifugal force (g). In this context, "g" is sometimes also denoted as RCF (Relative Centrifugal Force). The calculated relationship between these two units (where *r* represents the radius) is as follows:
When we encounter a centrifuge, the first questions that come to mind are: What exactly is a centrifuge? And what are its underlying principles? We have provided a detailed explanation of these principles in our article, "Low-speed Centrifuge: Principles, RCF Conversion, Rotor Types & Practical Usage Tips"—so if you are unfamiliar with the subject, we recommend reading that piece first. However, when using a centrifuge for the first time, one question frequently arises: How do you ensure the centrifuge remains balanced? In the article that follows, we will explore this very topic together!
I. Why Balancing a Centrifuge Is Crucial
An unbalanced centrifuge rotor can lead to consequences ranging from minor issues—such as machine wear and sample loss—to severe outcomes, including serious injury or even fatalities. Mastering the correct balancing techniques is therefore paramount: fixed-angle rotors should adhere to the "center-symmetrical method"; horizontal rotors require careful consideration of both the center of gravity of the swinging buckets and the principle of symmetry; and for an odd number of tubes, a flexible combination of "2x + 3x" balancing sets should be employed. Ensuring that every set of centrifuge tubes is properly balanced is the only way to safeguard both experimental integrity and personal safety, thereby preventing accidents before they occur.
Since the invention of the centrifuge in the 19th century, the risks associated with unbalanced rotors have been a persistent concern. At the milder end of the spectrum, an imbalance can cause excessive wear on the machine's drive shaft—thereby shortening the centrifuge's service life—or result in tubes dislodging and samples being lost; at the severe end, it can lead to serious injury or loss of life. The most effective way to mitigate the dangers posed by an unbalanced rotor is to learn and apply the correct balancing procedures!
II. How to Balance a Centrifuge
1. Balancing Fixed-Angle Rotors
For balancing fixed-angle rotors, one generally needs only to keep the "Center-Symmetry Method" in mind. Taking the 12-place fixed-angle rotor shown in the figure below as an example, samples can be positioned by following this principle:
2. Balancing Horizontal Rotors
In practical applications, the task that causes the most headaches for researchers is balancing horizontal rotors. This is because one must not only ensure the symmetry of the centrifuge tubes *within* a single bucket but also ensure that the tubes in the *opposing* bucket are balanced relative to it. When balancing under these circumstances, two key principles must be strictly observed:
When placing centrifuge tubes within a single bucket, ensure that the bucket's center of gravity remains precisely at its geometric center;
When placing centrifuge tubes in the opposing bucket, use the placement configuration of the first bucket as a reference point, strictly adhering to the principle of symmetry relative to the rotor's central axis, and then position the tubes accordingly.
Taking the 4×14-hole horizontal bucket rotor shown in the figure below as an example, samples can be loaded by following the aforementioned principles:
3. Balancing an Odd Number of Tubes
Furthermore, situations involving the balancing of an odd number of tubes are frequently encountered. When faced with such a scenario, most students typically opt to insert a "dummy" balancing tube (filled with water). While this method is simple and straightforward, it can prove rather cumbersome in experiments involving multiple centrifugation cycles or where sample masses and volumes vary significantly. So, how exactly should one proceed when encountering such a situation? Using a 24-place fixed-angle rotor as an example, the process can be categorized into three distinct scenarios:
If there is only 1 tube or exactly 23 tubes: In this specific instance, there is no alternative; one must simply prepare a balancing tube filled with water to achieve equilibrium.
The "Triple-Multiple" Balancing Method: When the number of tubes constitutes an odd multiple of three—such as 3, 6, 9, 15, or 21—they should be arranged in a clockwise pattern, as illustrated in the diagram below.
The 2x + 3x Balancing Method: When dealing with a configuration of 5, 7, 11, 13, 17, or 19 units, one may adhere to the principle of central symmetry by first balancing a group of three units, and then balancing the remaining two units; in this way, the combined result remains balanced overall.
Ultimately, the balancing of a centrifuge rotor is governed by the principle of symmetry: the relative masses of the two centrifuge tube cups situated on a single moment arm must remain identical. In practice, by simply loading the rotor wells sequentially—following a pattern such as 2 + 2 + 2... + 3—and ensuring that each set remains balanced, you can guarantee a foolproof and error-free operation. The illustrative diagram below demonstrates this principle with even greater clarity:
Only by mastering the proper balancing techniques for centrifuges can we ensure both experimental and personal safety, thereby preventing accidents before they occur.
Huatai Hehe Mini-Classroom:
Three Common Centrifugation Methods: Differential Centrifugation; Rate-Zonal Centrifugation; Isopycnic Centrifugation.
1. Differential Centrifugation
The operational procedure typically involves separating the supernatant from the precipitate following the initial centrifugation step. The supernatant is then subjected to further centrifugation at a higher rotational speed to isolate a second fraction of precipitate. This iterative process continues—with the rotational speed being successively increased at each stage—to sequentially isolate the desired substances. Differential centrifugation offers relatively low resolution; particles with sedimentation coefficients falling within the same order of magnitude are difficult to separate effectively. Consequently, this technique is most commonly utilized for the preliminary, crude extraction of biochemical samples.
2. Rate-Zonal Centrifugation
Rate-zonal centrifugation is a form of incomplete sedimentation separation; its effectiveness is significantly influenced by the physical dimensions of the substances themselves. It is typically applied in situations where substances differ in size but share the same density. The technique operates on the principle that, under the influence of centrifugal force, particles to be separated will exhibit varying sedimentation velocities within a density gradient medium. Consequently, following centrifugation, particles with different sedimentation rates settle into distinct density gradient layers, forming separate sample zones and thereby achieving their mutual separation.
For instance, when isolating mononuclear cells from venous blood, the separation medium Ficoll causes all mononuclear cells (lymphocytes and monocytes) within the blood to settle into a single layer, allowing for their simultaneous extraction. In contrast, the separation medium Percoll resolves the lymphocytes and monocytes in the blood into two distinct gradient layers, enabling their separate extraction. During the centrifugation process—as well as during subsequent sampling—the gradient medium serves as both a supporting matrix and a stabilizer, preventing the resuspension of the separated, layered particles caused by mechanical vibrations.
When employing the rate-zonal centrifugation method, it is essential to strictly control the centrifugation duration. The time allotted must be sufficient to allow the various particles to form distinct zones within the gradient medium, yet not so prolonged as to cause any of the target particles to settle completely into a precipitate.
3. Isopycnic Zonal Centrifugation
Isopycnic zonal centrifugation is a centrifugation method applied to liquid dispersion systems containing particles with differing buoyant densities. Under the influence of a centrifugal force field, these particles either sediment downward or float upward along a density gradient until they reach a position where their density precisely matches that of the surrounding medium—known as the isopycnic point—thereby forming distinct zones.
The efficacy of isopycnic zonal centrifugation depends on the difference in buoyant density among the particles; the greater this density difference, the more effective the separation. This separation outcome is independent of the particles' size and shape, although these latter two factors do determine the rate and time required to reach equilibrium, as well as the width of the resulting zones.
Key characteristics of the isopycnic zonal centrifugation method include:
(1) The separation is dependent upon the density of the sample particles;
(2) The separation is independent of the sample particles' size and other parameters;
(3) Provided that the rotational speed and temperature remain constant, extending the centrifugation time will not alter the final zonal positions of these particles.
Now that you have gained an understanding of the centrifugation methods employed by centrifuges, do you have a general idea of how to go about selecting one? In this article:How to Choose the Right Centrifuge: A Guide to Size, Capacity, and Application
we provide a detailed guide on how to choose the right centrifuge—let's explore this topic together.
Centrifuge Balancing – Q&A
How to Balance a Centrifuge with an Odd Number of Tubes
To balance a centrifuge with an odd number of tubes, use dummy tubes filled with water to match sample weight and volume, creating an even, symmetric load. Distribute tubes evenly around the rotor to avoid vibration and damage.
How to Balance a Centrifuge with 5 Tubes
Balance 5 tubes by adding 1 dummy tube to make 6 total, then arrange them symmetrically at equal intervals around the rotor. All tubes must have identical weight and volume.
How to Balance a Centrifuge with 4 Tubes
Place 4 tubes directly opposite each other in symmetric pairs (e.g., 1&7, 2&8 in an 8-place rotor). Ensure equal volume and weight in all tubes for stable centrifugation.
How to Balance a Centrifuge with 2 Tubes
For 2 tubes, place them directly across from one another in the rotor. Confirm both tubes have the same liquid volume to maintain balance.
How to Balance a Centrifuge with 3 Tubes
Arrange 3 tubes evenly spaced 120° apart in a triangular pattern. All tubes must match in weight and volume for proper balance.
How to Balance 5 Tubes in a 12-Place Centrifuge
In a 12-place rotor, balance 5 tubes by using 1 dummy tube to create 6 balanced positions. Place tubes symmetrically in pairs and one single position with its dummy opposite.
How to Balance a Swinging Bucket Centrifuge
Balance a swinging bucket centrifuge by loading opposite buckets with equal-weight tubes. Never leave one bucket loaded while its pair is empty; use dummy tubes if necessary.
Centrifuge Balance Chart for 24-Place Rotor
2 tubes: 1 & 13
4 tubes: 1 & 13, 2 & 14
6 tubes: 1 & 13, 2 & 14, 3 & 15
8 tubes: 1 & 13, 2 & 14, 3 & 15, 4 & 16
Odd tube counts: Add dummy tubes to reach an even symmetric number.
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