Centrifuge

From Wikipedia, the free encyclopedia

This article is about the scientific device. For the Christian camp, see Centrifuge (camps).

A laboratory tabletop centrifuge

A centrifuge is a piece of equipment, generally driven by a motor, that puts an object in rotation around a fixed axis, applying force perpendicular to the axis. The centrifuge works using the sedimentation principle, where the centripetal acceleration is used to separate substances of greater and less density. There are many different kinds of centrifuges, including those for very specialised purposes.

1 Theory
2 History and predecessors
3 Types and uses
4 Use and safety
5 Calculating relative centrifugal force (RCF)
6 See also
7 External links

[edit] Theory

Protocols for centrifugation typically specify the amount of acceleration to be applied to the sample, rather than specifying a rotational speed such as revolutions per minute. The acceleration is often quoted in multiples of g, the acceleration due to gravity at the Earth's surface. This distinction is important because two rotors with different diameters running at the same rotational speed will subject samples to different accelerations.

The acceleration can be calculated as the product of the radius and the square of the angular velocity.

[edit] History and predecessors

English military engineer Laval (1707-1751) invented a whirling arm apparatus to determine drag, and Antonin Prandl invented the first centrifuge in order to separate cream from milk to make churning butter much easier.

[edit] Types and uses

There are basically four types of centrifuge:

Tabletop/clinical/desktop centrifuge or microcentrifuge
High-speed centrifuge
Cooling centrifuge
Ultra-centrifuge

[edit] Biology and biochemistry

Simple centrifuges are used in biology and biochemistry for isolating and separating biomolecules, cell organelles, or whole cells. They vary widely in speed and capacity. They usually comprise a rotor containing two, four, six, or many more numbered wells within which centrifuge tips may be placed.

The rotor is covered by a plastic cover. The cover is usually interlocked to prevent the motor from turning the rotor when it is open, and from allowing the cover to be opened before the rotor stops for several minutes. The cover protects the user from being injured by touching a rapidly spinning rotor. It also protects the user from fragments in case the rotor fails catastrophically.

To ensure that the rotor does not turn unbalancedly, it must be balanced by placing samples or blanks of equal volume opposite each other. Some centrifuges may stop turning when wobbling is detected.

The centrifuge tips are usually made of plastic or glass; they may vary in capacity from tens of mililiters, to much smaller capacities used in microcentrifuges used extensively in molecular biology laboratories. These microcentrifges typically accommodate microcentrifuge tubes with capacities from 250 μL to 2.0 mL.

The ultracentrifuge was invented in 1925 by Theodor Svedberg, which by use of very high acceleration, and allowing the observation of sedimentation rates for macromolecules, allowed for the determination of their approximate molecular weights. Svedberg's 1926 Nobel Prize in Chemistry was earned for work made possible using his ultracentrifuge.

[edit] Isotope separation

Other centrifuges, the first being the Zippe-type centrifuge, are used to separate isotopes, and these kinds of centrifuges are in use in nuclear power and nuclear weapon programs.

Gas centrifuges are used in uranium enrichment. The heavier isotope of uranium (uranium-238) in the uranium hexafluoride gas tend to concentrate at the walls of the centrifuge as it spins, while the desired uranium-235 isotope is extracted and concentrated with a scoop selectively placed inside the centrifuge. It takes many thousands of centrifuges to enrich uranium enough for use in a nuclear reactor (around 3.5% enrichment), and many thousands more to enrich it to bomb-grade (around 90% enrichment).

The 20 G centrifuge at the NASA Ames Research Center

[edit] Aeronautics and astronautics

Exceptionally large centrifuges are used to test the reactions and tolerance of pilots and astronauts to acceleration above those experienced in the Earth's gravity.

The use of large centrifuges to simulate a feeling of gravity has been proposed for future long-duration space missions. Exposure to this simulated gravity would prevent or reduce the bone decalcification and muscle atrophy that affect individuals exposed to long periods of freefall. An example of this can be seen in the film 2001: A Space Odessy.

[edit] Commercial applications

Standalone centrifuges for drying (hand-washed) clothes - usually with a water outlet.
Centrifuges are used in the attraction Mission: SPACE, located at Epcot in Walt Disney World, which propels riders using a combination of a centrifuge and a motion simulator to simulate the feeling of going into space.
In soil mechanics, centrifuges utilize centrifugal acceleration to match soil stresses in a scale model to those found in reality.
Large industrial centrifuges are commonly used in water and wastewater treatment to dry sludges. The resulting dry product is often termed cake, and the water leaving a centrifuge after most of the solids have been removed is called centrate.

[edit] Use and safety

The load in a laboratory centrifuge must be carefully balanced. Small differences in mass of the load can result in a large force imbalance when the rotor is at high speed. This force imbalance strains the spindle and may result in damage to centrifuge or personal injury.

Centrifuge rotors should never be touched while moving, because a spinning rotor can cause serious injury. Modern centrifuges generally have features that prevent accidental contact with a moving rotor.

Because of the kinetic energy stored in the rotor head during high speed rotation, those who have experienced the loss of a rotor inside of an ultracentrifuge compare the experience to having a bomb explode nearby.

[edit] Calculating relative centrifugal force (RCF)

Relative centrifugal force is the measurement of the force applied to a sample within a centrifuge. This can be calculated from the speed (RPM) and the rotational radius (cm) using the following calculation.

g = RCF = 0.00001118 × r × N²

where

g = Relative centrifuge force

r = rotational radius (centimetre, cm)

N = rotating speed (revolutions per minute, RPM)