A centrifuge is a piece of equipment, generally driven by a motor, that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis. The centrifuge works using the sedimentation principle, where the centripetal acceleration is used to evenly distribute substances (usually present in a solution for small scale applications) of greater and lesser density. There are many different kinds of centrifuges, including those for very specialized purposes. It can be used for viable counts, when shaking the culture e.g. yeast, out of suspension.

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 standard 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. Traditionally named "Relative centrifugal force" (RCF), it is the measurement of the acceleration applied to a sample within a centrifuge and it is measured in units of gravity (times gravity or × "g"). It is given by

$\backslash mathrm\{RCF\}\; =\; r\backslash ,\; (2\backslash pi\; N)^2/g\; \backslash ,$

where

$\backslash textstyle\; g$ is earths gravitational acceleration,

$\backslash textstyle\; r$ is the rotational radius,

$\backslash textstyle\; N$ is the rotating speed, measured in revolutions per unit of time.

When the rotational speed is given in revolutions per minute (RPM) and the rotational radius is expressed in centimetres (cm) the above relationship becomes

$\backslash mathrm\{RCF\}\; =\; 1.118\backslash ,\; \backslash times\; 10^\{-5\}\backslash ,\; r\_\backslash mathrm\{cm\}\; \backslash ,\; N\_\backslash mathrm\{RPM\}^2\; \backslash ,$

where

$\backslash textstyle\; r\_\backslash mathrm\{cm\}$ is the rotational radius measured in centimetres (cm),

$\backslash textstyle\; N\_\backslash mathrm\{RPM\}$ is rotating speed measured in revolutions per minute (RPM).

History and predecessors

Types

There are at least five types of centrifuge:

• preparative centrifuge
• analytical centrifuge
• angle fixed centrifuge
• swing head centrifuge
• haemacrotic centrifuge

Industrial centrifuges may otherwise be classified according to the type of separation of the high density fraction from the low density one :

• Screen centrifuges, where the centrifugal acceleration allows the liquid to pass through a screen of some sort, through which the solids cannot go (due to granulometry larger than the screen gap or due to agglomeration). Common types are :
• Pusher centrifuges
• Peeler centrifuges
• Decanter centrifuges, in which there is no physical separation between the solid and liquid phase, rather an accelerated settling due to centrifugal acceleration. Common types are :
• Solid bowl centrifuges
• Conical plate centrifuges