Centrifuge

Centrifuge, a laboratory machine designed for the rapid separation of suspended and agitated particles from liquids or for separating liquids from bodies that they permeate. The centrifuge belongs to centrifugal machines. The principle of operation of such a machine is based on creating significant centrifugal force, in the field of action of which the object being processed is placed. Centrifugal force can be expressed as follows: F = mv²/r, where m is mass, v is peripheral velocity in m/sec., and r is radius. From this it is clear that at one radius and speed, particles with greater mass will receive absolutely greater centrifugal force and will be located in the liquid at the periphery of the circle, while lighter ones will be closer to the center. Peripheral velocity in cm/sec. can be expressed as follows through the number of revolutions per minute n: v = 2πrn/60. Substituting this expression into the formula for centrifugal force, we get: F = m(2πrn/60)²/r = m(4π²r²n²/3600)/r = mπ²rn²/900.

FIG. 1.

Analyzing this expression, we have: 1) with constant mass and number of revolutions, the centrifugal force is stronger the greater the radius, 2) with constant mass and radius, the centrifugal force increases in proportion to the square of the velocity. There is a huge number of different systems of centrifugal machines serving for various purposes. In all these machines there is a vertical rotating axis, which carries a greater or smaller number of metal tubes (fig. 1), into which glass test tubes of various shapes can be inserted: rounded, conical, pointed, with caps, with graduations, etc. (fig. 2). The centrifuge can work well if the test tubes with liquid, located at the ends of the diameter, are balanced with each other; this ensures uniform load on the axis, ensuring uniform and smooth operation of the centrifuge. For this balancing, special balances are often used (fig. 3).

Centrifuges can be divided into several groups: first, by the number of revolutions they give per minute, and second, by the volume of liquid that can be subjected to centrifugation at the same time. 1) Centrifuges giving 2,000-3,000 revolutions per minute; volume of liquid in each test tube 15 cm³, total 2-4 tubes. Rotation in these centrifuges is often done by hand (fig. 1). 2) Centrifuges giving from 3,000 to 4,000, in exceptional cases up to 5,000 revolutions per minute; volume of liquid varies: from 4 test tubes of 15 cm³ each to 4 test tubes of 50 cm³ each (there are models with a larger number of tubes or with volume of each up to 100 cm³). Rotation is mostly done by an electric motor. These centrifuges are currently manufactured in Moscow, Leningrad (fig. 4), Saratov. Saratov centrifuges (3,000 revolutions per minute) in form resemble the centrifuge shown in fig. 5, but are suspended in a special stand on four legs. Suspension of Kolletz centrifuges is done to a special bracket on the wall or to a special massive stand (fig. 6). Figure 5 shows the connection with a hematocrit (a device for determining the volume of erythrocytes). While the centrifuge gives 3,000 revolutions, the hematocrit rotates at a speed of 10,000 revolutions per minute. 3) Centrifuges giving from 3,000 to 4,000 revolutions, but designed for a larger volume of liquid—from 6 test tubes of 50-100 cm³ each to 8 test tubes of 1 liter each (there are models giving up to 6,000 revolutions with a relatively small amount of liquid—6 test tubes of 50 cm³ each). In these centrifuges, the axis is for the most part not connected directly to the motor (fig. 7). Often in biochemical laboratories, it is very convenient to replace the slow process of filtering large volumes of liquids with rapid centrifugation. For serological purposes, an insert allowing centrifugation of a large number of test tubes at the same time can be used in a centrifuge (fig. 8). Recently, the Kolletz firm has produced high-speed centrifuges (Ecco-Super-Rapid), giving 10,000 revolutions per minute. These are small machines for 4 test tubes of 1.5 cm³ each, which will probably find wide application in biochemical research laboratories.

Figure 6.

Figure 8.

cm³. Derbiz.