Page 94 - The Veterinary Laboratory and Field Manual 3rd Edition

The selection, use, maintenance and quality control of laboratory equipment and supplies 63

• Check the rim of the body where the lid gas- A laboratory centrifuge consists of an electri-
ket seats for damage to the metal casting. cally driven motorized spindle that spins a rotor
• Check the draw-off cock for signs of leaking head containing tubes inside a heavy metal case.
around the seals and from the valve. The unit has a timing control mechanism and
• Always use a sterilization indicator for each modern units have a cut-out switch incorporated
cycle: autoclave tape (Figures 2.23–2.25), in the lid mechanism enabling them to operate
Bowie and Dick test sheets, helix test. only when the lid is closed.
• Use a biological indicator at least once per Most small diagnostic laboratories will have
year to check the sterilization cycle. Biological one or two standard centrifuges and possibly a
indicators are test systems containing viable portable unit. Field centrifuges can be used with
microorganisms providing a defined resis- a generator for parasitology work or for determi-
tance to a specific sterilization process. nation of haematocrit value.
A biological indicator provides information on Centrifuges commonly used in a regional
whether necessary conditions were met to kill laboratory will operate at maximum speed of
a specified number of microorganisms for a approximately 6000 rpm. If the centrifuge is not
given sterilization process, providing a level of used properly at this high speed the instrument
confidence in the process. Bacterial spores (for is potentially dangerous. It is therefore, very
example, Geobacillus stearothermophilus spores) important to read the manufacturers operating
are the microorganisms primarily used. instructions and carefully follow a few simple
5
operating rules.

centrifuges

A centrifuge is used to spin tubes that con-
tain substances suspended in liquids, at a high
speed. The suspended material will be depos-
ited, in order of molecular weight, with heavier
substances at the bottom of the tube. Speed,
spinning time and the difference in density of
suspended particles and the medium will all
affect the settling of particles (Figure 2.26).
The speed of a centrifuge is usually referred to
in terms of revolutions per minute (rpm). That
is the number of times the centrifuge head com-
pletes a cycle each minute. The actual force that
the material is subjected to is called the relative
centrifugal force (RCF) and this is measured for
each centrifuge by using the formula:

RCF = 1.118 × R × rpm × 10 –5
2

where R = the radius in centimetres measured
from the centre of the centrifuge shaft to the tip Figure 2.26 Centrifuge. Photo: Willy Schauwers,
of the extended centrifuge tube. Provincial Institute for Hygiene, Antwerp, Belgium.

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