bioMerieux DENSIMAT, User Manual, Page: 27 / 50

User's Manual DENSIMAT 4-1
4 P r i n c i p l e
DENSIMAT enables precise determination of bacterial density by two
measurements.
An incident beam of light is passed through the ampoule and two subsequent
measurements are obtained:
−
Scattered light S (two photosensors),
−
Transmitted light T (one photosensor).
The ratio S/T is directly proportional to the density of the bacterial suspension.
The results are displayed on a mobile graduated scale on a liquid crystal
display. The number of squares displayed is proportional to the McFarland
value. The corresponding numerical value is shown on the right-hand side of the
display.
Correspondence McFarland scale/Bacterial concentration/Optical density:
STANDARD
McFARLAND
SCALE
BACTERIAL
CONCENTRATION (1)
x10
8
/ml
THEORETICAL
OPTICAL
DENSITY (2)
at 550nm
0.5
1
2
3
4
5
6
7
1.5
3
6
9
12
15
18
21
0.125
0.25
0.50
0.75
1.00
1.25
1.50
1.75
1) The bacterial concentration varies according to the size of the micro-
organisms. The figures which are reported represent a mean, valid for the
bacteria. For the yeasts, due to their bigger size, these figures have to be
divided by 30.
2) The values correspond to the optical densities of bacterial suspension. The
Barium Sulphate (BaSO4) solutions do not present the same optical density
since the size and the form of the particles are different from those of
bacteria. The light is also diffracted differently.