2. General Information
Densitometer DEN-1 is designed for solution turbidity measurement in the range of 0.3 -
6
6
5.0 McFarland units (100x10 - 1500x10 cells/ml). DEN-1 is capable of measuring
solution turbidity in a wider range (5.0 - 15.0 McFarland units) however, it is important to
be aware that in this case the standard deviation values increase.
Densitometer DEN-1 is used for determining the concentration of cells (bacterial, yeast
cells) in the fermentation process, for the detection of susceptibility of microorganisms
against antibiotics, for identification of microorganisms with various test systems, for
measuring optical density at fixed wavelength and for quantitative evaluation of
substance concentration.
The operation principle is based on optical density measurement with digital result
representation in McFarland units.
The unit is calibrated at the factory and saves calibration data when switched off.
However, it can be calibrated by 2-6 points in 0.5 - 5.0 McFarland unit range if
necessary. Both commercial standards (e.g. produced by bioMerieux, Lachema, etc.)
and the standards prepared in the laboratory can be used for calibration. Table 1 shows
the data provided by the McFarland standard manufacturer bioMerieux.
Table 1. Interpretation of McFarland Standard results into corresponding numeric values of
bacterial suspension concentration and their optical density at 550 nm.
*Bacterial concentration depends on microorganism size. The numbers represent an
average value valid for bacteria. For yeasts, which are larger in size, these numbers should
be divided by about 30.
**Values correspond to optical densities of bacterial suspensions. The BaSO solutions
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optical density values differ, because the particle size and form differ from those of
bacteria and light is diffracted differently.
McFarland
Standard
0.5
1
2
3
4
5
Composition
Concentration
BaSO
4
-5
2.40 10 mol/l
-5
4.80 10 mol/l
-5
9.60 10 mol/l
-4
1.44 10 mol/l
-4
1.92 10 mol/l
-4
2.40 10 mol/l
Bacterial concentration*
6
150 x 10 cells/ml
6
300 x 10 cells/ml
6
600 x 10 cells/ml
6
900 x 10 cells/ml
6
1200 x 10 cells/ml
6
1500 x 10 cells/ml
Theoretical optical
density at 550 nm**
0.125
0.25
0.50
0.75
1.00
1.25
Interpretation
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DEN-1
Operating instructions
Version 1.03 - July 2013
