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SAMPL’AIR LITE MANUEL UTIILSATION / USER’S MANUAL MA107501
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7.5.3 Cleaning procedure
After cleaning the device (See “Care and maintenance” chapter), vacuum or dry the device with warm air in
order to dry the sampling head.
7.6 Counting and calculation
After incubation, count the number "n" of CFU (Colony Forming Units) which have grown.
In order to take into account the probability of having coinciding impactions of 2 or more micro organisms
(implying only one countable colony), an algorithm is applied according to Peto and Powel work
(1)
:
{
}
1 n T
1
......
2 T
1
1 T
1
T
1
En
+ −
+ +
−
+
−
+ =
T = number of holes in the sampling head
n = number of numerated colonies
E
n
is the estimated number of viable micro organisms giving n positive impacts, when using a grid whose
number of holes = T.
This formula suggests that the micro organisms flow stops when a micro organism reaches the hole n.
This flow being irregular, the estimated number of particles, when n impacts are observed, has to be equal or
superior to E
n
, but inferior to E
(n+1)
.
The adjusted number N is obtained using:
N =
{
E
2
1
n
+ (E
n +1
–1)
}
Consult the table in the appendices section (appendix number 2). A table indicates the n value (number of
colonies actually numerated) corresponding to N (probable number of viable impacted micro organisms - either
estimated or adjusted).
Once N is identified, the controlled volume (V = sampling duration x 0.1 m3 / min) and the airborne
contamination level can be obtained using
V
N
Colony Forming Units / m
3
.
Example
After sampling and incubation, 120 colonies are counted on Petri dish. Then n=120.
N = 161
V = 5 min x 0.1 m
3
/ min = 0.5 m
3
Level of airborne contamination:
5 . 0
161
= 322 CFU / m
3
.
(1)
Peto S. and Powel E. O. (1970). The assessment of aerosol concentration by means of the
Andersen sampler. J. App. Bacteriol. 33,582-598.
Follow carefully the operation instructions and sampling plan described by
the team that analyses the results. File the test report carefully after every
analysis and identify the Petri dish.
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