SAMPL'AIR LITE
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
1
En
=
+
+
+
T
T
−
1
T
−
2
T = number of holes in the sampling head
n = number of numerated colonies
E
is the estimated number of viable micro organisms giving n positive impacts, when using a grid whose
n
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
, but inferior to E
n
The adjusted number N is obtained using:
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
Example
After sampling and incubation, 120 colonies are counted on Petri dish. Then n=120.
N = 161
V = 5 min x 0.1 m
Level of airborne contamination:
(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.
48
MANUEL UTIILSATION / USER'S MANUAL
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.
1
}
......
+
T
−
n
+
1
.
(n+1)
1
{
E
N =
+ (E
n
2
N
Colony Forming Units / m
V
3
/ min = 0.5 m
161
= 322 CFU / m
0
5 .
}
–1)
n +1
3
.
3
3
.
MA107501
(1)
: