Casella CEL Airborne bacteria sampler User Manual
Page 13
6.
THE SELECTION OF SAMPLING CRITERIA
When petri dishes of agar are being prepared and handled into and out of the
instrument, random contamination may occur which may amount to at least
1 colony in every two plates. Therefore suitable sample times and volumes
should be selected to reduce the errors created by such contamination.
Between 40 and 200 colonies on a plate is normally easily countable,
although higher numbers are possible to enumerate. Such numbers will be
achieved under normal conditions, using a sample time of 2 minutes, when a
single revolution of the plate can be used, and a single open slit.
Such a timing and instrument configuration will give a sample volume of 350
litres. In highly contaminated air, for example many people in a small room, a
sample time of 0.5 minutes, using one open slit and a single revolution of the
plate may be sufficient, producing a sample volume of 87 litres.
While for well-ventilated large rooms with few people, longer sample periods
up to five minutes, using the same instrument configuration, may be more
suitable producing sample volumes 875 litres.
For greater purity air or for higher accuracy, longer sampling times (with the
plate rotating continuously) and higher flow rates (by increasing the number
of open slits) may be required. For example, sampling with all four slits open
at flow rates of 700 litres/minute, would allow air volumes of up to 7 cubic
metres to be sampled in 10 minutes, or if only 1 slit was used at a flow rate
of 175 litres per minute the same volume could be sampled over 40 minutes.
This would allow a low number of bacterium per cubic metre, for example
between 2 and 10, to be determined with reasonable accuracy.
Sampling Criteria
Page 13 of 16
BACTERIA SAMPLER
User Manual