Casella CEL Airborne bacteria sampler User Manual

Page 13

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