Settle plates are cheap, that’s a bonus, but that is where the positives end and if you want to truly monitor your process and be confident in your process then active air sampling is the only way to really achieve this. They have their place in well documented airflow areas but in reality they are a lucky dip and here is why:
Settle plates are not generally considered active air samplers and simply rely on organisms dropping onto them. This is pure physics and not microbiology.
A settle plate in a room with no airflow only has gravity as the downforce, therefore all particles will fall eventually. All particle sizes are equally capable of landing on the 90mm plate and all have equal probability of doing so. This means in theory settle plates have a d50 value of 0µ – the perfect air sampler right? (Unsure what a d50 value is, click here)
The above is true until you move it to the real world and under a HVAC where air is continually impacted against the agar from above. At this point, the plate is effectively an active air sampler. There is a known down flow or air which is the impact velocity onto the surface of the agar and there is a known inlet size which is the diameter of the plate.
Now particles are not really settling through gravity but rather being blown and this causes an air pocket to form within the rim of the plate which then behaves like a boundary causing airflow to pass over and around it rather than through it meaning only the very largest particles make it to the agar surface.
So what is the true d50 value of a settle plate? We can calculate it:
In this example, the true d50 value of a settle plate is 87.69µ
And this is proven further by the many studies that have shown significantly lower counts versus any active air sampler. A study performed by AstraZeneca showed even the poorest performing sieve air sampler was 19 times more efficient than a settle plate and our own ImactAir was 43 times more efficient.
When we need a d50 value of 1µ and settle plates are in the order of 87! With even the worst active air sampler on the market 19 times more efficient than a settle plate; can you really choose a settle plate as your method?
For more information on our range of active air monitors please click here.
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