### Scales

February 08, 2019

#### by Meghan Kelley

Flow rate is important to consider when selecting a particle counter for a critical environment. The flow rate of a particle counter is the rate at which a pump draws air through the sample chamber, measured in cubic feet per minute (cfm). Two common flow rates for particle counters are 1 cfm and 0.1 cfm. Depending on the application, each flow rate can be valuable.

## 1 cfm versus 0.1 cfm

Let's think about it this way: the highway versus the back roads. If you're going to take a long trip or have a schedule to stick to, you'll want to take the highway. But if you're just running to the store a couple streets over, you would take the back roads.

In terms of cfm, a particle counter with a 1 cfm flow rate is like taking the highway. Its higher flow rate allows for a faster sample collection. A particle counter with a 0.1 cfm flow rate is like taking the back roads. It can achieve the same results in applications where time is not a constraint.

Flow rate does play a significant role in how quickly a particle counter performs its job, but it is important not to ignore the sensitivity of the particle counter. If a particle counter isn't sensitive enough to measure small particles as specified in the ISO Classification of the particular critical environment, then the flow rate doesn't matter.

## Applications for Each Flow Rate

The current standard for particle counting requires a sample of 1 cubic meter. For an inspector, then, a 1 cfm flow rate makes sense. A 1 cfm flow rate would allow an inspector to collect the sample information they need in just over half an hour. A flow rate of 1 cfm means an inspector can collect samples from all the areas they need to within their schedule.

However, an employee doing a spot check or a continuous monitor in a space doesn't require the same amount of power. A technique called sequential sampling allows the total required volume to be divided into equal parts and essentially the proportion of required particles is equally divided throughout each portion. A large sample volume does not have to be sampled if the sub-samples show sufficient evidence of cleanliness. With sequential sampling, a particle counter with a 0.1 cfm flow rate, like Setra's 8000 series handheld, can be used to ensure a clean room or other critical environment meets standards during a spot check.

For continuous monitoring of a space, Setra's 7000 series AQM is ideal. A particle counter and environmental monitor with a flow rate of 0.1 cfm, the 7000 series AQM is the most versatile remote air quality monitor available. When integrated into a building automation system, a 0.1 cfm particle counter can provide continuous monitoring without the need for a higher flow rate.