The Conditions for Data Comparison
When looking at data from two different particle counters, you’ll need to know if your sampling was performed with isoaxial and isokinetic airflow. But what do these terms mean? First let’s take a look at an example.
Say you’re working in an area where airflows move from top to bottom. To minimize variation between aerosol particle counts, there are two important sampling setup conditions:
- Sample probe inlet orientation
- Size of the probe’s inlet (cross-sectional area)
These conditions can also be replicated during sample collection when particle size distributions are compared. Variation is reduced by using the same sample inlet accessories, such as the bare tube or probe, during samples that are to be compared. Also, when possible, the sample inlet opening should still face upward so that the counter’s vacuum pulls in a downward direction, matching the settling direction of relatively larger particles (≥ 0.1 µm) so these larger particles will not need to overcome their downward momentum and change direction into the probe, which leads to under sampling.
Laminar flow is all about controlling the lateral migration of airborne particles. Any generated particles are removed from the cleanroom as quickly as possible by taking the shortest path possible, which is a downward straight line through perforated floor tiles, or downward-diagonally to side wall return intakes. In the laminar environment, it is critical to use the optimal sampling configuration that leads to conditions referred to as:
- isoaxial—attained from optimal orientation, and
- isokinetic—a result of the correct probe inlet sizing.
How the counter’s inlet probe is oriented determines particle sampling efficiencies. The probe inlet should be pointed directly up in all use cases, known as the isoaxial orientation. This minimizes differences in counter results.
Isoaxial sampling ensures the counter’s internal vacuum draws air directly downward. If the probe opening is angled away from this orientation the particles’ downward inertia may prevent accurate counting (i.e., the particles don’t make it into the sampling probe). In certified cleanrooms with laminar air flow at velocities ranging from 60 – 90 ft/min it’s critical to ensure minimal horizontal travel. When comparing counts, regardless of counter airflow velocities, the air intake must be oriented in the isoaxial position.
Other Forms of Isoaxial and Isokinetic Airflow Sampling
For sub-isokinetic sampling, the velocity of air surrounding the sample probe is greater than the velocity of air entering the probe and a “sheath-layer” volume of air bends around the probe. A particle’s kinetic momentum (sizes ≥ 5 µm) moves it top-down into the sample probe rather than remain in the sheath-layer. Larger particles are over-sampled because the sheath-layer volume is not calculated in the “sampled” volume for each sample interval. However, these larger particles are counted and added to the sampled particle population concentration calculation.
In super-isokinetic sampling, the velocity of air surrounding the sample probe is less than the velocity of air entering the probe. In this case, the room air laminar flowlines deform into the probe. This results in larger particles (≥ 5 µm) being under-sampled because their momentum carries them past the sample probe rather than being counted as part of the sampled particle population. These counts are not included in the calculated sampled volume.
Understanding what kind of airflow you’re working with is key to understanding your particle data.
Learn more… Get the full paper here.