Breathability Testing
Since we began testing the filtration performance of different materials, one question that has come up is breathability. How much effort is required to breath through the material? If the material is very good at blocking particles, if it is too difficult to breath through that is makes it not useful.
The approximate basis for our test setup was this paper. However, with several simplifications because we could find a stable particle generator that didn't saturate our particle counter. We also didn't have large sample of all materials that were flat can could be wrapped over a large port.
Below is a schematic of our breathability test setup:
The simplifications are:
- We couldn't find a regulatable particle generator. The fog machine, and humidifier both trigger oversaturation of our particle counter. So, we just used ambient air on the intake.
- Our particle counter uses a small pump to pull air into the sampling chamber, but when we had the particular counter on the negative pressure side of the filter, it was giving us erratic readings. Likely, the pump in the particle counter was not strong enough to overcome the negative pressure and possible air was being pulled backwards through the sampling chamber. So, we decided to separate breathability from our existing particular filter results at 3L/min of flow.
- The reference paper uses sampling area of ~59cm^2. However, we didn't have large flat samples of all of our materials and pre-formed masks are hard to flatten without creating wrinkles that would create leaks. So, our sampling area is 20mm in diameter, which is approximately 3.14cm^2. It is our current opinion is that, this should still lead to valid relative results. But, will generate higher negative pressures to achieve the target flow rate.
- Our target flow rate was 90L/min, which 3.17 ft^3/min also from the reference. Which is comparable to a strong breath.
Here is a picture of the test fixture tasking a sample.
