Particle Capture Tests
For the particle capture tests, CK used a Met One GT-531 particle counter. This particle counter is best for HEPA outlet tests because it measures down to 0.3 micron particles—the hardest particles to capture. CK noted 0.3 micron and 2.5 micron particle counts with and without the HEPA.
Importantly, this particle counter has a pump to regulate air intake. That’s important because increasing the air flow in a particle counter will mean the particle counter counts more particles, simply because more air has passed through it!
There was decent variation in the 0.3 micron efficiency between the 8 HEPAs. We randomly inspect several HEPAs from each batch we receive from the manufacturer to make sure they capture 99.9% of particles 0.3 microns and above. Yet some of the readings of these old HEPAs were below 99.9%.
I can think of two reasons for that variation. First, there are slight variations in the test—particularly in how close the air inlet of the particle counter gets to the HEPA. That affects whether any air from near the HEPA (not coming through the HEPA) gets in. That doesn’t let many particles in, but when you’re trying to tell the difference between 99.9% and 99.0%, a few particles make a huge difference.
Second, these HEPAs were used, strapped in, then unstrapped, shipped to Beijing by Smart Air volunteer DIYers, and strapped on to another fan. All that movement and handling could make some of the gaps between HEPA fibers larger.
The Met One tests for 60 seconds and reports output as the cumulative number of particles counted (rather than the average).
Air Speed Tests
CK used a Pyle anemometer to take wind speed measurements at four points on each HEPA—north, south, east, and west. Wind speed varies a fair amount between the different HEPAs. That can be due to the dynamics of the air coming out of the fan (it’s surprisingly not uniform!), damage from being shipped and handled, and how much particulate is stuck in the filter.
CADR (in meters cubed per hour) was estimated using the following formula:
AVERAGE(wind speed in m/s)*AVERAGE(percent particle capture of 0.3 micron particle capture)*area of the outlet*3600/100
For reference, the smoke CADR of a Blue Air 203 is 155 (according to AHAM).
The original data is available as an Excel file and as an image below:
P.S. What about using compressed air to clean the filter? Check out the end of Thomas’s Quora post for the answer.
Paddy graduated in aeronautical engineering from Bristol University, and now runs Smart Air’s operations from Beijing. He’s an advocate for open data, free information and transparent business.