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Comment by etchalon

3 years ago

It's a 10x difference. It's not small.

In a sealed environment, you're right, you'd eventually end up with all particles filtered.

But homes are not sealed environments.

26 comments

etchalon

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hexane360  3 years ago

(0.9005 - 0.90005) / 0.90005 = 0.00049997222

It's a 0.049997% difference, not a 10x difference.

In an unsealed environment, the steady state will be related to amount filtered * % filtered / amount exchanged for any given time period. The difference in % filtered is not a significant factor in the above ratio.

diffeomorphism  3 years ago

What? Where are you getting the 10x from? Both numbers are about 0.9 and the difference is about 0, not 10. If you are refering to the sticker number, yeah the whole point of that calculation is that a 10x sticker number does absolutely not translate to a 10x difference.

> but homes are not sealed.

Correct, but neither are they ultra high throughput (at which point any filter sitting in the room would be useless anyway, since you never get the filtered air). So "not sealed" is too vague to make any conclusion.

kbelder  3 years ago

No, a 10x difference would be between 0.9 and 0.09. What was given was about a 1.0005x difference. If you had a child that was .9005 meters tall and one that was .90005 meters tall, you couldn't tell which was taller without a precision ruler.

  • etchalon  3 years ago

    A E12 filter filters out 99.5% of particles above 0.3 microns.

    An H13 filter filters out 99.95% of particles above 0.3 microns.

    Assuming a volume of 10000 particles above 0.3 microns:

    An E12 filter will leave 50 particles.

    An H13 filter will leave 5 particles.

    That's a 10x difference.

    • rcoveson  3 years ago

      The only point the author was making with the 0.9005 vs 0.90005 example was that if you're only processing 10% of the air, then the efficiency of your filter doesn't matter. The entire section honestly would have been better without numbers, because they cause some amount of confusion and they don't really help make the point since it's obvious. If your room's air is recycled with outside air fully over the course of one day, and your filters take ten days to work through the volume of air in your room, then the efficiency of your filter doesn't matter.

      That's it. Yes, one filter is 10x as efficient. It doesn't matter because in this example they aren't moving enough air relative to the room size/leakiness for it to matter.

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    • stephen_g  3 years ago

      So? We're talking about practical effectiveness here. The difference really only matters if you only have one chance to filter the air, like the filter in a ventilation system bringing air into a cleanroom (the article goes into this).

      Since the air purifier intakes and exhausts in the same space (meaning filtered air gets re-filtered), all the slightly worse filter means it that you'd need to run it for a couple more minutes to get the room down to a similar concentration of particulate per unit volume... So the difference in particulate concentration would likely not be anywhere near 10x at steady state, it would be much smaller (but depends how much air leaks into the room from outside, the particulate content of the outside air, the volume of air you're getting through the purifier per unit time, etc.)