7 ms·

> The idea that the difference between 0.9005 and 0.90005 is "small" is … weird. it is. that's one minute of filtration and the difference is minuscule. over

> The idea that the difference between 0.9005 and 0.90005 is "small" is … weird. it is. that's one minute of filtration and the difference is minuscule. over time, this would trend to zero. in 10 minutes you'd expect to be near the steady state of the room. (obviously not completely steady state since you are filtering some already filtered air and probably introducing more particulates but close enough for an approximation)

etchalon 4y 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.
- deleted 4y ago
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- diffeomorphism 4y 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.
- hexane360 4y 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.
- kbelder 4y 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 4y 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 4y 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.
      - etchalon 4y ago
        Saying, "OK, but as a total percent of the air in your home at any given time, the difference is tiny" is … weird. It's still a 10x difference. Using small numbers to make the difference look small doesn't hide that.
        amluto 4y ago
        Sorry, you’re still missing the point. If you are taking air, running it once through a filter, and using the air that comes out for an application that needs very few particles, then a 99.99% filter is “10x” as efficient as a 99.9% filter in the sense that the air coming out will have 1/10 as many particles. For example, a 99% efficient face mask is “10x” as efficient as a 90% efficient mask (assuming both fit perfectly, which they don’t, although a PAPR approximates a perfect fit). But an air purifier doesn’t do this at all. It continuously sucks in air, removes particles from it, and sends the filtered air right back into the room to mix with all the other air. The performance of a 95% filter in this context is barely distinguishable from that of a hypothetical 100% filter. Your characterization would have the 100% air purifier being “infinitely” more efficient.
- weaksauce 4y ago
  in practice the filtration in a room goes down exponentially and quite quickly even with budget filters that only filter out 90%. even in shops where you are sanding. https://woodgears.ca/dust/dylos.html https://woodgears.ca/dust/dylos.html

etchalon 4y 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.

deleted 4y ago
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diffeomorphism 4y 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.
hexane360 4y 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.
kbelder 4y 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 4y 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 4y 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.
    - etchalon 4y ago
      Saying, "OK, but as a total percent of the air in your home at any given time, the difference is tiny" is … weird. It's still a 10x difference. Using small numbers to make the difference look small doesn't hide that.
      - amluto 4y ago
        Sorry, you’re still missing the point. If you are taking air, running it once through a filter, and using the air that comes out for an application that needs very few particles, then a 99.99% filter is “10x” as efficient as a 99.9% filter in the sense that the air coming out will have 1/10 as many particles. For example, a 99% efficient face mask is “10x” as efficient as a 90% efficient mask (assuming both fit perfectly, which they don’t, although a PAPR approximates a perfect fit). But an air purifier doesn’t do this at all. It continuously sucks in air, removes particles from it, and sends the filtered air right back into the room to mix with all the other air. The performance of a 95% filter in this context is barely distinguishable from that of a hypothetical 100% filter. Your characterization would have the 100% air purifier being “infinitely” more efficient.
weaksauce 4y ago
in practice the filtration in a room goes down exponentially and quite quickly even with budget filters that only filter out 90%. even in shops where you are sanding. https://woodgears.ca/dust/dylos.html https://woodgears.ca/dust/dylos.html