8 ms·

Agreed: the Wirecutter's emphasis on HEPA is not right for a purifier that sits in a room. Once you get to reasonably high removal efficacy (even 90%, let alon

by jefftk 4y ago

Agreed: the Wirecutter's emphasis on HEPA is not right for a purifier that sits in a room. Once you get to reasonably high removal efficacy (even 90%, let alone 99.5% vs 99.97%) flow rate matters far more than filter spec. I also wish the Wirecutter would publish more detailed logs. They just check the particle density after half an hour, which is generally super low. Instead they could show the particle density curves, or the minute-over-minute decreases (ex: https://www.jefftk.com/p/testing-air-purifiers https://www.jefftk.com/p/testing-air-purifiers)

99_00 4y ago
>HEPA is not right for a purifier that sits in a room Why not? I don't know anything about HEPA, or quality, air flow, etc.
- jefftk 4y ago
  The article explains it well: Here’s a thought experiment: Take a 1000 cubic feet room and a purifier that processes 100 cubic feet of air per minute. (I follow Wirecutter in using vulgar imperial units.) Assume pessimistically that all particles are the worst-case size. If you run that purifier with an E12 filter, the fraction of particles that will remain after one minute is .1 × (1-.995) + .9 = 0.9005. That’s because 10% of the air goes through the purifier and has 99.5% of particles removed, while 90% of the air doesn’t go through the purifier at all. Meanwhile, if you run that purifier with an H13 filter instead then the fraction of particles that remain will be .1 × (1-.9995) + .9 = 0.90005. If you noticed that 0.9005 and 0.90005 are almost identical then congratulations—you understand air filters better than the Wirecutter. Both 99.5% and 99.95% are close enough to 100% that performance is almost entirely determined by the volume of air they process.
  - etchalon 4y ago
    The idea that the difference between 0.9005 and 0.90005 is "small" is … weird. The moment I read that I checked out on the rest of the authors opinions.
    - bramblerose 4y ago
      Why do you feel it is weird? They are both 90%, because 90% * 100% + 10% * "effectively 0%" is completely dominated by the first term.
    - 1986 4y ago
      Why? It's a 0.05% difference, seems pretty small to me.
      - etchalon 4y ago
        The difference between 99.5% and 99.95 is the difference between an event happening 1 in 200 times and happening and 1 in 2000 times. It's a 10x difference. The author's "I'll just times .1 by the percent of flow, and produce very small numbers that look fine! See! The numbers are so small!" trick is just … wrong. The author implies that the difference can be made up by the volume of air being processed, but that would only be true of a sealed environment, where no new pollutants are added to the air. Setting aside the basic misunderstanding of probability, and ignoring that home purifiers don't operate in sealed environments, the IKEA unit does not process 10x the amount of air as the other units, so the point is mute.
        snowwrestler 4y ago
        Probabilities and amounts are not comparable even though they both use % notation. In this case they are measuring the % of particles captured (an amount), not the likelihood a particle is captured (a probability). The parent is right, it’s a tiny difference.
        rootlocus 4y ago
        Consider a purifier that purifies 99.995%. According to your "probabilities", that's a 100x improvement. Now consider this purifier purifies 1 cubic millimeter of air per hour. That is to say, each hour 1 cubic millimeter of air is 99.995% purified (no probability). Would you say that this purifier is 100x better than the IKEA one with 99.5% purification at 1 cubic feet of air per minute? Considering air flow is not a trick.
  - rootlocus 4y ago
    Another application of Amdahl's law.
    - jefftk 4y ago
      "the overall performance improvement gained by optimizing a single part of a system is limited by the fraction of time that the improved part is actually used" Thanks for teaching me the name for this principle!
      - Gigachad 4y ago
        This is how I feel every time HN suggests rewriting every website in C while ignoring the fact the database takes most of the time for average web apps. Or the articles about how Python is causing climate disaster while the author continues to drive an oversized SUV.
  - asojfdowgh 4y ago
    I really don't like this math, no one actually stops at that point, you take the output, la, "0.9005" and re-run it (0.9005 × .1) × (1-.995) + (0.9005 × .9) and again, and again, etc, point being that over time it does cycle the entire room, due to entropy, and then suddenly the differences start to stack up a bit, not a lot, but when one filter is letting 10x the particles through vs the other filter, it'll show
    - jefftk 4y ago
      > suddenly the differences start to stack up a bit, not a lot, but when one filter is letting 10x the particles through vs the other filter, it'll show It doesn't show: the difference is too small to notice at every stage: https://www.jefftk.com/remaining-particles-by-minute-995-vs-9995-big.png https://www.jefftk.com/remaining-particles-by-minute-995-vs-... Even comparing 95% vs 99.95% it's barely noticeable: https://www.jefftk.com/remaining-particles-by-minute-95-vs-9995-big.png https://www.jefftk.com/remaining-particles-by-minute-95-vs-9... Sheet: https://docs.google.com/spreadsheets/d/1wVIdWR0lWgZRt4gbnVaHqZNrYfBZyznEn-xv486OBgk/edit https://docs.google.com/spreadsheets/d/1wVIdWR0lWgZRt4gbnVaH...
      - asojfdowgh 4y ago
        fair
- highwaylights 4y ago
  Almost all of these review sites, not understanding the physics involved, believe a HEPA filter sieves particles down to a size of 0.3 microns, which implies that anything smaller passes on through. This is utterly false. HEPA filters are measured at the efficiency of what’s known as the MPP (the Most Penetrating Particle size). It’s the hardest particle size to capture as it can get by the two methods used to capture large particles (impaction), and smaller particles (diffusion). Considering almost none of the air in a room is passing through the filter at a given moment, the efficiency of the filter is less important than how much air it moves through the filter media per minute, which IKEA have favoured here. Essentially this filter performs close to par with more expensive units, while using less energy, and having dramatically lower costs for filter replacements when due. What they don’t do is give reviewers either kickbacks or basic physics lessons.
  - weaksauce 4y ago
    > Almost all of these review sites, not understanding the physics involved, believe a HEPA filter sieves particles down to a size of 0.3 microns, which implies that anything smaller passes on through. To be fair, it took a pandemic for me to go to the literature of mask effectiveness and finally found the "on the filtration efficiency of fiberous filters" paper that showed the u shaped curve. it's not something that they scream from the hills about in their product brochures. That said it should be screamed from the hills.
    - highwaylights 4y ago
      You might also enjoy this: https://www.youtube.com/watch?v=cagRuiyAsio https://www.youtube.com/watch?v=cagRuiyAsio
      - weaksauce 4y ago
        that's a president really standing by their product... impressive.
  - DantesKite 4y ago
    Thank you for this comment. Really gave me a lot of clarity for how to think about air filters.
- s0rce 4y ago
  HEPA makes sense if you filter all the air, ie. the filter is inline like in a laminar flow cabinet/cleanroom or directly inserted in an air stream filtering 100% of the downstream air. In those cases you care a lot about how many particles make it through since they will cause yield loss or contamination in the processes.
  - seoaeu 4y ago
    Yeah, the difference is whether you can run the same air through the filter multiple times.
- Lazare 4y ago
  For EU standards, a filter removes X% of the hardest particle size to remove (called the Most Penetrating Particle Size or MPPS). That size varies between filters, but is often around 0.3 microns. Filters will do better than X% for particles both larger and smaller than the MPPS. So for the EU standard, we're saying "it'll do X% worst case, and better than X% for all other cases, meaning it'll always filter out more than X% of all particles, regardless of size". If X% is 99.95% or higher and below 99.9995%, it's technically HEPA; 99.9995% and above it's ULPA, and below 99.95% (but 85% or higher) it's EPA. So let's say you've got an E12 filter that removes 99.6% of particles. Technically not HEPA, but after one pass through the filter and you've got 0.4% of the particles left. Two passes and you've got 0.0016% left, three passes and it's 0.0000064% left. An H13 filter might remove 99.97% of particles. One pass and you've got 0.04% left, two passes and you've got 0.000016% left. Or in other words, one pass through a technically HEPA filter might leave you with 25 times more particles than two passes through a technically non-HEPA filter. So if the air is cirulating back through the filter (as it would in a closed room), what matters is both how good the filter is and how much air it can filter. And since at the high end the filters are all so good, the volume of air processed dominates. A filter that processes twice as much air is vastly better than one that filters out an extra fraction of a percent of particles. (US standards are similar, but the cutoff for HEPA is 99.97% of 0.3 micron particles, not 99.95% of "whatever the filter is worst at". But the difference is generally irrelevant.)
asojfdowgh 4y ago
> which is generally super low. Except when it isn't, which is kinda the point: Its a fan and a filter, if the fan is improperly fitted, path of least resistance starts playing, if the filter is improperly fitted, blah blah making a fan spin to the point of getting the most volume allowed through a filter, is probably the easiest bit of the entire process
deleted 4y ago
[deleted]

by jefftk 4y ago

99_00 4y ago

>HEPA is not right for a purifier that sits in a room Why not? I don't know anything about HEPA, or quality, air flow, etc.

jefftk 4y ago
The article explains it well: Here’s a thought experiment: Take a 1000 cubic feet room and a purifier that processes 100 cubic feet of air per minute. (I follow Wirecutter in using vulgar imperial units.) Assume pessimistically that all particles are the worst-case size. If you run that purifier with an E12 filter, the fraction of particles that will remain after one minute is .1 × (1-.995) + .9 = 0.9005. That’s because 10% of the air goes through the purifier and has 99.5% of particles removed, while 90% of the air doesn’t go through the purifier at all. Meanwhile, if you run that purifier with an H13 filter instead then the fraction of particles that remain will be .1 × (1-.9995) + .9 = 0.90005. If you noticed that 0.9005 and 0.90005 are almost identical then congratulations—you understand air filters better than the Wirecutter. Both 99.5% and 99.95% are close enough to 100% that performance is almost entirely determined by the volume of air they process.
- etchalon 4y ago
  The idea that the difference between 0.9005 and 0.90005 is "small" is … weird. The moment I read that I checked out on the rest of the authors opinions.
  - bramblerose 4y ago
    Why do you feel it is weird? They are both 90%, because 90% * 100% + 10% * "effectively 0%" is completely dominated by the first term.
  - 1986 4y ago
    Why? It's a 0.05% difference, seems pretty small to me.
    - etchalon 4y ago
      The difference between 99.5% and 99.95 is the difference between an event happening 1 in 200 times and happening and 1 in 2000 times. It's a 10x difference. The author's "I'll just times .1 by the percent of flow, and produce very small numbers that look fine! See! The numbers are so small!" trick is just … wrong. The author implies that the difference can be made up by the volume of air being processed, but that would only be true of a sealed environment, where no new pollutants are added to the air. Setting aside the basic misunderstanding of probability, and ignoring that home purifiers don't operate in sealed environments, the IKEA unit does not process 10x the amount of air as the other units, so the point is mute.
      - snowwrestler 4y ago
        Probabilities and amounts are not comparable even though they both use % notation. In this case they are measuring the % of particles captured (an amount), not the likelihood a particle is captured (a probability). The parent is right, it’s a tiny difference.
      - rootlocus 4y ago
        Consider a purifier that purifies 99.995%. According to your "probabilities", that's a 100x improvement. Now consider this purifier purifies 1 cubic millimeter of air per hour. That is to say, each hour 1 cubic millimeter of air is 99.995% purified (no probability). Would you say that this purifier is 100x better than the IKEA one with 99.5% purification at 1 cubic feet of air per minute? Considering air flow is not a trick.
- rootlocus 4y ago
  Another application of Amdahl's law.
  - jefftk 4y ago
    "the overall performance improvement gained by optimizing a single part of a system is limited by the fraction of time that the improved part is actually used" Thanks for teaching me the name for this principle!
    - Gigachad 4y ago
      This is how I feel every time HN suggests rewriting every website in C while ignoring the fact the database takes most of the time for average web apps. Or the articles about how Python is causing climate disaster while the author continues to drive an oversized SUV.
- asojfdowgh 4y ago
  I really don't like this math, no one actually stops at that point, you take the output, la, "0.9005" and re-run it (0.9005 × .1) × (1-.995) + (0.9005 × .9) and again, and again, etc, point being that over time it does cycle the entire room, due to entropy, and then suddenly the differences start to stack up a bit, not a lot, but when one filter is letting 10x the particles through vs the other filter, it'll show
  - jefftk 4y ago
    > suddenly the differences start to stack up a bit, not a lot, but when one filter is letting 10x the particles through vs the other filter, it'll show It doesn't show: the difference is too small to notice at every stage: https://www.jefftk.com/remaining-particles-by-minute-995-vs-9995-big.png https://www.jefftk.com/remaining-particles-by-minute-995-vs-... Even comparing 95% vs 99.95% it's barely noticeable: https://www.jefftk.com/remaining-particles-by-minute-95-vs-9995-big.png https://www.jefftk.com/remaining-particles-by-minute-95-vs-9... Sheet: https://docs.google.com/spreadsheets/d/1wVIdWR0lWgZRt4gbnVaHqZNrYfBZyznEn-xv486OBgk/edit https://docs.google.com/spreadsheets/d/1wVIdWR0lWgZRt4gbnVaH...
    - asojfdowgh 4y ago
      fair
highwaylights 4y ago
Almost all of these review sites, not understanding the physics involved, believe a HEPA filter sieves particles down to a size of 0.3 microns, which implies that anything smaller passes on through. This is utterly false. HEPA filters are measured at the efficiency of what’s known as the MPP (the Most Penetrating Particle size). It’s the hardest particle size to capture as it can get by the two methods used to capture large particles (impaction), and smaller particles (diffusion). Considering almost none of the air in a room is passing through the filter at a given moment, the efficiency of the filter is less important than how much air it moves through the filter media per minute, which IKEA have favoured here. Essentially this filter performs close to par with more expensive units, while using less energy, and having dramatically lower costs for filter replacements when due. What they don’t do is give reviewers either kickbacks or basic physics lessons.
- weaksauce 4y ago
  > Almost all of these review sites, not understanding the physics involved, believe a HEPA filter sieves particles down to a size of 0.3 microns, which implies that anything smaller passes on through. To be fair, it took a pandemic for me to go to the literature of mask effectiveness and finally found the "on the filtration efficiency of fiberous filters" paper that showed the u shaped curve. it's not something that they scream from the hills about in their product brochures. That said it should be screamed from the hills.
  - highwaylights 4y ago
    You might also enjoy this: https://www.youtube.com/watch?v=cagRuiyAsio https://www.youtube.com/watch?v=cagRuiyAsio
    - weaksauce 4y ago
      that's a president really standing by their product... impressive.
- DantesKite 4y ago
  Thank you for this comment. Really gave me a lot of clarity for how to think about air filters.
s0rce 4y ago
HEPA makes sense if you filter all the air, ie. the filter is inline like in a laminar flow cabinet/cleanroom or directly inserted in an air stream filtering 100% of the downstream air. In those cases you care a lot about how many particles make it through since they will cause yield loss or contamination in the processes.
- seoaeu 4y ago
  Yeah, the difference is whether you can run the same air through the filter multiple times.
Lazare 4y ago
For EU standards, a filter removes X% of the hardest particle size to remove (called the Most Penetrating Particle Size or MPPS). That size varies between filters, but is often around 0.3 microns. Filters will do better than X% for particles both larger and smaller than the MPPS. So for the EU standard, we're saying "it'll do X% worst case, and better than X% for all other cases, meaning it'll always filter out more than X% of all particles, regardless of size". If X% is 99.95% or higher and below 99.9995%, it's technically HEPA; 99.9995% and above it's ULPA, and below 99.95% (but 85% or higher) it's EPA. So let's say you've got an E12 filter that removes 99.6% of particles. Technically not HEPA, but after one pass through the filter and you've got 0.4% of the particles left. Two passes and you've got 0.0016% left, three passes and it's 0.0000064% left. An H13 filter might remove 99.97% of particles. One pass and you've got 0.04% left, two passes and you've got 0.000016% left. Or in other words, one pass through a technically HEPA filter might leave you with 25 times more particles than two passes through a technically non-HEPA filter. So if the air is cirulating back through the filter (as it would in a closed room), what matters is both how good the filter is and how much air it can filter. And since at the high end the filters are all so good, the volume of air processed dominates. A filter that processes twice as much air is vastly better than one that filters out an extra fraction of a percent of particles. (US standards are similar, but the cutoff for HEPA is 99.97% of 0.3 micron particles, not 99.95% of "whatever the filter is worst at". But the difference is generally irrelevant.)

asojfdowgh 4y ago

> which is generally super low. Except when it isn't, which is kinda the point: Its a fan and a filter, if the fan is improperly fitted, path of least resistance starts playing, if the filter is improperly fitted, blah blah making a fan spin to the point of getting the most volume allowed through a filter, is probably the easiest bit of the entire process

deleted 4y ago

[deleted]