16 ms·

>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.

by 99_00 4y ago

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.
        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. The "rootlocus" filter would leave 0.5 particles. So yes, I would say your filter is 100x better because it literally is.
  - weaksauce 4y ago
    > 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
        [deleted]
      - 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.
  - csours 4y ago
    The author explicitly states that it's small in the home use context. If you're talking about medical or cleanroom manufacturing contexts, yes it's a huge difference.
    - etchalon 4y ago
      Small home or not, homes are not sealed environments. A 10x difference is a 10x difference. Using one small number or produce another small number, so the difference looks small, doesn't hide the 10x change.
      - jefftk 4y ago
        How is there a 10x change? I see a 0.1% change.
      - dan-robertson 4y ago
        0.90005 times 10 is 9.0005, not 0.9005 (I.e. the two fractions presented are 90.005% and 90.05%). Even if you look at the complement you get 9.995% vs 9.95% which is small. One could imagine that these differences could also arise from eg obstructions to airflow or positioning in the room or the direction of the wind outside. The point is that the difference is dominated by air flow in a typical environment rather than filtering differences.
      - infinityio 4y ago
        Is it a 10x difference? If you used the better filter, you would still have 99.95% of the particles you would have had if you used the worse filter
    - rcoveson 4y ago
      The difference between 0.9005 and 0.90005 is not huge in a medical context, or in a chip fab context, or in any other practical context. We're not talking about the difference between 0.0005 and 0.00005. The numbers in question are 0.9005 and 0.90005, and the point being made is that the 0.9 problem dwarfs the 10x efficiency difference way over in the thousandths place.
  - evancox100 4y ago
    This entire subthread is the math version of "most programmers can't even do fizz buzz".
  - aidenn0 4y ago
    This is how it works because the room is not sealed, nor is the filter being used to filter outside air into a positive pressure area. It is (hopefully) easy to see that e.g. a filter that removes 99.5% of particles, but moves twice as much air per minute will remove almost twice as many particles per minute as a filter that removes 99.95% of particles. Using the numbers from TFA (20% of the room for the 99.5 rather than 10%): .2 × (1-.995) + .8 = 0.801 vs .1 × (1-.9995) + .9 = 0.90005 Thus proving the point in TFA that the airflow matters more than E12 vs H13. The fact that the steady state (given that "dirty" air is being introduced somehow) is lower for the filter that moves more air follows from the fact that it removes particles at a faster rate.
- 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.)

>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.

by 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.
      - 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. The "rootlocus" filter would leave 0.5 particles. So yes, I would say your filter is 100x better because it literally is.
- weaksauce 4y ago
  > 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
      [deleted]
    - 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.
- csours 4y ago
  The author explicitly states that it's small in the home use context. If you're talking about medical or cleanroom manufacturing contexts, yes it's a huge difference.
  - etchalon 4y ago
    Small home or not, homes are not sealed environments. A 10x difference is a 10x difference. Using one small number or produce another small number, so the difference looks small, doesn't hide the 10x change.
    - jefftk 4y ago
      How is there a 10x change? I see a 0.1% change.
    - dan-robertson 4y ago
      0.90005 times 10 is 9.0005, not 0.9005 (I.e. the two fractions presented are 90.005% and 90.05%). Even if you look at the complement you get 9.995% vs 9.95% which is small. One could imagine that these differences could also arise from eg obstructions to airflow or positioning in the room or the direction of the wind outside. The point is that the difference is dominated by air flow in a typical environment rather than filtering differences.
    - infinityio 4y ago
      Is it a 10x difference? If you used the better filter, you would still have 99.95% of the particles you would have had if you used the worse filter
  - rcoveson 4y ago
    The difference between 0.9005 and 0.90005 is not huge in a medical context, or in a chip fab context, or in any other practical context. We're not talking about the difference between 0.0005 and 0.00005. The numbers in question are 0.9005 and 0.90005, and the point being made is that the 0.9 problem dwarfs the 10x efficiency difference way over in the thousandths place.
- evancox100 4y ago
  This entire subthread is the math version of "most programmers can't even do fizz buzz".
- aidenn0 4y ago
  This is how it works because the room is not sealed, nor is the filter being used to filter outside air into a positive pressure area. It is (hopefully) easy to see that e.g. a filter that removes 99.5% of particles, but moves twice as much air per minute will remove almost twice as many particles per minute as a filter that removes 99.95% of particles. Using the numbers from TFA (20% of the room for the 99.5 rather than 10%): .2 × (1-.995) + .8 = 0.801 vs .1 × (1-.9995) + .9 = 0.90005 Thus proving the point in TFA that the airflow matters more than E12 vs H13. The fact that the steady state (given that "dirty" air is being introduced somehow) is lower for the filter that moves more air follows from the fact that it removes particles at a faster rate.
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.)