Experiments on a $50 DIY air purifier that takes 30s to assemble • Hacker News Board

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#11

Post by cyounkins » Thu, Dec 17, 2020, 2:43 AM UTC

rckoepke wrote: post #9

cyounkins wrote: post #6

Earlier quoted context omitted.

Most things that smell are very very small particles not captured by even HEPA filters and require a different kind of filtration. Most commonly activated carbon is used, either as granule or deposited on fibers.

HEPA filters are actually more efficient at capturing very very small particles ( More precisely, many odors are not particles at all but rather gases/vapors. Activated charcoal can capture larger organic molecules because it has an incredibly jagged/porous surface - just 100 grams of activated charcoal provides over 1,000,000 square feet of "effective" filtration surface area.

Yes, that's true that HEPA is very efficient at particles in the 0.01 to 0.1 micron range. It may also be below that, but I personally haven't seen that data. But because HEPA fails to filter gases/vapors, its performance must fall off before 1 angstrom = 0.0001 micron.

> odors are not particles at all but rather gases/vapors

I think the confusion comes from people referring to "particulate" as particles of >= 0.01 micron, whereas in physics we say that gases/vapors are also composed of particles. They may be single atoms floating (can't actually come up with an example though) or small molecules like O^2, CO, etc, but I'd still call those particles.

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#12

Post by xnyan » Thu, Dec 17, 2020, 5:02 AM UTC

Havoc wrote: post #10

artvark11 wrote: post #3

Are diy filters effective at removing smells or only removing the measurable particles?

Not really. To remove smells ozone generators are used. That stuff is really dangerous though

This is not correct. Activated carbon is very effective and in wide use for odor removal.

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#13

Post by xnyan » Thu, Dec 17, 2020, 5:03 AM UTC

artvark11 wrote: post #3

Are diy filters effective at removing smells or only removing the measurable particles?

To echo everyone else, you want activated carbon. Put it in your filter stack behind the other filter and you’re golden.

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#14

Post by Rebelgecko » Thu, Dec 17, 2020, 5:38 AM UTC

I wish there were multiple reps of each experiment, especially with how noisy the results were and the difficulties of reproducing smoke

Regardless, this project actually makes me feel better about buying my fancy air filter

Instead of buying 2 filters (both HEPA), a fan, and a poorly calibrated air quality sensor for $150 like the author, I bought an air purifier that contains 2 filters (1 HEPA, 1 activated charcoal) a fan and a poorly calibrated air sensor for $153.

However since my air quality sensor is integrated with my purifier, it only pulls ~60 watts at the times when I'm cooking or if something is on fire. That probably saves me $50/year on electricity

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#15

Post by kylebenzle » Thu, Dec 17, 2020, 8:58 AM UTC

With Covid-19 Rates Rising Adding an Air Purifier To Your Home is the Single Most Important Thing You Can Do to Improve Your Health [0]

[0] https://kylebenzle.medium.com/adding-an-air-purifier-to-your...

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#16

Post by WalterBright » Thu, Dec 17, 2020, 9:00 AM UTC

Another way to do it is to bubble the air through water. As a kid, we did an experiment where a cigarette was "smoked" by attaching a tube to the cigarette filter, the tube exited at the bottom of a flask filled with water, and a pump would suck the air from the top of the flask.

It didn't take long before the water would turn black.

Quite a turnoff for wannabee smokers, which was the obvious point of the experiment.

It's the same idea as the Rainbow vacuum cleaner, which uses a tank of water as its filter.

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#17

Post by totalZero » Thu, Dec 17, 2020, 9:01 AM UTC

This is nutty. I love it.

The limitation for me would simply be the appearance factor; I want my filter to blend in quietly, not to draw attention. I think appearance and noise are part of why people pay so much for fancy purifiers.

Also, I imagine a centrifugal fan would draw less electricity per CFM under load (air resistance through the filter).

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#18

Post by xiphias2 » Thu, Dec 17, 2020, 9:03 AM UTC

cyounkins wrote: post #6

artvark11 wrote: post #3

Are diy filters effective at removing smells or only removing the measurable particles?

Most things that smell are very very small particles not captured by even HEPA filters and require a different kind of filtration. Most commonly activated carbon is used, either as granule or deposited on fibers.

I don't know any way to filter NO2 effectively from air, and I have tried practically all commercial air filters with the largest carbon filters on the market. The worst parts of air pollution cannot be filtered out without industrial devices (that are part of the air filteration system) as far as I know.

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#19

Post by chopin » Thu, Dec 17, 2020, 9:10 AM UTC

What I am wondering about: I worked with and designed clean room equipment in the nineties. I remember that HEPA requires a considerable pressure difference which cannot be delivered by an axial fan. You'd need a radial one which can deliver a considerable differential pressure, at cost of throughput (where the axial fan shine). If you strap a HEPA to an axial fan and seal it, the fan runs effectively "empty", i.e. it displaces only little air which most likely even doesn't pass the filter. That's why HEPA filters work in vacuum cleaners since they have radial fans. I am astounded by the results of the post.

I tested simply with a laser. In a normal room you can see the laser beam in dark. If you have a good purifier, you can't.

Re: Experiments on a $50 DIY air purifier that takes 30s to assemble

#20

Post by chopin » Thu, Dec 17, 2020, 9:16 AM UTC

totalZero wrote: post #17

This is nutty. I love it. The limitation for me would simply be the appearance factor; I want my filter to blend in quietly, not to draw attention. I think appearance and noise are part of why people pay so much for fancy purifiers. Also, I imagine a centrifugal fan would draw less electricity per CFM under load (air resistance through the filter).

With the same throughput radial fans are much bigger than an axial fan. But they can deliver differential pressures in the range of several 100 Pa, which axial fans cannot.