Fake HEPA filters are easy to spot using DIY PM sensor and DIY Air Purifier (we just need to turn the fan ON, place the sensor's tube in front of it, and read the data).

But what about carbon filters?

I bought a filter like this to clean the air, but I wonder if it really works (there are many fakes out there).

I'm looking for some tricks to know if it works, and (much better) to evaluate its efficiency (using Arduino sensors or any basic ways).

  • 1
    $\begingroup$ Well, the AliExpress link look very suspicious (instead of photos there is a bunch of pitch black rectangles). I think the classical method of testing would be determining the surface area of activated charcoal by Langmuir adsorption isotherm. $\endgroup$
    – andselisk
    Jan 3, 2019 at 11:01
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    $\begingroup$ Thank you! I'll look at it. (I bought a HEPA filter from that brand, and it works ―amongst many others which did not work. So the seller might be honest). $\endgroup$
    – JinSnow
    Jan 3, 2019 at 11:22

2 Answers 2


You might try a something like a VOC sensor before and after your filter.

Just let some gasoline (or diesel, or nail polish, or whatever) evaporate in a fixed volume box (you could measure the amount by weight for instance). You can also have a fan in that box to ventilate the mixture, take the base reading with the VOC sensor. Add your filter to the fan an look what happens to the VOC reading.

It might be complicated to quantify the reduction rate professionally but could give a good impression of its effectiveness.

Edit: take 2 base readings, one of the box with "clean" air, one with contaminant. If you ever reach zero again you may even calculate absorption rates, you could also determine total absorption capacity of the filter, etc.

  • $\begingroup$ What sensor do you use (I tried few years ago, and it did not work at all)? $\endgroup$
    – JinSnow
    Dec 30, 2022 at 9:04
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    $\begingroup$ There are a few digital VOC sensors that seem to work relatively well (such as Sensirion SGP40 and modules based on it). $\endgroup$
    – datenheim
    Dec 30, 2022 at 10:45

⚠ EDIT (after writing this answer). Be careful, when a carbon filter becomes "full", it can then release the molecules to which it has less affinity to capture the one to which it has more affinity.

The big question is: does it affect the test below? Can iodine take the place of nasty stuff and thus give the impression that the active carbon isn't full? Anyone can answer?

1/ Use a TDS meter:

It's easy if you want to test the efficiency of a water filter: Use a TDS meter to test the efficiency of a carbon filter This Video from Project Farm use it to compare the efficiency of different water filters.

2/ The iodine method:

I found the answer on this video (from Cody's lab YouTube channel). Cody's show how to make activated carbon/charcoal and how to test its efficiency.

It's quite easy. The test below measures the absorption's efficiency of your charcoal filter (activated or not). (⚠ don't use it to test a water filter unless you throw it away after, because the smell/taste of iodine won't go away).

Quick facts:

  • Cody use iodine but it seems to work with any dye (it works with methylene blue and even red Fanta.
  • Non activated carbon (normal charcoal) does filter, but it's much less efficient.
  • To check if the carbon is activated or not just put it in some water, activated carbon is bubbling.


  1. Take 1 gram of active charcoal (from your filter)
  2. Put it in a test tube
  3. Add 2ml hydrochloric acid (to make sure the charcoal is acidified, because otherwise, the alkaline ash in the charcoal will react with the iodine).
  4. warms it a little to help the reaction.
  5. Add 25ml of iodine (iodine with alcohol). Cody uses ≈ 4mg/ml.
  6. Wait 24h, then look at the color of your solution: the lighter the color the more effective is the charcoal (the lighter, the more it has absorbed iodine which is dark). You could compare it with some active carbon you bought, or normal charcoal enter image description here

To measure more precisely the efficiency of your charcoal:

The titration method:

  1. Take 10ml of the solution you made above.
  2. Add some thiosulfate to it (drop after drop) until your solution becomes clear.
  3. Measure the quantity of thiosulfate that was needed.
    enter image description here
    See Cody's video for more details.

Someone in a comment says:
"When titrating iodine, it pays to use starch as an indicator. The change from dark blue to colorless makes the endpoint much clearer than the slow fading out of yellow you were dealing with. This would also be much clearer on camera."

3/ The Formaldehyde (or VOIC) method (requires a Formaldehyde Meter):

If you want to know if your activated carbon filter works, you can also test it using Thomas Talhelm method (he is the most famous air pollution DIY expert). But this technique requires a solution of formaldehyde, and a device to mesure the quantity of formaldehyde in the air.

  1. Buy a formaldehyde solution.
  2. warm it (e.g., using a rice cooker) in a closed room.
  3. measure the level of formaldehyde
  4. turn ON your activated carbon filters, and keep measuring.

If your filter is working you should see that: enter image description here

With a fan only (red line), formaldehyde levels stayed high. But with a carbon filter on the fan (blue line), formaldehyde levels went down.

  • $\begingroup$ What is the concentration of iodine $\ce{I2}$ used in the first Iodine method ? $\endgroup$
    – Maurice
    Mar 14, 2022 at 12:43
  • $\begingroup$ @Maurice Cody uses ≈ 4mg/ml (*) but it only matters if you use the nitration method (to simply compare an unknown with a known/good active carbon it probably doesn't matter). *: youtu.be/GNKeps6pIao?t=833 $\endgroup$
    – JinSnow
    Mar 17, 2022 at 10:52

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