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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).

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  • $\begingroup$ 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 '19 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 '19 at 11:22
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1/ 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 an easy task. The test below measures the absorption's efficiency of your charcoal filter (activated or not).

● Take a gram of your active charcoal (from your filter)
● Put it in a test tube
● Add 2ml hydrochloric acid (to make sure the charcoal is acidified, because otherwise, the alkaline ash in the charcoal will react with the iodine)
● just warm it a little to help the reaction.
● Add 25ml of iodine (iodine with alcohol).
● Wait for 24h, 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:
● Take 10ml of the solution you made above.
● Add some thiosulfate to it (drop after drop) until your solution becomes clear.
● Measure the quantity of thiosulfate that was needed.
enter image description here See Cody's video for more details.


2/ The Formaldehyde (or VOIC) method:


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

  • Buy a formaldehyde solution.
  • warm it (eg. using a rice cooker) in a closed room.
  • measure the level of formaldehyde
  • 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.

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