New DIY Design Increases CADR 15%

After I made my first DIY, a design magazine called me up to interview me. I said, “You realize this has no design in it, right? It’s just a fan and a filter.” Now I actually have a design improvement! I’m calling it the DIY 1.1. 

The Problem

The data shows the Original DIY significantly reduces particulate pollution in the home (1, 2). But it’s not perfect. If you put your hands on the sides of the machine, you can feel that some air hits the HEPA and then comes out the side of the machine.

So we wondered what would happen if we put a guard on the side. If that forces more air through the HEPA, it should improve performance. In true DIY spirit, we first made a couple DIY versions:

After some positive initial test results, we designed a “fancy” plastic side guard to go around the back. Here’s what it looks like:

But does it work better? We put it to the test using real-world room tests and third-party CADR tests.

The Room Test

We tested the DIY 1.1 using the same room and method as my earlier tests. These tests use real air pollution in a normal apartment to see how much particulate the purifier can remove in the situation people use them most frequently—while sleeping. Click here to learn more about the testing methods.


Compared to my previous tests of the Original DIY in the same room, the DIY 1.1 did a little better: 84% versus 86% of 0.5 micron particles, and 92% versus 94% of 2.5 micron particles.

More details about the test and the original data are available in the supplemental materials.

The CADR Test

One thing I like about room tests is that they’re in real life conditions, exactly how normal people use purifiers. That means room tests take real-life factors into account like leakage from windows and a person moving in the room.

In CADR tests, the tester burns (or sprays) particles into a sealed room rather than using naturally occurring pollution in the home. CADR tests are less realistic, but the big benefit of CADR tests is that they are more tightly controlled. They give more precise estimates of the differences between two machines than the real-world tests.

With that in mind, we asked a third-party testing organization in Fujian to conduct CADR tests with the Original DIY and the 1.1. They burned a cigarette in a standard 30m3 sealed room for the 20-minute CADR test (more details on CADR tests in this guide).

CADR Results

Remember that CADR can be reported in cubic feet per minute or cubic meters per hour. It’s easy to confuse the two, especially since people often don’t report the units. These results are in cubic meters per hour. Here’s what the CADR tests found:

The 1.1 design increases the CADR by 15%. That’s not bad for a plastic fin! A CADR of 101 cbm/hr is equivalent to the Blueair S1 as measured by independent AHAM CADR tests.

Effectiveness is the most important thing to a numbers guy like me, but the new version also has two other small improvements.

It’s Safer!

We added a metal mesh in front on the fan blades. That makes the fan safer for from children’s fingers and doggie paws. It’s something we’ve always had people worry about. With the added mesh, we think the DIY1.1 will fit perfectly in a child’s bedroom.

And Easier

The DIY has always been really simple. It’s a filter strapped to a fan. With the original DIY, the filter would sometimes slip off, but the 1.1 has slits that keep the strap in place. For people with clumsy hands like me, the slits make it easier to strap on the HEPA, and stops it slipping off if it gets bumped or nudged.

After seeing the data, I’m ready to release the new DIY 1.1 on the Smart Air homepage and Taobao store for 228 RMB.

Open Data

As always, I’m publishing more details on the methods and the original data for fellow nerds here.

Thomas Talhelm

Thomas is a new Assistant Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.

5 thoughts on “New DIY Design Increases CADR 15%

  1. With the Cannon model, have you thought about introducing a simple plastic part to go on the output end to convert the shape from circular to square, something like this shape?

    It would fit your filters better, perhaps removing the need to strap them on, and more importantly I would expect it to improve performance/longevity of the filters. Judging by the images of your used filters, according to my maths the Cannon isn’t using 21.5% of the available filter area.

    1. Hi Sam,

      Yes, we’ve definitely thought of this! Using more of the HEPA area would increase the efficiency, but in fact more of the HEPA is actually being used than you think.
      We’re yet to come out with a modification, because the downside is that creating a simple plastic part like that would cost us thousands of USD to create the machine mould. Paying up front for this would mean we’d need to inflate the price of the Cannon. Since the Cannon is aimed as a low cost DIY air purifier this is not something we want to do right now – keeping it as affordable as possible is our main aim.

  2. Forgive me if this has already been answered on your site, but why is the filter placed after the fan rather than before the fan (from the air movement’s point of view). Most other filters in commercial models are placed before the fan. Thanks!

    1. Hey Josh, good question! The main reason for doing this is because it’s the front of the fan that’s flat. The back of the DIY1.1 isn’t flat which means there wouldn’t be a good seal if we placed the filter there.
      The main advantage of putting the filter on the back is that it means clean air passes over the fan, which means the fan won’t get dirty. It shouldn’t affect airflow, and we’ve actually done tests on the Cannon proving that airflow is worse with the filter on the back (for the Cannon, it does have a flat back).

  3. On another page on this website, it says that the DIY 1.1 was tested in a room size of 15 sqm with 92-95.5% effectiveness. The description for the Cannon says “From our testing, we’ve seen that the Cannon is 96 – 97% effective against PM2.5 in a room size of 15sqm. So the Cannon works in a room about twice as large as the Original DIY.”

    So does this mean, that with the new improvements, now the DIY 1.1 can be used in a room of 15 sqm just like the Cannon? Can the DIY 1.1 be used for a room that size, whereas the Original DIY couldn’t be used for a room that size?

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