Clean air delivery rate

Measure your filter box's CADR

Measuring a DIY filter box's CADR with a phone barometer

Estimate how much clean air your DIY filter box actually delivers, using nothing more than the barometer built-in to most smart phones. When the fans run, the box pulls air in through its filters, and the filters resist that flow, so the air inside the box sits at a slightly lower pressure than the room. That pressure drop tells you how fast air is moving through the filters. Combined with how well the filter media captures particles, it gives a rough clean air delivery rate (CADR). Take one pressure reading with the fans off and one with them on, pick your filter layout and size, and read the estimate below.

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Calculator

Pick the same filter layout and size you built with, then enter the two pressure readings from a phone sealed inside the box. Readings are in hPa, exactly as phyphox shows them.

How many filters the air passes through in parallel.
Same options as the builder. All are MERV-13 except STARKVIND (a PM2.5 filter), whose different media makes its estimate rougher.
The baseline: fans turned off, no flow.
Fans at your normal running speed.
0CADR, CFM

How to run the test

Seal an Android or iOS phone inside the box, read the pressure with the fans off and then on, and enter both numbers in the calculator above. Work quickly, the barometer drifts as it warms.

1Set up phyphox

Install phyphox (free) on a phone that has a barometer (many Android and most iOS models do), open the Pressure experiment, press play, and turn on Remote Access so you can read it live from another device on the same Wi-Fi network.

phyphox Pressure experimentphyphox measurement runningphyphox Remote Access enabled

2Seal the phone inside the box

Put it in before you close up the last filter so the box stays as sealed as it is in normal use. The phone reads the pressure inside the box.

Phone sealed inside the filter box

3Read the baseline (fans off)

With the fans off, read the pressure and write it down. This is your baseline; with no flow, the inside of the box is at room pressure.

Reading the baseline pressure with the fans off

4Read with the fans on

Turn the fans on at the speed you normally run them. Wait 5 seconds for them to reach full speed, then read the pressure again.

Reading the pressure with the fans runningReading the pressure with the fans running

5Work quickly

Phone barometers drift as they warm, so take both readings within 20 seconds or so, and write down the center of the small range the reading wanders through rather than waiting for it to settle.

6Enter your readings

Enter both readings, the layout, and the filter size in the calculator above.

Why inside the box? The number that matters is the pressure drop across the filters. With the fans running, the room is at the higher pressure and the sealed box is pulled below it. The phone inside measures that difference directly.

The math

The pressure drop across the filters is just the difference between the two readings:

ΔP (Pa) = |P_off − P_on| × 100

A MERV-13 filter's resistance follows a known curve, so a given ΔP corresponds to a particular face velocity (the speed of air through the media, in feet per minute):

ΔP (inH2O) = A × V² + B × V,   A = 4.73e-7,   B = 3.38e-4

Solving that for V and multiplying by the total effective media area of all the filters gives the airflow through the filters. The clean air delivery rate is that airflow times the filter's single-pass capture efficiency:

CADR = V × filter area × efficiency

The resistance curve, the effective-area model (deeper filters pleat more media in), and the MERV-13 efficiency (about 81%, averaged across the 0.3 to 10 µm range) are the same ones the builder uses to predict CADR. Here they run in reverse, from a real measurement instead of a fan-and-filter model.

What it tells you, and what it does not

  • It estimates clean air delivery rate: the volume of filtered air per hour, counting only what the filters actually clean. Higher is better.
  • It assumes MERV-13 media, the filter type the builder is designed around. A different filter has a different resistance and efficiency, so the estimate would be off.
  • It assumes the box is reasonably sealed, so the pressure the phone sees really is the drop across the filters. Big leaks will throw it off.
  • Phone barometers read in steps of about 1 Pa with a few Pa of noise and drift, so treat the result as a ballpark, most useful for comparing builds or fan choices rather than as an exact figure.
  • This is a rough field estimate, not a certified CADR chamber test.
This is the same CADR quantity the builder predicts in its Performance panel. Measuring your finished box is a good way to check the prediction against the real build.