DIY testing of air cleaning is practical, and thoughtful experimental design
can substitute for high-quality sensors including for evaluating air purifier
setups that give >100,000x particle reductions.
I've done a lot of DIY testing over the years ( 1, 2, 3,
4,
5, 6). The
goal is generally to understand how well something removes particles from the
air. A professional particle counter (example)
costs thousands of dollars, and they're amazing devices, but what you're paying
for is convenience, reliability, calibration, and dynamic range. If we're
willing to give up on convenience and buy multiple devices for reliability, we
can cheaply address calibration and dynamic range with experimental design.
The cheapest ready-to-go option for DIY work today is probably the Temtop
P600 which I see as $70. While I haven't tried it, it's a stripped-down
version of the Temptop
M2000 which I bought several years ago to use for my DIY experiments. If
you want to make something cheaper, you can get a
PMS5003,
which I see as $21, and connect it to a cheap SoC (~$10) or to an Android phone
(adapters in the $15 range). At scale I think you could get this down below
$15: a PMS5003 or clone at high volume would be ~$7, the phone adapter would be
under $1 at this scale, software <$1, then a box, assembly, and some QC.
The Temtop and PMS5003 are somewhat calibrated, but fortunately we don't need
to know absolute particle counts. We just need some number that is, within a
reasonable range, linearly proportional to particle counts. As long as the
meter is stable over time we can look at ratios. For example, if you're trying
to see how quickly something can clear smoke from a room you don't need to
generate a target amount of smoke or know exactly how much smoke you've
generated: you can just measure the half life. This gives you relative
efficacy directly or CADR if you have a sealed room of known volume.
Dynamic range is harder, but still doesn't require professional sensors. Let's
say you want to measure the efficacy of a DIY cleanroom setup. (Note: if
you're excited about this Coefficient Giving might be willing to fund
you). You have some kind of outer room where you'll fill the air with
particles, and some kind of inner area where you want to ensure you're keeping
particle counts down. The sensors I've talked about above can measure particle
concentrations over a ~500-1,000x range, but if you're trying to assess whether
you've successfully achieved a larger reduction a simple experiment won't have
the range. A level of particles you can measure outside will give "below
range" inside, and a level you can measure inside will give "above range"
outside. What can you do?
The simplest option is just to wait longer. This is really not bad! Particle
counters are really very good at only reporting a particle when there is one,
which means you can get 10x the sensitivity by running for 10x as long. Still,
if we have 1000x range and want to measure a 100,000x reduction those are some
long waits. We can speed it up (or extend our range further) by bringing air
concentration into the range of our sensors.
The next simplest option would be to have one sensor inside and one outside,
along with an air purifier outside. Calibrate the sensors to each other ahead
of time and then start off the experiment with a very high concentration (above
range outside, within range inside). Let your air purifier bring levels down
outside. After passing through a middle region (above range outside, below
range inside) you get within range outside (but below range inside). Here's an
example of what an idealized version of this experiment might look like:
There's no time during which we have both the internal and external
measurement, but we can extrapolate our curves and estimate that when the
inside sensor is reading 10 the outside sensor would read 1,000,000.
Instead of relying on the air purifier to remove a consistent 10% of particles
from the outside each minute, however, we can add a third sensor. A MERV-16
fan removes at least 95% of the particles, so we can make a box with a fan and
a MERV-16 filter and measure counts inside that box. The box should not be
sealed; positive pressure from the fan is enough to ensure we're only measuring
the post-MERV concentration:
Unfortunately this is still not enough to connect our Inside and Outside curves,
but we can add a fourth and final link in the chain with a HEPA filter to
remove at least 99.97% of particles:
Now we have substantial temporal overlap between each pair of sensors and can
plot their ratios:
The parts of this plot we care about are the horizontal sections: that's where
the values reported by each sensor are moving proportionally. Sloped (and
ratio=1) sections aren't meaningful, since they're cases where a sensor
is out of range.
We can then read off a 20x reduction for the MERV-16, a 167x reduction from the
MERV to the HEPA, and a 30x reduction from HEPA to inside the cleanroom. These
stack to give the expected 100,000x reduction end to end.
Of course real data would be much messier, but the basic idea should be solid.
Additional logistical notes:
The particle levels we're talking about here are really high, and you
don't want to be breathing them. Ideally you can set it up so you run the
whole experiment from outside a sealed room, monitoring levels remotely. If
you do need to go inside, use a well-fitting P100 (and keep in mind that they
don't work with
beards).
I've used smoke, but smoke is sticky and poorly behaved. Better to use
aerosolized salt. You can get it in the air with an ultrasonic humidifier and
salt water, and as long as the relative humidity is below 45% the droplets will
dry out to pure salt crystals. If you're doing this in a humid place you could
use a dehumidifier.
Even levels much lower than this will set off your smoke alarm, and
levels this high might break it. I'd remove it, or at least turn it off and
seal it well with plastic.
Apparently the salt gets everywhere and is mildly corrosive (like living
by the beach for a long time). Take everything out of the room, and either
encase the room in disposable plastic sheeting (thin painter's sheeting is very
cheap) or wipe down all surfaces with a wet cloth after.
DIY testing of air cleaning is practical, and thoughtful experimental design can substitute for high-quality sensors including for evaluating air purifier setups that give >100,000x particle reductions.
I've done a lot of DIY testing over the years ( 1, 2, 3, 4, 5, 6). The goal is generally to understand how well something removes particles from the air. A professional particle counter (example) costs thousands of dollars, and they're amazing devices, but what you're paying for is convenience, reliability, calibration, and dynamic range. If we're willing to give up on convenience and buy multiple devices for reliability, we can cheaply address calibration and dynamic range with experimental design.
The cheapest ready-to-go option for DIY work today is probably the Temtop P600 which I see as $70. While I haven't tried it, it's a stripped-down version of the Temptop M2000 which I bought several years ago to use for my DIY experiments. If you want to make something cheaper, you can get a PMS5003, which I see as $21, and connect it to a cheap SoC (~$10) or to an Android phone (adapters in the $15 range). At scale I think you could get this down below $15: a PMS5003 or clone at high volume would be ~$7, the phone adapter would be under $1 at this scale, software <$1, then a box, assembly, and some QC.
The Temtop and PMS5003 are somewhat calibrated, but fortunately we don't need to know absolute particle counts. We just need some number that is, within a reasonable range, linearly proportional to particle counts. As long as the meter is stable over time we can look at ratios. For example, if you're trying to see how quickly something can clear smoke from a room you don't need to generate a target amount of smoke or know exactly how much smoke you've generated: you can just measure the half life. This gives you relative efficacy directly or CADR if you have a sealed room of known volume.
Dynamic range is harder, but still doesn't require professional sensors. Let's say you want to measure the efficacy of a DIY cleanroom setup. (Note: if you're excited about this Coefficient Giving might be willing to fund you). You have some kind of outer room where you'll fill the air with particles, and some kind of inner area where you want to ensure you're keeping particle counts down. The sensors I've talked about above can measure particle concentrations over a ~500-1,000x range, but if you're trying to assess whether you've successfully achieved a larger reduction a simple experiment won't have the range. A level of particles you can measure outside will give "below range" inside, and a level you can measure inside will give "above range" outside. What can you do?
The simplest option is just to wait longer. This is really not bad! Particle counters are really very good at only reporting a particle when there is one, which means you can get 10x the sensitivity by running for 10x as long. Still, if we have 1000x range and want to measure a 100,000x reduction those are some long waits. We can speed it up (or extend our range further) by bringing air concentration into the range of our sensors.
The next simplest option would be to have one sensor inside and one outside, along with an air purifier outside. Calibrate the sensors to each other ahead of time and then start off the experiment with a very high concentration (above range outside, within range inside). Let your air purifier bring levels down outside. After passing through a middle region (above range outside, below range inside) you get within range outside (but below range inside). Here's an example of what an idealized version of this experiment might look like:
There's no time during which we have both the internal and external measurement, but we can extrapolate our curves and estimate that when the inside sensor is reading 10 the outside sensor would read 1,000,000.
Instead of relying on the air purifier to remove a consistent 10% of particles from the outside each minute, however, we can add a third sensor. A MERV-16 fan removes at least 95% of the particles, so we can make a box with a fan and a MERV-16 filter and measure counts inside that box. The box should not be sealed; positive pressure from the fan is enough to ensure we're only measuring the post-MERV concentration:
Unfortunately this is still not enough to connect our Inside and Outside curves, but we can add a fourth and final link in the chain with a HEPA filter to remove at least 99.97% of particles:
Now we have substantial temporal overlap between each pair of sensors and can plot their ratios:
The parts of this plot we care about are the horizontal sections: that's where the values reported by each sensor are moving proportionally. Sloped (and ratio=1) sections aren't meaningful, since they're cases where a sensor is out of range.
We can then read off a 20x reduction for the MERV-16, a 167x reduction from the MERV to the HEPA, and a 30x reduction from HEPA to inside the cleanroom. These stack to give the expected 100,000x reduction end to end.
Of course real data would be much messier, but the basic idea should be solid.
Additional logistical notes:
The particle levels we're talking about here are really high, and you don't want to be breathing them. Ideally you can set it up so you run the whole experiment from outside a sealed room, monitoring levels remotely. If you do need to go inside, use a well-fitting P100 (and keep in mind that they don't work with beards).
I've used smoke, but smoke is sticky and poorly behaved. Better to use aerosolized salt. You can get it in the air with an ultrasonic humidifier and salt water, and as long as the relative humidity is below 45% the droplets will dry out to pure salt crystals. If you're doing this in a humid place you could use a dehumidifier.
Even levels much lower than this will set off your smoke alarm, and levels this high might break it. I'd remove it, or at least turn it off and seal it well with plastic.
Apparently the salt gets everywhere and is mildly corrosive (like living by the beach for a long time). Take everything out of the room, and either encase the room in disposable plastic sheeting (thin painter's sheeting is very cheap) or wipe down all surfaces with a wet cloth after.
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