Agreed that measuring inside negative-pressure respirators is harder, but it might be more feasible to measure particle concentrations inside improvised positive-pressure respirators.
One thing that might need additional R&D on top of existing low-cost particle sensors: making sure that they can meaningfully measure very-low concentrations, e.g. air that’s 1000x cleaner than usual (which might be close to the noise floor of these sensors).
Measuring air from in a positive pressure respirator or clean room that is more than three logs cleaner than the room air is doable if you’re thoughtful about experimental setup. Fundamentally, each sensor gives you about three logs of range, you don’t have to use the sensor raw. Here’s an example with four sensors:
You have your outer room, where you’re going to have a very high particle concentration, and you put a sensor here. This sensor will read out of range (too high) for most of the experiment.
In your outer room, you put a fan with a MERV-16 filter, blowing air into a bax (plenum) at very very slight positive pressure, where you put another sensor. This will read within range for your whole experiment if you’re lucky, or perhaps just a second 2⁄3 of the experiment.
Do the same as (2) with a HEPA filter. This will read within range for your whole experiment if you’re lucky, or perhaps just the first 2⁄3 of the experiment.
The final sensor goes in the place you’re trying to measure. It will read out of range (too low) at the end of the experiment.
Also put a small air purifier in your outer room, so that the particle count will decrease over time in a smooth way.
Before starting, calibrate your sensors by putting them all the same room and seeing how they handle the same input.
To begin the experiment, you put a huge amount of particles into the air of the outer room, perhaps dried salt. You continuously monitor your four sensors to do math on afterwards. You’re going to want to do it on count bins, not mass, because that should be more stable over time and the experiment isn’t able to measure everything simultaneously.
You should be able to calculate ratios between all of these sensors, either directly or by chaining. What you want to know is the ratio between your initial room (1) and your clean room (4), which you can get chained from (2) and (3).
If you use salt for your particles, the most you can get in the air is probably 100-500 mg/m3 and the sensors can do ~1-1000 ug/m3, this gives us 5-6 logs of dynamic range.
You can also get additional sensitivity by running longer, but I think sensitivity increases with the square root of time, so 10x sensitivity means 100x longer, which is pretty annoying. [EDIT: actually I think it’s linear, so this is a good way to extend your range.]
Agreed that measuring inside negative-pressure respirators is harder, but it might be more feasible to measure particle concentrations inside improvised positive-pressure respirators.
One thing that might need additional R&D on top of existing low-cost particle sensors: making sure that they can meaningfully measure very-low concentrations, e.g. air that’s 1000x cleaner than usual (which might be close to the noise floor of these sensors).
Measuring air from in a positive pressure respirator or clean room that is more than three logs cleaner than the room air is doable if you’re thoughtful about experimental setup. Fundamentally, each sensor gives you about three logs of range, you don’t have to use the sensor raw. Here’s an example with four sensors:
You have your outer room, where you’re going to have a very high particle concentration, and you put a sensor here. This sensor will read out of range (too high) for most of the experiment.
In your outer room, you put a fan with a MERV-16 filter, blowing air into a bax (plenum) at very very slight positive pressure, where you put another sensor. This will read within range for your whole experiment if you’re lucky, or perhaps just a second 2⁄3 of the experiment.
Do the same as (2) with a HEPA filter. This will read within range for your whole experiment if you’re lucky, or perhaps just the first 2⁄3 of the experiment.
The final sensor goes in the place you’re trying to measure. It will read out of range (too low) at the end of the experiment.
Also put a small air purifier in your outer room, so that the particle count will decrease over time in a smooth way.
Before starting, calibrate your sensors by putting them all the same room and seeing how they handle the same input.
To begin the experiment, you put a huge amount of particles into the air of the outer room, perhaps dried salt. You continuously monitor your four sensors to do math on afterwards. You’re going to want to do it on count bins, not mass, because that should be more stable over time and the experiment isn’t able to measure everything simultaneously.
You should be able to calculate ratios between all of these sensors, either directly or by chaining. What you want to know is the ratio between your initial room (1) and your clean room (4), which you can get chained from (2) and (3).
If you use salt for your particles, the most you can get in the air is probably 100-500 mg/m3 and the sensors can do ~1-1000 ug/m3, this gives us 5-6 logs of dynamic range.
You can also get additional sensitivity by running longer, but
I think sensitivity increases with the square root of time, so 10x sensitivity means 100x longer, which is pretty annoying. [EDIT: actually I think it’s linear, so this is a good way to extend your range.]Expanded this and my earlier comment into a post: https://www.jefftk.com/p/high-dynamic-range-diy-air-testing