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Domukin t1_jcfk1ic wrote

I wonder how this compares to purple air sensors.

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Tactically_Fat t1_jcfscg5 wrote

Purple Air sensors are...problematic and really cannot be relied upon.

There's no way to test/check them. Therefore, there's no way to determine if they're working properly. They are absolutely not approved in any fashion by the USEPA as any kind of reliable sampler.

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Ut_Prosim t1_jcg6eaq wrote

The Purple II achieved excellent results in AQMD's most recent testing. The PM10 score was not quite as good, but it still seemed to be the best PM2.5 sensor in the price range.

I have yet to find any gas sensors that weren't utter trash or cost thousands. I wonder how good this MIT built tool's NO2 sensor is.

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Tactically_Fat t1_jcgatc7 wrote

The purple air samplers still can't be tested / audited, though.

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hazpat t1_jcgdz3o wrote

I would believe they can be tested in a very similar matter to professional sensors. My top of the line detectors are just calibrated to zero, which i assume is extremely easy to do on the purple. You simply apply a filter and make sure levels drop to 0. There are no mid range calibration for particulate matter. It is nearly impossible to produce a standard concentration aerosol.

Now if it is also testing for gasses or vapors, it would need to be calibrated with standards

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Tactically_Fat t1_jcgf4ya wrote

Need to be able to audit flow.

And flow is related to temperature and pressure.

Agreed that a simple leak check can be peformed with a HEPA filter.

PM samplers that can/have met Federal Equivalency Methods status are all able to be calibrated to atmospheric conditions as well as having these things audited / verified.

A lot of this is done so that the data generated can be defensible should the need arise.

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hazpat t1_jcghwce wrote

Flow meters are usualy built in and auto adjust. Checking flow with a rotometer is fast and simple.

Most meters operate under factory calibration factors that are acceptable under most conditions. You can, but are not required by any regs, to set user calibration factors based on local conditions. This is a very tedious process that you don't typically see people do unless they are in abnormal conditions like constant fog or whatever. On my meters temp and pressure sensors are built in, no idea about the purple, but I would trust the particulate data if it passed a flow and zero check.

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Maktube t1_jcgd3lr wrote

They're not really accurate to the extent that you'd want the government basing health-related decisions on their readings (I'm not sure any crowd-sourced thing could be reliable enough, either), but they're fantastic for what they are, which is 1, widely distributed and 2, consistent.

Kind of like citizen weather stations, they're not a replacement for the government data, but they're an excellent supplement to it.

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Tactically_Fat t1_jcgfe5y wrote

Some things can be an excellent supplement - but when purple air readings are way off from properly calibrated, maintained, and audited Federal Equivalency Methods or Federal Reference Methods instruments - they leave a LOT to be desired. A lot.

ETA: things that involve moving air at rates that are supposed to be constant - and/or able to compensate for changing atmospheric conditions aren't quite the same as solid-state temperature sensors, rain gauges, or even wind vanes.

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Maktube t1_jche2z8 wrote

When the right conversion is applied -- as it is by default -- PA2 sensors are actually pretty close to the official EPA sensors. Like, they produce the exact same AQI 90-95% of the time, and they're within 5ug/m^3 >98% of the time. They predict the wrong category (good/moderate/UHSG/etc) basically never (<1% of the time) and when they do it's typically because the value was right on the line between two categories.

Even if that weren't the case, though, they fill in a major gap in the EPA sensor setup that no one talks about. There aren't that many EPA sensors out there, but if you go to the EPA website to look at air quality, it will show you a value for everywhere on the map. It does this by interpolating between sensor stations and taking into account weather data. This is often not just wrong, but so wildly wrong that I think it's irresponsible to even show it. The PA2 sensors could be a factor of 2 off the official values and still be more useful than that map, because they're everywhere and they're consistent. They would regularly report dangerous air quality values in regions that the EPA map does not, which is a lot more valuable than being right on the money in terms of the actual numbers (though again, they pretty much are always right on the money).

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TricoMex t1_jchq3wh wrote

But didn't you hear? If they can't be calibrated and tested they're useless! /s

I don't know where these people with absolutist views come from honestly.

It's like amazing bills and laws being rejected because they don't resolve an issue 100%.

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-peas- t1_jcgv5ro wrote

I made & coded my own array of various sensors and have a blower/laser pm2.5 sensor, and I care not about its scientific accuracy, but I care about its ability to be able to tell me when air quality on my deck gets worse. It does that immediately with an LED that shines into my window with various colors depending on EPA air quality math. It could be huge percentages off scientifically accurate, but its going to tell me that the air quality got much worse. Its pm2.5 numbers are close to other stations around me regardless, but it definitely isn't a scientific instrument.

I'm not sure if most people buying these things care about its scientific accuracy, mainly just if things are getting worse outside.

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CARLEtheCamry t1_jchpkey wrote

I live less than 20 miles downwind from East Palestine. In the immediate aftermath local subs were full of panic posts about "omg death cloud, look at PurpleAir!".

Turns out it just got cold, and people burned wood. Happens frequently with the Cracker Plant as well, people try to correlate PurpleAir with it, it's always wood burners.

You're using the sensors right. It should be more of general guidance, leave the actual testing to scientists. Like, I wouldn't walk into a hazmat scene with my air purifier if it's sensor was green.

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Tactically_Fat t1_jcgvy1s wrote

> scientific accuracy

But that's, like, the only real way to know for sure. Otherwise - it's either speculation or generalization?

Accuracy, repeatability, and defensibility.

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what595654 t1_jcgzrjl wrote

Did you not read what he said? You are just looking at whether it goes up or down. Being perfectly accurate is not necessary.

Its like if you had a weight scale. If it told you, tomorrow that you gained 15.3 lbs, and you repeated... and it said 13.9 lbs... 17.6 lbs, so on. It doesnt matter the exactness. The point is, your weight went up a lot in one day. That is good enough to make decisions on. Not for scientific studies.

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Hyperi0us t1_jcgadoc wrote

I have a dual-range purple air system on my home. It consistently reads 10ppm low compared to the others in the neighborhood, but the high-range above 100ppm seems somewhat accurate at least.

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