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Ut_Prosim t1_jcg6eaq wrote

Reply to comment by Tactically_Fat in Open-source tool from MIT’s Senseable City Lab lets people check air quality, cheaply. by chrisdh79

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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