Question: How do I calculate error for averages?

wu_ming2 is asking a question about purpleair: Follow this topic

wu_ming2 asked on February 02, 2019 03:27
236 views | #18243


I just installed the device indoor to check PM2.5 air quality in our flat and how the new air purifier works. After a while I am planning to move it outdoor.

As mentioned elsewhere I developed a micro service to notify me when readings cross a threshold. I have those values expressed as a range since we have the error range for the sensor.

PA-II comes with two PMS 5003 laser counters to estimate air particulate concentration. Specifications sheet among other things cite a maximum consistency error for PM2.5. This tells me the internal precision w/o reference to a true or known value. But what kind of distribution is assumed for this precision estimate? And what is the significance? As example I would have expected to read +-10% with a significance of 2.5 standard deviations for a normal distribution.

Without this piece of information I wouldn't know what probably range to assign and how to calculate the precision for averages between the two sensors and for different timeframes. Am I overthinking it and +-10% is just fine enough? Thanks.



2 Comments

I'm going to share this on the Public Lab Twitter account to see if we can get some more input from others

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Hi @wu_ming2 - sorry we weren't able to connect you with some resources on this. This comment from @BrandonFeenstra may have some useful information:

If you would like to learn more about Air Quality Sensors and their performance against regulatory instrumentation, I would recommend a visit to the South Coast Air Quality Management Districts's (SCAQMD) Air Quality Sensor Performance Evaluation Center (AQ-SPEC) web page. The purple air sensor has been evaluated in both the field and the laboratory. Program's website: http://www.aqmd.gov/aq-spec PA-II field report: http://www.aqmd.gov/docs/default-source/aq-spec/field-evaluations/purple-air-pa-ii---field-evaluation.pdf?sfvrsn=4 PA-II laboratory report: http://www.aqmd.gov/docs/default-source/aq-spec/laboratory-evaluations/purple-air-pa-ii---lab-evaluation.pdf?sfvrsn=4

Ah, and i see you found @guolivar's post here: https://publiclab.org/notes/guolivar/01-08-2018/thoughts-on-low-cost-air-quality-sensors#c21833

Thank you!

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