Hi,
These are calculations based on the pm2_5_cf_1, pm2_5_cf_1_b, current_humidity
and current_temp_f values returned from calling http://<purple-air>/json
Specifically, method is in the code here:
@staticmethod
def compute_pm2_5_us_epa_correction(pm2_5_cf_1: float, pm2_5_cf_1_b: float,
current_humidity: int, current_temp_f: int) -> float:
# PM2.5=0.541*PA_cf1(avgAB)-0.0618*RH +0.00534*T +3.634
val = 0.541 * (pm2_5_cf_1 + pm2_5_cf_1_b) / 2.0 - 0.0618 *
current_humidity + 0.00534 * current_temp_f + 3.634
return val if val >= 0.0 else 0.0
This extension does not provide the 12-hour NOW cast.
> On Mar 24, 2021, at 10:07 AM, [email protected] <[email protected]> wrote:
>
>
> This post is intended to get information and feedback from John Klein, the
> author of the weewx-purple extension.
>
> I recently set up two pupleair outdoor sensors and installed the weewx-purple
> extension. All is working well, but I have some questions about the
> calculation of AQI and the US EPA correction for PM2.5 used in the extension.
>
> I reviewed the reference document
> (https://cfpub.epa.gov/si/si_public_file_download.cfm?p_download_id=539905&Lab=CEMM)
> and it proposes the following averaging operations:
>
> 1. Average each column to 1-hr average (e.g. 8:00-8:59= 8am average)
> 2. Exclude hour if less than 90% of the measurements are available in the
> hour average
> 3. Exclude hour if 1-hr A&B averages are different by BOTH: 5 µg m-3 & 70%†
>
> My question is if the extension applies these averaging and data exclusion
> operations? From the graphs produced it appears that the 1 hour average is
> not used, because the graphs show peaks that are not present when compared
> with the 1 hour average graphs obtained directly from the purpleair website.
>
> My suspicion is that the correction applied is limited to the following
> formula
> PM2.5=0.541*PA_cf1(avgAB)-0.0618*RH +0.00534*T +3.634
>
> Thanks in advance for any feedback.
>
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