For the past few months my Purple air II has been reporting less than 100 percent concordance between A and B. I’ve cleaned it out multiple times with a vacuum and even sprayed compressed air into the 8 holes. Both A and B data curves of pm2.5 basically look the same but the A line is always a bit higher than the B line. Does this mean that my A sensor is dirty/bad? When is it time to replace the sensors?
I am sharing the general steps we do in case this could help your decision:
We like to do a linear regression of channel A (x-axis) and channel B (y-axis).
We have seen one channel report slightly higher then the other but have an R2 >0.9.
This is considered ‘good’ for a PurpleAir sensor.
If the R2 is <0.7 for a prolonged period of time (typically more than 2 weeks), 1st cleaning and then replacement if cleaning doesn’t improve the R2 value.
Another consideration for replacement is if one of the channels gets stuck on reporting very high values (>1000) for PM2.5.
Note, one can replace the laser counters instead of the whole sensor. The procedure on how to do that is in other community forum posts.
Thanks. It’s been stuck at 88 to 94 percent no matter how much cleaning I do.
I’m wondering if I should pull out the sensors as if I was replacing them and clean them out when they are out of the white shell? And then if that doesn’t help then replace them…m. I bet there’s a bug or something stuck in there.
I took it apart and gave it a thorough cleaning and a good suction with the vacuum cleaner. How long should I wait before declaring success or failure?
just as a follow up I took it apart and gave it a good cleaning with my vacuum cleaner, and now it’s reporting even lower – 74%. I ordered new laser counters, coming tomorrow. We’ll see how that fairs. I think it’s been several years since I’ve installed them, so maybe it’s time…
Is there a way for me to send back the counters to purple air (I can pay for shipping) and have them conduct a post-mortem, or is that not helpful for anyone?
Hi, I may be a bit late in replying. We have some generalized criteria for the R2 of the channel A and B scatter plot with linear regression. If an R2 >= 0.9; we’ve been considering that ‘good’; an R2<0.9 but >0.7 something to watch, and if prolonged R2<.7 likely need to clean and/ or replace. We typically do a channel A / B linear regression weekly.
Here is a another good resource page on the community forum:
it’s up to 96%. By R2>=0.9, does that translate to the 90%+ reported by the Purple Air? (ie if mine is reporting 96%, then R2 = 0.96?
I do not know how PurpleAir calculates their confidence score - the % you see reported.
They do need two counters to determine a confidence score, a channel A and B.
The confidence score may be related to the R2 of a linear regression analysis. I did a quick search in the community forum and didn’t see an answer as to how the confidence score is calculated, just that high is ‘good’ and low means likely a problem with one or both channels. Perhaps a question for PurpleAir as to how the confidence score is calculated.