Hi @nshapiro - these are all good questions, let me try to go through them one by one, thanks!
the documentation needs to be vastly improved
I'm hoping that we can get into a lot the "what's this good for" through this question, and to build out documentation as we go.
What do you mean by "high precision" in describing the sensor? In my understanding Plantower doesn't use that language when describing the product. How might that language be misleading to non-experts?
Sorry, I don't know where the "high precision" text is, can you point it out?
Oh, i see - on the store page -- i believe that was just copied from the page where we bought the sensor, but I agree we should change that. I'll edit that page now, thanks!
How does a single PMS 1003 sensor differ from the Purple Air (so that people don't think they are buying just a cheaper version of the purple air w/out quant and logging features)?
The Purple Air is not open source, so it's hard to know exactly what's going on in software, but it is based around two Plantower PMS5003 sensors, and part of our interest in DIY/open source devices is to provide transparency and accountability around what exactly closed-source devices are doing. I think there are people who've spoken directly with the Plantower and Purple Air teams and have more information on what's going on inside, so this is one way to get at this question. The Purple Air uses 2 sensors for redundancy, but I'm not sure if it averages the data, or just throws it out if they don't match.
What does it mean that these sensors are factory calibrated?
Since they're not open source, it's hard to say, but I know that folks organizing big Purple Air monitoring projects are re-calibrating them after purchase. There is a lot of information about this in the context of Purple Air's use of them in:
What conditions are these sensors good for and what conditions make them wonky?
I'm interested in developing not only materials around this, but also trainings and event models to help people plan study designs, and since the Simple Air Sensor uses the same sensor as the Purple Air, I hope it will make for a good platform to empirically determine how conditions affect readings and build knowledge around this in a group context.
What kind of particulate matter do these sensors work best in (ie they tend to preform worse in traffic-dominant pm than wood smoke-pm)? How fast does factory calibration deteriorate?
These are all great questions for any device that uses Plantower sensors, and because the Purple Air does, there are some threads about this connected to the Purple Air topic:
What are the cut off points for the different colored lights and why?
The intent of the light isn't to tell you what is safe or not, but to show relatively what the sensor is detecting. Because of the cost and complexity of the Purple Air, it's harder to understand the effects of, say, a small breeze, or the position (1ft off the ground vs. 4ft) so we are hoping people can try these kinds of things with the Simple Air Sensor to get a better understanding of what the Plantower sensor is "good at". But you can look in the source code on lines 70-75 to see how we map the
pm25_standard value to a color wheel. And, if you connect it to the Arduino IDE, you can read the numeric data on the serial port.
Which would also lead to a question of error (which remains unanswered here: https://publiclab.org/questions/wu_ming2/02-02-2019/how-do-i-calculate-error-for-averages) as the user might want to know what the lights mean and if it changes to a scarier color how accurate + precise that change is.
I agree some people will want to know this, but perhaps they would need to read the data over serial, or work with a different device. This certainly isn't the best device out there, and what we're trying to offer is a kit which can help people with questions or use cases that are quite different from that of another device. But we're open to ideas on how some of these things could be done with a Simple Air Sensor. Maybe it could blink, or make a noise, or something?
As to the error, I guess this is a good question for all Plantower-based devices, and I'll see if we can connect that question you linked to up with some of the other threads regarding Purple Air devices, and also see if we can get some folks who have offered input on this in the past to chime in.
Why should one be interested in just the 2.5 data if the sensor counts (estimates) three sizes?
Well, the way it's set up now, we can only output one type of data. I'm interested in how to change this, so if you have any ideas, I'd love to hear them. We could maybe use 3 lights, although it'd be harder to read, or we could try adding a switch. Or we could think about some kind of blinking pattern.
Could you specify on the store page who this sensor is for? I sincerely don't understand how this sensor can be "not intended for data collection" while here you are asking for how it can be used for an air quality project.
It's for some of the use cases described on this page, and some listed at https://publiclab.org/simple-air-sensor#Uses -- and we are hoping to build these out into well-documented activities that people can do. Broadly, it's intended to help people who do not want to program a sensor or build electronics, but want to try using one of the recent generation of Plantower-like sensors in scenarios where immediate feedback is helpful, and non-quantitative data is still useful. This might be alongside more expensive devices, in an event to build knowledge about the effects of conditions like wind, temperature, humidity on sensor readings, or in a classroom setting.
Nick, I'm going to reach out because I'm hearing some frustration on your part. We're grateful for your input and thank you for asking some great questions.