I had not seen this before with this (fairly new) sensor-kit, but that was before it was inside the Stevenson (and this particular sensor-kit had not seen such high humidity values yet).
Anyway, I took it out (and indoors) a few hours ago, and the PM values dropped to normal again. But honestly, this is really bad. RH above 80% values are very common here during nights, itās in a field outdoors, so essentially this means we canāt put the sensors outdoors.
Is there any experience with this by others? Are these misreading related to humidity/condense on electronics, or are they related to the actual SDS sensor not getting it when thereās fog on top of its membrane or something?
Yes, this is normal behaviour. With a humidity of 70% and up the particles are absorbing water and the readings become higher. I am currently experimenting with different techniques (calibrating with multiple lineair regression, heating the airflow to reduce humidity (take care of the T/H sensor!) ).
A lot has been written about this. Do soem extensive searching on the internet on the use of low cost sensors and you will find a lot of information.
I cannot recall seeing such high PM values in high humidity. Maybe if there is fog but otherwise not. Have you checked official data and/or other sensors near your place? Maybe the PM values are realistic. At my place, there was a pronounced PM increase (ājustā up to 35) for several hours. I also suspected a humidity effect but the official PM station data showed that the pollution is real (actually for the whole North of Germany).
I am seeing occasional dropouts in the plots, too. Sometimes itās a local Wifi issue. Most of the time the data is visible in my local influxdb, hence I believe itās a communication problem with the remote server or a problem on the remote server itself.
OK, I have placed the sensor-board in a drier place (with more air-flow - and an outlet from a warmer dryer indoor area) and hope it will make the readings more reliable. The sensor inlet hose and SHT31 are still in outdoor air this way, shielded from the āheatedā board, so that should work.
Either way, having it inside our Stevenson was not a good idea, itās just too humid there. It would just break down giving strange readings and drop-outs each time there was a fog of some sort.
HECA was designed for use in Nettigo Air Monitor, but version 2 is also compatible with inline tube like the one used in Sensor.Community. Itās small. BOM is cheap. Parts are widely available.
I wasnāt sure that the sensor community supported heating the air input as there isnāt a way of recognising this in the data for the community map. It would be really useful if this could be added to the data as where we live the humidity is usually high.
The real problem is water droplets in very high humidity. Many have tried to build an algorithm to compensate for this but none have succeeded. I believe the only solution is a heater.
Have we come to a definitive answer on the approach to high humidity and obtaining more accurate PM data?
I still think that Bernd Laquai et al ((PDF) Calibration Method for Particulate Matter Low-Cost Sensors Used in Ambient Air Quality Monitoring and Research) have a good method. The issues with high humidity being 2 fold: that of water droplets and hygroscopic growth on particulates. The article suggests heating the air to remove the water droplets and then monitoring the humidity of the exhaust to determine the amount of water still associated with the particulates. An algorithm can then be used to determine the actual particle size.
As the type of particulate matter varies with location this needs to be calibrated vs a reference system for optimised accuracy. Is this possible to achieve? Or is the community happy with the reduced confidence in the high humidity results? I am considering lobbying my local council to purchase a portable reference system but implementing the Laquai changes would mean a diversion away from the sensor community preferred set up. What do you suggest as the best way forward?
