We launched our indoor air quality monitor AirGradient ONE at the beginning of this year. We measure PM, CO2, TVOCs, temperature and humidity. When we started the development we would have never thought that achieving an accurate temperature measurement was by far the most difficult thing.
It took us 8 hardware iterations to get it right! We experienced with different enclsoure shapes, orientations and PCBs (component placement).
Our experience is similar to the author of this article:
Your biggest enemy is internal heat produced by active components, mainly the MCU but also by other sensors like the PM sensor and CO2 (NDIR).
How we solved it:
1) Minimize heat from energy by using components that have low energy consumption and make extensive use of sleep functions of the MCU as well as sensor components (e.g. PM sensors can be put to sleep).
2) Make use of physics. Our monitor is wall mounted and uses the convection mechanism. So nearly all components that heat up like LEDs, MCU etc. are located at the very top inside the enclosure. Optimize the vents at the top and bottom of the enclosure and create an unobstructed air flow through the enclosure based on natural convection.
3) Plan for enough space. To get temperature right, you need a lot of air around the temperature sensor that is free from radiation from other heat sources. So we specifically designed the PCB to have the temperature sensor at a 'finger' at the bottom with a lot of space to other components and ensured there is a good air flow around.
If you are interested to see our design, have a look at our open-source, open-hardware DIY Pro monitor [1].
We integrated a lot of the things we learned designing the commercial monitor mentioned above into these build instructions [2] that allow you to build your own highly accurate monitor and then also be able to change the firmware and adjust to your needs. In fact, our kit contains exactly the same plastic injected enclosure we use for our commercial monitor (but you can also just 3D print the one from the build instructions and get the other parts by yourself).
In our journey to get the temperature right, we made extensive use of thermal imaging and compared our prototypes with competitors.
The images showed that one of these sensors got extremely hot during operation but the temperature was nearly identical to the real ambient temperature. We couldn't believe this.
So we made a test and put a strong fan onto this sensor. The temperature then dropped by more than two degrees Celcius BELOW ambient which showed clearly that they made a software offset.
It can be debated if such an offset is a legitimate design choice (and I personally believe it has a lot of risks) but it was something we wanted to avoid in our design.
Guilty as charged. Based on design validation testing, I ended up making the design choice to partially rely on a software offset, a strong fan blowing into the sensor not being a supported scenario.
But the placement / air flow / multiple openings definitely took us more design iterations than we had planned for.
Looks like non-professionals have easier time with this. :) I did put DHT22 into a breadboard, some atmel mcu on the other side to convert from DHT's protocol to UART and half a meter of wires to UART of Orange Pi SBC which serves as a logger/controller. Open air design, lot of space, far away from anything that heats up air.
Sensors hanging off wires may look ugly, but it has benefits.
Your product looks interesting but I'm curious why you have a "contact for pricing" for your AirGradient One, are you targeting B2B sales with that device?
I'm looking at your build kits which are pretty cool. I'd probably get the "PRO Pre-soldered" kit. Couple questions about them:
> PCB has slots for TVOC, pressure or other sensors (sensors not included)
Do you sell these sensors separately? or have recommendations on sensors you trust and work with this kit? Your AirGradient One comes with TVOC sensing built in so you definitely have something
> 24 months AirGradient data platform (or connect it to your own server, e.g. home assistant)
Can I use the device without connecting to a server?
I live in an area prone to frequent fog for many months out of the year.
I learned that the fog will thoroughly disrupt the accuracy of pm2.5 sensors.
After some research I found that running the air through a heated pitot or similar before taking a measurement will almost entirely correct this for somewhat obvious reasons; the fog is evaporated.
As far as I can tell, none of the mid to high range weather stations have such a system and they will all be corrupted by a sight fog.
I’ve resigned myself to making one… Does such a sensor even exist?
We launched our indoor air quality monitor AirGradient ONE at the beginning of this year. We measure PM, CO2, TVOCs, temperature and humidity. When we started the development we would have never thought that achieving an accurate temperature measurement was by far the most difficult thing.
It took us 8 hardware iterations to get it right! We experienced with different enclsoure shapes, orientations and PCBs (component placement).
Our experience is similar to the author of this article:
Your biggest enemy is internal heat produced by active components, mainly the MCU but also by other sensors like the PM sensor and CO2 (NDIR).
How we solved it:
1) Minimize heat from energy by using components that have low energy consumption and make extensive use of sleep functions of the MCU as well as sensor components (e.g. PM sensors can be put to sleep).
2) Make use of physics. Our monitor is wall mounted and uses the convection mechanism. So nearly all components that heat up like LEDs, MCU etc. are located at the very top inside the enclosure. Optimize the vents at the top and bottom of the enclosure and create an unobstructed air flow through the enclosure based on natural convection.
3) Plan for enough space. To get temperature right, you need a lot of air around the temperature sensor that is free from radiation from other heat sources. So we specifically designed the PCB to have the temperature sensor at a 'finger' at the bottom with a lot of space to other components and ensured there is a good air flow around.
If you are interested to see our design, have a look at our open-source, open-hardware DIY Pro monitor [1].
We integrated a lot of the things we learned designing the commercial monitor mentioned above into these build instructions [2] that allow you to build your own highly accurate monitor and then also be able to change the firmware and adjust to your needs. In fact, our kit contains exactly the same plastic injected enclosure we use for our commercial monitor (but you can also just 3D print the one from the build instructions and get the other parts by yourself).
[1] https://www.airgradient.com/open-airgradient/kits/ [2] https://www.airgradient.com/open-airgradient/instructions/di...