When working from home I was curious about the air quality of the environment I was putting myself through for many hours of the day. I know the room I work in is relatively small so I thought about low oxygen potentially messing with my thought process.

The build was relatively simple. I wanted to use an older ESP8266 NodeMCU V2 I had spare and in retrospect think it was a great choice, I decided to try Home Assistant and ESPBuilder because the process seemed easy and it was.

Once I flashed ESPHome from my PC to the ESP and added it to Home Assistant it was fairly simple overall to iterate and test my design. I didn’t have home assistant either but setting everything up took a little over an afternoon.

Hardware selection was fairly simple, I just looked what had good support in ESPHome and went with the SCD41 from Aliexpress.

For the sensing hardware I had a DHT22 sensor for Temperature and Humidity, which the SCD41 also supports. I was primarily interested in CO2 levels but I thought I may as well throw it into the mix to measure the performance of the DHT22.

The Results

Once I had tweaked with the temperature and humidity calibration settings I created a dashboard in home assistant. Again this was something I’d not done before and it was very easy. I have really been pleasantly surprised with how everything has integrated well.

I created a 7 day and 24 hour view,

It became clear to me a number of things,

• CO2 levels regularly rise above 1000ppm in the room I work in when not ventilated, and they rise a lot quicker than I expected. I thought it would take all day to really get to what I consider a harmful level, but actually its more like an hour. After research online 1000ppm is what I would consider too high. It does seem like naturally detect these events and ventilate the room by instinct but its useful insight to see that.
• Ventilating the room equally drops the readings much more rapidly than expected, and leaving the window or door open is enough to bring the levels back to ~650ppm/400ppm relatively quickly.
• High CO2 levels seem more noticeable to me now, previously I had no perception or concept of when it might happen but I feel its much more noticeable now I know what it feels like, if that makes sense.
• I would like to log the data for a more long term view. Say a year or more. I think it would interesting to see if this affects humidity, temperature etc.
• Overall this was easy to setup and provided valuable insight I think.

Here is the 7 day view for those interested, I added the light switch being on or off state at the top of the graph too.

And here is the humidity readings, It was interesting seeing spikes on the graphs, I can easily correlate these with opening doors, having the heating on etc.

Try this yourself

Here is the final version of the YAML I applied to my ESP. I did some testing with sliding window averages but I actually found it was easier to just ignore the first few readings off the sensors to reduce the noise in the graphs.

esphome:
  name: tco2s
  friendly_name: tco2s

esp8266:
  board: nodemcuv2

# Enable logging
logger:

# Enable Home Assistant API
api:
  encryption:
    key: "WWVhaCB0aGlzIGFpbnQgcmVhbA=="

ota:
  - platform: esphome
    password: "SSBjaGFuZ2VkIHRoaXMgbG9s"

mdns:
  disabled: true

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

  manual_ip:
    static_ip: 192.168.4.22
    gateway: 192.168.4.1
    subnet: 255.255.255.0

  # Enable fallback hotspot (captive portal) in case wifi connection fails
  ap:
    ssid: "Tco2S Fallback Hotspot"
    password: "U2xlZXB5"

captive_portal:


i2c:
  - id: bus_a
    sda: D4
    scl: D5
    scan: true

# DHT22 Sensor
sensor:
  - platform: dht
    pin: D2
    model: DHT22
    temperature:
      name: "DHT22 Temperature"
      filters:
          - skip_initial: 3
    humidity:
      name: "DHT22 Humidity"
      filters:
          - offset: -20.0
          - skip_initial: 3
    update_interval: 60s
  - platform: scd4x
    co2:
      name: "SCD41 CO2"
      filters:
          - skip_initial: 3
    temperature:
      name: "SCD41 Temperature"
      filters:
          - skip_initial: 3
    humidity:
      name: "SCD41 Humidity"
      filters:
          - skip_initial: 3

I think based on the data I will improve the ventilation to see if it has any effect on my overall fatigue and concentration.

Thank you for reading.