Enviro+ / Ohms to PPM

Hi there,

have my Enviro+ up and running and set up a custom .py to read values and send them to MQTT. From there on I further use the values in openHAB (Smarthome).

My question is about the Enviro+’ MICS6814 sensor. Right now it reads…

  • 566 kOhm redu
  • 25k kOhm oxid
  • 369 kOhm NH3

Is there a chance to convert these resistances to PPM? I’ve stumbled upon a data-sheet (https://www.mouser.fi/datasheet/2/18/1143_Datasheet-MiCS-6814-rev-8-1144828.pdf) where there are diagrams for RS/R0 over PPM, but the RED sensor diagram only reaches to 10.00 kOhm on Y-axis.

Many thanks in advance!

The gas sensors seem to be totally dependent on air temperature, pressure and humidity so I can’t see how anything accurate can be read from them. I have two boards. On in my garage near Manchester current reads as follows:

This is in my lounge in Tenerife:

Ignore the flat section where my RPI crashed.

Hi @nophead, thanks for your reply.

I don’t have any issues with sensor data, just wanted to know if someone here managed to convert the Ohm’s to PPM.

Have a nice & KR!

@nophead what did you use to graph the data from the Enviro+?

It is code I wrote myself and published here: https://github.com/nophead/EnviroPlusWeb

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Nice! It will be this week’s project to see if I can make it play with luftdatden.py
Thanks.

I’ve managed to convert the ohms readings from the gas sensors to ppm using the following algorithms.

red_in_ppm = math.pow(10, -1.25 * math.log10(red_rs/red_r0) + 0.64)

oxi_in_ppm = math.pow(10, math.log10(oxi_rs/oxi_r0) - 0.8129)

nh3_in_ppm = math.pow(10, -1.8 * math.log10(nh3_rs/nh3_r0) - 0.163)

Where each respective rs/r0 is the current reading in ohms divided by the baseline air reading in ohms. They therefore require an r0 calibration measurement to be made upon startup and after a 100 minute warmup period.
They’re embedded in the code here: https://github.com/roscoe81/enviro-monitor

That code also includes some temperature, humidity and air pressure compensation of the gas sensors.

Hope that’s of value to others on this thread.