Enviro+ Oxide Level slow climb

Back to the oxide sensor topic I now have full day of results without any interruptions.

The huge peak that is well over 100K is from cooking four rhubarb and ginger cakes in a gas oven.

One curious pattern I have noticed is all three gas sensors drift up over night and then fall in the morning when the central heating comes on for about 1/2 an hour. While I know temperature affects them it should affect the oxide sensor in the opposite direction. I.e. when it is cold its resistance gets lower. This is apparent when the program is stopped for a short time, oxide drops and the other two rise.

The other things that are changing is relative humidity and light. The gas sensors seem very sensitive to relative humidity but the two step response in the morning seems to indicate a strange combination of temperature and humidity.

The other odd thing is I can see why the temperature rise causes a fall in relative humidity but why does it rise again shortly afterwards? And why does it fall over night when temperature is actually falling. Is it in fact my presence in the room that is raising the humidity when I arrive with my morning coffee?

It is had to separate real physical changes from senor instability!

I’m seeing odd behaviours with the gas sensors too.
First, my outdoor system has been running since June - initially feeding Luftdaten, but no longer doing that since they banned me because the Pimoroni software was sending data too often. Here’s the plot for the last week

Note that I’ve changed some of the axis values to accommodate the very high readings for ammonia and oxidising - especially with the outdoor one.

There’s some variation throughout the day, but the gas results are very odd to say the least!

And here’s the indoor one, as I’m only allowed one image per post.

Yes I think the dramatic spikes in the gas readings are just a reflection of the temperature or humidity or both. It is hard to tell because temperature and relative humidity are so closely related.

I think there needs to be some compensation. It would also be interesting to display absolute humidity rather than relative.

My oxide sensor continues to climb and I will need to re-scale the axis because it goes above 100K.

I wonder if it will ever stop climbing. I probably need a years worth of data to see if it is seasonal variation of pollution or the sensor wearing out.

I have set up a second Enviro+ in the same room. Although the gas sensors show the same variations their starting values are a long way apart, particularly the oxide sensor which has started its slow climb from about 3K and only got to 40K so far.

I haven’t got a fan on the second board so its temperature and humidity are off. I placed it near an open window where there is a cold breeze this morning and the humidity goes up and down. The other two gas sensors mirror it very closely.

My next task is to make a 3D printed cases for them both and place them next to each other to see how close they match. They are not really portable at the moment.

So I took one of my Enviro+ sensors to Tenerife and was surprised to find the oxide and nh3 readings went off the scale. I am sure that there is less NO2 pollution here as I don’t have a runny nose, whereas I always do in the UK when I go outside. There is sometimes a smell of bad eggs here, so there must be some H2S.

Oxide is approaching 400K when it is supposed to be 20K max. Obviously it is much hotter here and by turning the fan off I can show that the gas sensors are greatly affected by temperature.

I also think they are affected by air pressure and humidity.

Here is a second unit I left at home in the garage:

Not sure what the odd dips in all the gas sensors are on Sat Nov 2 but pressure was very low then.

The first particle peak is the taxi that took us to the airport. The big peak in the middle was a bunch of cars leaving an event in the museum the other side of our garden fence. The peak on the right was the next door neighbour returning home. The garage is integral to the house and the door is sealed but the brick work needs a bit of pointing in one corner. Seems to let in a lot of traffic pollution!

I don’t think the gas sensors can be correlated with absolute readings because they are so affected by other things. I think they need temperature stabilisation and / or compensation for temperature. humidity and air pressure.

That was one of my conclusions… temperature has a massive impact on the readings for gas. I was getting resistance levels far out of tolerance. I started to think that maybe for the gas readings to be useful you might need to enclose the device and control environmentals. You could pass the air through it and then get better gas level readings? That was the reasoning I had for poking around the heater crawling up in temp and thereby making my gas levels climb with it.

I wonder if the heater has a significant temperature coefficient. If it does one could measure the voltage across it with the ADC channel and use PWM on the heater to regulate the temperature.

Great idea, obviously you wouldn’t want to adjust airflow since the airspeed would impact the gas readings.