In a previous post some early tests were performed on one of our two Air Quality Eggs. This post will continue that process but show the two of them together.
Here’s a picture of the two Liverpool FoE Air Quality Eggs (AQEs), which I’ve named Tweededee and Tweedledum!
In this mode they are static AQEs, connected to power, monitoring local air quality and reporting NO2, CO, temp and relative humidity to the opensensors data store every 4 seconds. Below is a picture of the pair of Eggs reporting their data taken 17 November 2016 at 15:32 GMT.
This picture shows one of the challenges of devices such as this. The two Eggs are next to each other but Tweedledee is reporting an NO2 figure of 44 ppb, whereas Tweedledum is reporting 63 ppb.
Investigating this further, the photo below includes another AQE that I have had for a year or so. Again it’s in the same location and picture was taken within minutes of the photo above.
Tweedledee now reads 52 ppb, Tweedledum 74 ppb (a similar delta), whereas the other AQE on the right gives a reading of 22 ppb. The temperature, Carbon Monoxide and Relative Humidity figures are all identical or very close. It’s the NO2 numbers that have some variation.
There are various technical answers to this variation in the data. The Eggs were calibrated before they were shipped, but it’s possible that they need re-calibration, which is not a simple process at all. The key lesson for this project is that individual points from these sort of sensors should be treated with caution.
Before moving on this can be investigated a bit further. Below is a chart that shows the temperature readings for Tweedledee and Tweedledum from 12 November 2016 to 14 November 2016. As above, the sensors are indoors and next to each other. The blue line is Tweedledee and the orange line Tweedledum.
The chart shows the temperature readings slightly differing, but they definitely move in step with each other. The correlation of these data sets is 97% for those who like those sort of statistics!
The final chart in this post is for the same 2 Eggs and the same time period, but this time the data is the NO2 readings from each Egg.
The top line is Tweedledum and the lower one Tweedledee. The chart shows that Tweedledum consistently reports higher than Tweedledee. The movement of the two data sets is harder to see, but they do appear to move in step. The correlation between the two data-sets is 69%. Detailed technical transformations to analyse this data further will not be performed beyond this. The key point from these tests is that the data is potentially valuable, but individual data points should be treated with caution.