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VOC Sensors (Often MOX Construction) - What They Can and Cannot Detect.
Volatile Organic Compounds (VOCs) are perhaps more commonly relatable as smelly substances which are smaller than dust, and would include things like perfume, or if you smell a permanent marker. These sensors can be useful in the exploration of ‘general’ air quality sensing, though it should be stated, they are detecting a ‘broadband’ reading of VOCs. That is to say, if you say took the cap off a sharpie pen, opened a bottle of whiskey, and had just cut the cheese, the sensor would not be able to give discrete readings for these three things (let alone that they all contain more than one compound anyway). However, it can nonetheless say that you have a lot of a given type of compound in the room - and for certain applications, this is enough to have an ‘early warning’ or ‘proxy’ for either a response (turn on ventilation / purification of the air), or to fire up a more specialised analysis tool.
Given the tiny size and low cost of these devices, and how they often are used in creating an Air Quality Index (more on that later), it is often advisable to add one to your project, even if you take the reading with a ’pinch of salt’.
These were used in the ESDK - and are positioned next to the SHT40 Sensor, which measures Temperature and Relative Humidity (RH%).
As mentioned, the sensor is tiny, but nonetheless benefits from being shielded from sunlight and too much draught.
Sensirion also suggests a ‘Sanity Check’ where you can ‘benchmark’ a reading against a known solvent of VOC source, and calibrate readings from this. In many cases, you may not need to do this, if you are just wanting a ‘proxy’ or ‘early warning’ that the air is stuffy/poor, but the datasheets give some info. on how you might go about this.
As mentioned, the VOC sensor is looking at VOCs, and indeed at the time of making the ESDK, the SGP40 sensor was just exploring Organic compounds which - generally speaking - were ‘reducing gases’ - and this exchange of electrons create a corresponding change in resistance in the sensor’s Metal OXide layer.
However, after further development Sensirion released the SGP41 sensor, which has the capability to essentially ‘go both ways’, and it can essentially do the ‘inverse’ with ‘oxidising gases’. The most notable of these is NO2 or NOx compounds which are of particular interest in outdoor pollution from combustion of fossil fuels, e.g. Cars and Factory smog.
Sensirion gives some more details on the differences between the oxidising gases, (which are themselves reduced), and reducing gases (which are themselves oxidised) - and if you are not a Chemist, you’ll likely be lost with this (as I was as an undergraduate chemistry student learning about REDOX reactions), but fear not - you can simply google “is my [insert compound of interest] a reducing or oxidising agent” and you’ll likely get the answer in seconds.
Sensirion does give caveats to being able to give concentration levels of NO2/NOx in the field, but the MOX sensor is still a highly effective detector of changes in compounds which oxidise or reduce, and are likely an indicator of something smelly and volatile.
The Air Quality Shorts Series.
The following series will give you an overview of sensors of interest. Right now we have:
- Introduction & Readings
- Temp and Humidity (Heat and Moisture)
- VOC (smelly things) - This article
- CO2 (Combustion/Respiration)
- Particulate (dusty things)
But stay tuned, as we also have more sensors in development! So follow this Article for updates...