DIY air quality sensor (part I): sensor selection
18 November 2021
07 October 2016
DIY air quality sensor
Lots of people contact us wanting to buy sensors so they can make their own DIY air quality sensor device. Typically they are researchers who have limited funding and need a cheap way to get equipment together for a study. Sometimes they’re just people who like making things and an air quality sensor is their latest project.
Either way, this blog post is part one of a series to help you understand some of the challenges you need to overcome to make a decent DIY air quality sensor device. We’ll focus on air quality sensors for measuring outdoor (ambient) gaseous pollutants, because that’s what we know best.
This won't be easy
The reality is, measuring ambient levels of pollution in outdoor environments is extremely challenging. First, the pollutant levels can be low – often in the parts per billion (ppb) range. Then you have to account for other pollutants in the air which can interfere with your sensors. Then you have to deal with changes in temperature and humidity – over the day and throughout the year.
Finally, people are used to seeing data from instruments that cost tens of thousands, sometimes hundreds of thousands of dollars. These instruments (used by air quality agencies and approved by the US EPA etc) are laboratory grade instruments put into large air conditioned shelters. They are extremely accurate, able to measure in the ppb range, sometimes even in parts per trillion.
It’s hard for a sensor that costs under a hundred dollars to compete with that. In most communities this high quality data is made available to the public, so if you’re going to present data from your home made instrument to the community you need to know that people will always compare your results to the data that they already have. That means you need a high level of confidence in your data and need to be able to demonstrate its comparability to a known ‘reference’ standard.
Don't let that stop you
Here are a few things you need to keep in mind when deciding which sensors to use in your DIY air quality sensor.
There are many sensor technologies available: electrochemical, metal oxide, NDIR (near dispersive infrared). There are many different manufacturers. The main ones are SGX, Membrapore, Alphasense (Aeroqual makes sensors too, though these aren’t available for general sale).
The first thing to know is – not all sensors are created equal. An electrochemical sensor from one manufacture can vary significantly when compared to the electrochemical sensor of another manufacturer, especially once you get them out of the laboratory and into field conditions.
For that reason you should take time to evaluate a number of different sensors. Test them in the lab, and if that goes well, test them outdoors. The key criteria you should evaluate against are selectivity, sensitivity, and stability. (A word of warning; be wary of a sensor manufacturer’s specification sheet. These specifications are derived from lab tests, nothing like you will experience when you put the sensor out into a typical urban environment.)
Selecting a sensor
When designing an air quality device think of the 4 s’s of sensor selection.
Selective means how specific is this sensor to the particular gas you want to measure? Most gas sensors will respond to more than one gas. A highly selective sensor will respond very little to the other gases in the environment. So the question becomes, what other gases are likely to be present in the air I am measuring?
That’s where the application comes into it. For example, if you are doing a roadside study, you’ll likely get NO, NO2, and CO. If you have a sensor that is selective to CO, great. But if the sensor is also responsive to NO2 then that’s something you’ll need to think about.
Some manufacturers deal with this by giving a combined value for two pollutants which is indicative rather than an accurate measurement. For example, Cairpol’s Cairclip device gives a combined measure of O3 and NO2, because their NO2 sensor is also responsive to O3, and O3 and NO2 are often present together in urban environments.
In fact all electrochemical O3 sensors are strongly responsive to NO2 as well (and vice versa). The only way to get a selective NO2 measurement is to pair a NO2 electrochemical sensor with a selective O3 sensor like the one Aeroqual makes (and correct for the interference), or shell out $20K for a NO2 chemo-luminescence analyser.
Sensitive means how sensitive is the sensor to the target gas. A very sensitive sensor should be able to measure down to 10 ppb maybe even 1 ppb. Again this comes back to the pollutant of interest and the application.
Using our roadside example again, CO is going to be between 1 and 20 ppm. Most commercially available CO sensors should be sufficiently sensitive to deal with that. Ozone on the other hand is often in the 1-50 ppb range and there aren’t really any generally available sensors that can deal with that (Aeroqual’s ozone sensor being a notable exception).
So before you choose your sensor make sure you’ve thought about the levels of pollutant you expect to measure, and how low you need to go in order for your data to be useful.
The third criteria is stability. Stability means how stable is the performance of the sensor over time and under certain types of conditions (like changes in temperature and humidity). You will probably know that all sensors drift and that drift occurs at zero (‘baseline drift’) and throughout the measured range (‘span drift’).
The rate at which the sensor will drift has implications for your study. For example, if you are going to leave your sensor out in the field for a year then you need to be prepared for significant drift in the measurements over time. If the study is only for a few weeks then you probably don’t need to worry.
The speed of response of your sensor is a crucial consideration. Speed is often listed as the T90 of the sensor which is a fancy way of saying how quickly it takes to respond to a sudden change in pollutant concentration.
Let’s consider the roadside application again. You may wish to monitor the effects of passing traffic or buses stopping. This would require a very fast response from your sensor otherwise you will miss the ‘peaks’ from these traffic events.
The best air quality sensor is...
The very best sensor for your application is one that is selective AND sensitive AND stable AND speedy. Unfortunately very few such sensors exist, so you’ll need to make some trade-offs. Those trade-offs need to be driven by a clear understanding of your application and the objectives of your monitoring project.
You may also want to consider some mitigating measures. For example, there are ways to improve a sensor’s selectivity through mechanical methods or by using software. This is what happens when you take a simple sensor and start to design your air quality device. We’ll be writing another post soon giving some tips and tricks about some of the things to think about when designing your device so that the weaknesses of your chosen sensor are at least partly mitigated.
Of course buying an off the shelf instrument is always an option. Making your DIY air quality sensor starts out looking cheap but can end up costing much more than it’s worth. If you’re interested in evaluating one of the sensor based instruments we make, feel free to get in touch.