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Air Quality Shorts: Part 4 - Particulate Sensors

From BBQs to Workshops - Particulate Sensors Can Detect Truly Tiny Specks.

Humans are pretty good at worrying about things that they can *see* or ‘sense’ will kill them, from our cave-dwelling days running from Sabre-toothed Tigers to trying not to be one of the 1.35m people a year sadly killed by road accidents. However, the invisible killer that is Air Pollution (both indoor and outdoor) comes to 7 million per year, according to the WHO.

As much as this is a rather shocking premise to start a blog with on Particulate Sensors - as an Engineer and Technologist, I find it’s worth considering why the sudden interest in a given OEM product occurs in the first place. (Case in point CO2 sensors ‘spiking’ with the rise of Coronavirus, in the previous Blog). Particulates have seen a surge of interest in monitoring not just dust on our highways, but also in regard to an increase in allergies and conditions like Asthma.

When I was growing up, my father was a Builder, and sadly was likely exposed to a lot of fine dust and other particulates, which sadly may well have repercussions later in his life. Although when I was a kid, my Dad was keen to have me avoid his lack of awareness that Asbestos was bad, yet in my time also, I didn’t know MDF was bad, and likely made a good share of projects in school woodwork class without a PPF3 mask, or any awareness of the hazard in the first place. It seems we have a ‘generational wake-up call’ to a given material or another, so the more we can use science to help with prevention, rather than cure, the better.

Table explaining particulate size

Particulate Diagram, from Nature. (LINK).

As much as I find it highly irritating for people to cry for the ‘good old days’ and ‘it never did me any harm’, (for the record, my Dad is not one of them!). It's likely we will all live longer than our parent’s generation, to see the detrimental effects of particulates becoming more pronounced in our later years, so it’s worth knowing more about them now.

On a lighter note, as with so much of the Air Quality project, I believe you can focus on the Fear, or you can explore the Fun aspects also...either road will lead to better awareness and that’s no bad thing, and like the Good Air Canary, perhaps a mix of the two gives a laugh, and then some pause for thought and reflection, without being too preachy! (At least that was my intention!). If you especially enjoy this approach of ‘sugar for the medicine/facts to go down’, you may also like The IgNobel Awards! (Link).

Jude holding the SPS30 board

The Sensirion SPS30 is a very compact particulate sensor. There are many others like it, but this one is especially compact, and upon doing a teardown, you have to hand it to Sensirion, it looks amazing inside, with well-set laser diodes, mirror polished injection moulding, meticulously housed PCB, microfibre filters, and a top-notch SUNON fan to top it off. It’s a tour de force in OEM design.

Jude with the SPS30 Sensor Module

Safe to say, I go through the specs in the video, and how this measures the particulate grades PM10, 4, 2.5 and 1. It should be noted the definition of ‘ultrafine’ particulate is <0.1um, so the sensor has groups or ‘bins’ of bands of particulates - meaning it can detect the nearest size group of a given particle.

Best practice guide

It is tempting to think that MDF dust occupies an ‘exact’ particulate size of 0.3um, yet in reality, it is a spectrum of possible particles ranging from 0.1um to a few mm. It is just that 0.3um is an especially irritating size for our bodies to deal with, as it is neither small enough to just ‘pass through’, but also not big enough to just ‘sneeze out’. If this is combined with a ‘sharp’ surface, as many inorganic Silicas are, this essentially ‘cuts’ your soft tissue and causes immune response and irritation.

Graph showing particulate size against concentration

So when your sensor is giving readings for these 4 ‘bins’, these are the aggregate of particles in the above ‘ranges’. In short, if you ware wanting to look at 0.3um particles, you need to look at the PM1 ‘bin’, but respect that this is detecting 0.3-1.0um, and it could all be at 0.9-1.0um potentially, so some caution and follow-up analysis is needed. However, if you are wanting to get a ‘broadband’ understanding of fine to coarse dust in your workshop, this is certainly a good starting point for the price!

How Sensors Work

The sensor works by shining a laser at a carefully managed flow of air (with the particles inside it), and this light is scattered in front of an IR sensor, which converts these impulses of light into specific readings related to said particle groupings.

Sensirion ‘stress test’ image of a lot of fouling in the module

Lastly, I love that Sensirion posted this ‘stress test’ image of a lot of fouling in the module, but with a *clean sensor* shill showing. This is because they have a patented means of creating a ‘protective layer’ of air flowing over the sensor, which is coming from the pre-filtered region (the green cover has microfibre under it), and this prevents it from fouling - and is why the unit is valid for 8 years in the field. This is not to say pre-filtering of large debris like hairs is not advisable, but it is impressive to see this has been designed for the real world!

More on the SPS30 can be found here: (Link).

Although - hot off the press - at the time of writing I note that Sensirion have created a Particulate, Temp & RH%, and VOC all-in-one sensor, the SEN55: (Link).

Thanks for watching, do let us know what sensors you are working with, and if you would like to see any other additions to the ESDK!

The Air Quality Shorts Series.

The following series will give you an overview of sensors of interest. Right now we have:

But stay tuned, as we also have more sensors in development! So follow this Article for updates...

Winner of the 2020 Alastair Graham-Bryce "Imagineering" Award (IMechE), Jude thrives in high risk collaborations, uncertainty and pressure - drawing from global networks and experiences to deliver high profile campaigns and digital/physical products. A leading Creative Technologist & Physical Prototyping Expert, Jude has worked for NHS, Dyson, LEGO, and a number of start-ups. He is one of the eight featured inventors in BBC Two's Big Life Fix.