Thread: Developments in Dust Sensor tech

Originally Posted by BobL

If these cheap TH sensors are exposed to stuff like finishing vapours or cooking oils these sorts of problems can arise. The sensors (DHT11) used on the detectors are very cheap (a few $$) are commonly used in Arduino projects. The DHT22 sensor is a bit more accurate and precise (and a nit more $$) and are supposed to be more rugged although I have not made a side by side comparison about how vapours affect them.

Breathing on the sensor is one way of testing them (they should indicate a change within seconds) while the other way is just to put a clean finger on them, the small amount of water vapour coming of skin and raised temperature should also indicate a change quite quickly

I have just done some work with the SHT31...seems to work okay. Waiting for the go ahead from the client then i will use it to build a dehydrator controller. Not arduino but pic

Originally Posted by BobL

I've done this to show that these detectors aren't that accurate especially at low levels and even taking multiple readings may not give very precise results. The problems come if you were to be making dust extraction decisions based on readings from these detectors. you probably need to be operating at the +/- 10% level, and to be "safer", at the 2+/-20% level,

You get what you pay for and they don't cost much so don't expect much. As far as I am concerned fiddling around with cheap stuff like this gets cheap results and nothing more and I can't see any joy in it. Just my view and I realise others here look at things differently.

Originally Posted by Chris Parks

You get what you pay for and they don't cost much so don't expect much. As far as I am concerned fiddling around with cheap stuff like this gets cheap results and nothing more and I can't see any joy in it. Just my view and I realise others here look at things differently.

There's nothing wrong with the Plantower dust sensors used in these detectors. Plantower is a reliable brand and millions of them are widely used in monitoring air quality right across the world. Plantower, or similar budget level dust sensors (Sony, Sanyo, Honeywell etc) are used in hundred and even thousand dollar detectors eg the Plantower sensors are easily as good or some might say better than the sensors use in Dylos detectors.

ALL of these small detectors suffer from low level counting problems because there are physical limitations of not having sufficient numbers of particles in a given volume of air. To get around this the only way is to use bigger sensors, bigger pumps and to count for longer. There is no way around this.

Counting for longer with a small sensor is a poor persons substitute for larger sensors but not many people in workshop are going to make some dust and then twiddle their thumbs for a couple of minutes to wait for a reading.

Even if a more expensive meter is used it will still need to count for many minutes to get a reliable result which doesn't always mean much in terms of real situations because the dust levels can go up and down faster than the detector can measure them. Dust detector that can accurately measure dust levels in real time sit on lab benches and cost 10's of thousands of dollars.

I still recommend these small detectors as general indicators and they are definitely better than nothing. One problem with ANY detectors is most people being told a certain dust level is safe (eg 50µg/m^3) will run around like headless chooks when the levels reach 51µg/m^3 and start making decisions, sometimes involving lots of money, that are usually not needed. My post was not intended to say that these devices are useless but just don't treat them as absolute. They're a bit like having an analog clock that only has the 12, 3, 6 and 9 numbers on it, you can tell the time OK but don't expect to be able to read seconds with it.

Originally Posted by BobL

There's nothing wrong with the Plantower dust sensors used in these detectors. Plantower is a reliable brand and millions of them are widely used in monitoring air quality right across the world. Plantower, or similar budget level dust sensors (Sony, Sanyo, Honeywell etc) are used in hundred and even thousand dollar detectors eg the Plantower sensors are easily as good or some might say better than the sensors use in Dylos detectors.

ALL of these small detectors suffer from low level counting problems because there are physical limitations of not having sufficient numbers of particles in a given volume of air. To get around this the only way is to use bigger sensors, bigger pumps and to count for longer. There is no way around this.

Counting for longer with a small sensor is a poor persons substitute for larger sensors but not many people in workshop are going to make some dust and then twiddle their thumbs for a couple of minutes to wait for a reading.

Even if a more expensive meter is used it will still need to count for many minutes to get a reliable result which doesn't always mean much in terms of real situations because the dust levels can go up and down faster than the detector can measure them. Dust detector that can accurately measure dust levels in real time sit on lab benches and cost 10's of thousands of dollars.

I still recommend these small detectors as general indicators and they are definitely better than nothing. One problem with ANY detectors is most people being told a certain dust level is safe (eg 50µg/m^3) will run around like headless chooks when the levels reach 51µg/m^3 and start making decisions, sometimes involving lots of money, that are usually not needed. My post was not intended to say that these devices are useless but just don't treat them as absolute. They're a bit like having an analog clock that only has the 12, 3, 6 and 9 numbers on it, you can tell the time OK but don't expect to be able to read seconds with it.

As I said, you get what you pay for.

Originally Posted by Chris Parks

As I said, you get what you pay for.

I wouldn't be so dismissive.

The original and still by far the majority of users of these budget level dust detectors is for use in urban areas with high (50+ µg/m^3) dust levels, places like Beijing and Delhi. The intended use is to determine whether people should do things like let children and grandparents play outside or stay inside, or go for a walk or run or not. Many of these budget level meters have colour coded displays where the numbers change colours depending on the levels and these colour changes are plenty accurate enough to make those sorts of decision.

The mistake I see some people make is they try to use these detectors to within +/- 1ug/m^3. It's complete nonsense to use them to assess small changes to wood work dust extraction systems or processes. Even using a $10k meter would not help at this level without research lab grade procedures and setups. It's not nesseraily the price of the meter but how it is used. It might be better if these detectors displayed low dust levels in 10's or 20 of µg/m^3, eg 0 - 20 - 40 - 60 -80 etc. and some of the first ones that came out did do this. As I said above it might make more sense to ignore the numbers and use the changes in display colours to make decisions.

Thanks for acknowledging the short comings these cheap dust monitors have and their limitations.

Originally Posted by BobL

I still recommend these small detectors as general indicators and they are definitely better than nothing. One problem with ANY detectors is most people being told a certain dust level is safe (eg 50µg/m^3) will run around like headless chooks when the levels reach 51µg/m^3 and start making decisions, sometimes involving lots of money, that are usually not needed. My post was not intended to say that these devices are useless but just don't treat them as absolute. They're a bit like having an analog clock that only has the 12, 3, 6 and 9 numbers on it, you can tell the time OK but don't expect to be able to read seconds with it.

So, in summary, "they are definitely better than nothing"

Definitely better than when I only had my chronic lungs to tell me that I had received too much exposure, but only after the fact and most acutely about 4am the following morning!

What I have become more confident in is that they indicate quickly enough for my purposes in which direction the dust levels are going. Any lag or accuracy in the readings is not as important as the direction; ie, going up, staying up or going down.

For example, if I see the levels drifting up while sanding a larger bowl at the lathe I know that I don't have the bell mouth in the optimum position and can then adjust it so that the dust levels start to drop and the counter returns back to Ø, where it mostly sits while my DE is running.

I have no idea what that Ø reading means in terms of real dust levels... all I know is that is as low as I'm going to get with what I have and that my workshop is definitely running cleaner than the air quality outside the workshop. My lungs have also confirmed this.

There is a difference between the recommended long term dust exposure levels for otherwise healthy people and those with already compromised lungs. I wouldn't run around like a headless chook when the dust levels got up to 51µg/m^3, I'd have evacuated the workshop well before that!

Well it looks like I didn't look hard enough before committing words to the forum because the two detectors displays now agree.

What happened was I remembered that back in December my detector was not reading correctly - it was showing dust levels that were clearly too low given there was visible dust in the air. On inspecting the sensor I noticed fine threads of dust across the fan air inlets and outlets. I cleaned these off and the reading returned to agreed with a two of my other detectors.

So I decided to have another look at the fan air inlets and outlets and this is what I found.
Dustdetectordust.jpg

Those pesky dust threads are back!
What happens is if the dust threads are not completely cleaned they grow back much quicker than usual.
This detector spends most of it time in the kitchen so it could be that some oil vapour deposit onto the threADS making them sticky which causes more dust to stick to stick to the threads.
The small amount of ammonia in human breath can do the same thing. In offices the grey goo dust threads/flakes that grow on computer fan vents and ceiling fan vents are a mix of ammonia, skin cells, carpet and fabric dust, and dust brought in on clothing and shoes.
Or there's a teeny weeny spider living in that sensor

So I cleaned the threads off with alcohol and a cotton bud and gently blew compressed air across the outlets to help flush out any loose threads.

Then took an hours worth of readings, fortunately there is "burning off" going on somewhere in the Perth Hills so there plenty of dust around to measure.
Screen Shot 2022-04-18 at 12.08.37 pm.png

FWIW the average for detector A is 34.25, average for B is 34.17 µg/m^3, so there's nothing in it.

Lesson learned - check the inlets and outlets for dust threads, or teeny tiny spiders.

Sensirion have recently released a new air environment sensor model with a VOC sensor added - https://sensirion.com/media/document...heet_SEN5x.pdf

I have used a few of the SPS30 sensors without issue. Curious to know if the extra data from a VOC sensor would add any extra insight in a WW environment.

Originally Posted by pintek

Sensirion have recently released a new air environment sensor model with a VOC sensor added - https://sensirion.com/media/document...heet_SEN5x.pdf

I have used a few of the SPS30 sensors without issue. Curious to know if the extra data from a VOC sensor would add any extra insight in a WW environment.

They do look interesting what sort of $ are involved?

The >10Year Mean life hime between failures is perhaps a tad optimistic, maybe in Clean lab or similar, or the west coats of Tassie, but I doubt they will survive that long in a wood workshop. Professional level detectors require calibration at least annually but I guess they are in a different league.

I've have 5 Plantower sensors in service since 2017. The oldest is 5 years old and the youngest is 3 years old and the specified MLBFs is about 3 years but so far have only had to replace 1 sensor and that was easily identified as being left too close to a dust source - fortunately they only cost about $40 each to replace.

Using volatile finishes without adequate ventilation is an important health issue but I don't think it would add anything to the dust issue.

I use metho and kero on my metal work lathes and mill and several times I have felt light headed when using meths so I now employ one or both of my general ventilation extractors when doing this. Most organics like kero and oils have a misting/vapour OHS level equal to the old levels for hardwoods of ~5mg/m^3 which sees ludicrously high to my way of thinking given that is for an 8 hour continuous exposure. Formaldehyde is a lot lower 0.5 ug/m^3. Except for very old MDF there's not that much around that uses it these days. Methanol has a OHS limit of 100-200 ppm but I reckon it effects you well before it gets to the level., but I get woozy walking into a public bar these days so maybe its just me.

This reminds me of my daughter-in-law who is teacher and she asked me about CO2 meters to use in her classroom to assess general ventilation, so I suggested she get the same one I have. When she tried it out the thing was off the scale (>6000 ppm) and then I asked her how much hand sanitiser she was using and as he said bout a bottle a day (she teaches music). It turns out that budget meter uses a CO2 PLUS VOC sensor and generates a combined ingle digit result! The sanitiser would have acted as a double whammy - the alcohol would trigger the sensor as a VOC and then as it broke down into CO2 it would trigger the sensor again! So she's probably intoxicating the kids while she's trying to keep Covid at bay.

Originally Posted by BobL

FWIW the average for detector A is 34.25, average for B is 34.17 µg/m^3, so there's nothing in it.

Lesson learned - check the inlets and outlets for dust threads, or teeny tiny spiders.

More than close enough for my purposes!

Lesson noted, thanks. I'll check the next time I'm in my workshop.

Originally Posted by pintek

Curious to know if the extra data from a VOC sensor would add any extra insight in a WW environment.

Two-part epoxies and many glues (formaldehyde), solvent based polishes (including pine turpentine) and the solvents in paints/varnishes are all culprits. Also some woods like camphor (terpenoids). Acetone, mineral turps and range of other hydrocarbon based solvents are a few others.

One of my counters also has TVOC and HCOC (formaldehyde) sensors built in which do get quite excited when exposed to any of the above. I do also have an inbuilt sensor because most VOCs give me an instant headache, which probably relates back to when I was exposed for extended periods of time to screen printing solvents and oil painting many years ago.

So, I don't need a meter to know that I'm being exposed to VOCs, but less sensitive woodworkers may benefit from knowing their exposure levels. Although, if they are not feeling any effects and it is not for extended periods then just how useful those functions are is questionable. However, I do know from personal experience that extended exposure over time above the recommended limits does have health consequences.

OK - after all that hoo-haa I decided to drag out a couple of my Plantower 7300 sensor based detectors and run them alongside the budget level detectors.
The budget detectors use a Plantower 5300 sensor which when i have in the past compared them to a 7300, the 5300 read PM2.5 about 20% lower than the 7300.

I have a 7300 sensor based detector in the shed just above head height which keeps it out of direct sawdust streams but locating the budget detectors along side that one is tricky so I just decide to do it inside. The comparison is being made easier because there is someone burning something nearby because the AQI stations for Perth are reporting "good" air quality (2-10 µg/m^3) while the 7300 sensors were reading ~150 µg/m^3 when I got up at 5am. Its a coolish morning here and I suspect that a local may just have lit a wood fired heater - those things should be banned in suburbs.

Right now (9:30am) the 7300 sensors are reading about 62 µg/m^3. while the 5300 sensors are reading around 52µg/m^3 , again ~20% lower.
This aggressive with other testing which reports lower readings of between 10 and 20%.

What I really should be comparing them with is a pro detector but unfortunately I no longer have access to the one where I used to work.

Well I don't know what the AQI stations were reporting this morning because when I went to the shops at around 10am there was definitely smoke in the air and visibility was down to about 3 km.
I just checked again and the 2 nearest AQI stations are showing PM2.5 of 32 µg/m^3 whereas I'm still reading between 55 and 65 µg/m^3

Mind you these stations are not that near to our place, about 15 and 30 km away - there is no station anywhere near the CBD which we are about 2 km away from as the crow flies.