Hey everyone, I need some experience for a unexpected problem on the mq3 sensor. I planned to use it for simple "you're sober"/"you're drunk" project, and my first 2 prototype worked really well because both used funnel-shaped hole to place mouth and breath, making peoples instinctively breath "hot" (like when you try to warm you hand in winter).
The project worked, but it was not really hygienic and drunk peoples could put liquid alcohol in the funnel if they wanted (no one did fortunately).
So for this v3, I wanted to use disposable paper straw to blow into the sensor, but the problem is the following :
Instead of going a little up if sober and through the roof if drunk, the "cold" breath coming from the straw cooled the sensor, making it more resistant and the value drop instead of going up. Worse, once the cooling end, the re-heating make it go much higher than the expected value.
I would like to know what's your idea to overcome this problem before buying heating element and using high-voltage to try "heating" the "cold" breath before it touch the sensor.
I've already tried placing the straw farther or place it so that the air reaches the sensor indirectly (reducing it's speed). I thought of using a kind of grid like for the mic but I'm not sure it would work.
Finally the software solution is an idea, but because the sensor "re-heat" and go to high I'm scared of having too much false positive.
(I'm sorry if it's hard to understand or answer, It's not really an electronic problem, more an engineering one but I'm stuck and I hope some peoples already solved similar problem)
Edit : My project is entirely automatic, I can't be part of the process
So it's not automatic, i can't let the device unattended ^^'
Sorry if you misunderstood, I'll edit my post !
You want disposable spirometry tubes. Here's a dealer in Canada, where I live.
Looking at the sensor documentation, It requires constant heating while in use. Is this what you are doing?
Of course, like i said the mq3 sensor is working. My only problem is the sensor cooling because of the cool air when using a straw
Just so you know, that has nothing to do with detecting alcohol on someone's breath. Like, literally zero difference.
As @Paul_KD7HB I think is alluding to (but I don't speak for Paul, just a hunch), if the temp of someone's exhalations is changing the readings on that sensor in an unanticipated way, there's something wrong with the circuit (or your sensor, or both).
Did you over expose, ie, saturate the sensor with isopropyl alcohol vapors at any point in testing? Don't if you didn't because if you did, you probably ruined the sensor. The problem with alcohol is that it has high vapor pressure and it's easy to overheat these tin dioxide sensor filaments that way.
Another common error with these is ignoring the sensor burn-in time, which is 48 hours, continuous.
This page is a pretty good starting point in case you missed anything:
It's not that, I burned these sensors 72h and i didn't saturate with alcohol, I've only drink or blow vapors from distance.
When reading the documentation, you can read than these sensors need to heat to become semi-conductor (lower resistance mean higher analog value) then there is a small burnin time after short time without use which bring the sensor to a stable value (no alcohol in air). The problem is if i cool the sensor and element suddenly, the element is not a great semi conductor anymore, thus the resistance increase and the analog value drop, even if there is acid acetic due to alcohol presence.
Of course once the air blowing end, the sensor can heat again to the appropriate temperature, transforming alcohol into acid acetic etc
it's interesting, it's like straw but larger ?
Like slightly smaller than a toilet paper tube, 10x as sanitary and built for purpose. Spirometry is the study of pulmonary function. They are designed to be blown into to measure breath output in the medical field.
I'm a firefighter by profession and we have to blow into these things as hard and as long as we can (stop it 
) until we see stars every year (seriously, stop it! 
) as part of our medical exam.
