In Which Vapes May Yet Be Sensed

I’m coming to the “being a narc” thing a little late, since everyone is now having a panic about cutting agents in vapes killing people (Body count stands at ~15. The cops killed about 678 people so far this year, so y’know, keep vaping and avoid cops, it’s 45 times safer). At any rate, a school in my area was sold fantastically expensive devices that monitor areas for vaping, and report to the school administration when someone does it. The devices are on a subscription model, so they’re not just expensive once, they stay expensive.

This got me wondering what it actually takes to detect vape… vapor. Vape juice is mostly propylene glycol and glycerine, plus a dab of flavor and possibly nicotine. I have two theories about ways to detect this. One is that there will be some material produced by vaping that is detectable with a gas sensor. The other way is that phat clouds can be picked up by a particulate sensor, and the fact that it picks up smoke too is just fine, since the kids aren’t supposed to be smoking in the bathroom either.

MQ-2Combustable gasses, LPG, propane, hydrogen, probably also methane
MQ-8Just hydrogen
MQ-4Methane and natural gas
MQ-6LPG, iso-butane, propane
MQ-5LPG, natural gas, “town gas”
MQ-135Carbon monoxide, carbon dioxide, ammonia, nitrogen oxide, alcohols, “aromatic compounds”, “sulfide”, and smoke
MQ-9Carbon monoxide, LPG
CMJU-1100VOCs, toluene, formaldehyde, benzene, and so on

There’s a lot of overlap in these sensors, as well as a lot of ambiguity in their datasheets. “Sulfide” isn’t a thing without whatever it’s a sulfide of. Hydrogen sulfide is a toxic and terrible-smelling gas. Cadmium sulfide is a bright yellow solid. “Town gas” is typically a mix of carbon monoxide and hydrogen. LPG is also a mix, including propane, butane, and isobutane, so having it in a list with any of those is kind of redundant.

I haven’t tested yet, but I suspect that the sensors most likely to detect sick clouds are the MQ-3, the MQ-135, and maaaaybe the CMJU-1100. The MQ-3 claims to detect alcohol, as in a breathalyzer, but the family of alcohols is actually pretty large. There’s ethyl (drinkin’ alcohol) and isopropyl (cleanin’ alcohol) and methyl (killin’ alcohol), in addition to some stuff people don’t typically think of as alcohols, like the sugar alcohols, which includes glycerine. Since glycerine is in vape juice, perhaps the sensor will detect it.

The actual mechanism of these sensors is interesting. They appear to the circuit as a resistor that changes resistance in the presence of a gas. The resistor is made of tin dioxide, which has a resistance that drops when exposed to the gasses, but it only responds quickly if the device is hot, so there is also a built-in heater for the sensors.

Because the sensors have little heaters in them, A) they smell weird when they power up for the first time and B) they take a while to stabilize. I powered them up and watched them on the Arduino serial plotter until the outputs more or less leveled out. Then I vaped at them.

Pretty much all of the sensors had some sort of response, but the thing that seemed to vary between them was that some responded faster than others. The next step is going to be logging which ones are outputting what, so I can tell which ones had the fast response and which ones had the strongest response.

The sensors also have a slight response when I just blow at them with normal breath (I have a control! I’m a scientist, doin’ a science!). Interestingly, for some of the sensors, the reaction to my normal breath was a deviation downwards/towards a lower value, while the vape reactions was uniformly a deviation upwards/towards a higher value. This suggests that including one of the sensors that indicates vaping by the sign of its change would serve as a check, since only vaping would cause both sensors to go up, and normal breath would cause one to go up and one to go down.

int readings[9] = {0,0,0,0,0,0,0,0,0};
int index = 0;

void setup() {

void loop() {
  //Read all the analog inputs from A0-A8 (I think they're consecutive...)
  index = 0;
  for(int ii = A0; ii <= A8; ii++ ){
    readings[index] = analogRead(ii);

  for(int jj = 0; jj < index-1; jj++){