Question: I would like to build a electronic flatulence (fart) detector. I was thinking of methane because detectors are readily available, but I read <http://en.wikipedia.org/wiki/Flatulence> and it says:

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> However, not all humans produce flatus that contains methane. For example, in one study of the feces of nine adults, only five of the samples contained archaea capable of producing methane.
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Oxygen, nitrogen, carbon dioxide are listed but I think they would be too common in normal air. That seems to leave:

* Hydrogen
* Hydrogen sulfide
* Methyl mercaptan
* Dimethyl sulfide
* Dimethyl trisulfide

Does anyone know if practical sensors are available that detect those gases or have other ideas? I think somewhere around the $20 or less mark would be good, so I wasn't really seeking a full professional solution like gas chromatography that may normally be used.

The application is for under office type chairs, so maybe heat detection could be used although I'm not sure it would be posible to tell the difference between the desired event and someone just sitting down on a cold chair, although maybe a pressure sensor could be used along with some filtering to not trigger until the temperature had stabilized a bit.

Answer: 
It looks like Hydrogen is the major component: [Normal Flatus](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1378885/). 360mL perday. How much per fart will take some closer reading. 

Here is an Arduino flamable gas detector, it probably can sense Hydrogen: 
[LM393 MQ-9](http://dx.com/p/lm393-mq-9-flammable-gas-detection-sensor-module-for-arduino-red-black-151069), say, at 10ppm. (Some shopping legwork for a Hydrogen leak detector or flammable gas sensor is in order.) So a 36mL bolus of Hydrogen (I just guessed what volumes are emitted throughout the day to make up that 360ml, and guessed 1/10 of the total) must diffuse into a volume of 3600 Liters before it is below detection level of 10ppm. Your 10ppm sensor must be within about 100 centimeters. Looks like the under-the-seat location is the right spot.
Weird project. Chairs that detect farts? No thanks.

Anyway I would suggest you look at an off-the-shelf propane sensor. Propane (C3H8) and methane (CH4) are very similar. In fact many of them are described as Propane Methane sensors. They are cheap and made in the thousands for RV's and Boats. A friend of mine always said if he was too close the the sensor the alarm would go off.