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:

> 
> 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: 
Methane was used by another guy used in his office chair to detect his own flatus, but if that source is real, Futurlec seems to offer [dozens of gas sensors](http://www.futurlec.com/Gas_Sensors.shtml) for whatever gas you like.

As an example: [The Twittering Office Chair](http://www.instructables.com/id/The-Twittering-Office-Chair/) and [it's Twitter account](https://twitter.com/officechair) (apparently died of asphyxiation back in '09).
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.