Originally Posted by Splitrock
One year our son bought us a multi gas detector for use in our home. We proudly installed it near the bedroom door. My wife's hair spray set it off every time.
it was hair spray.
Methane ("natural gas") and propane are both odorless and have mercaptan added to give them what is usually described as a rotten egg smell. It smells like mercaptan to me, not rotten eggs.
Propane is heavier than air, methane is lighter than air.
Sewer gas comes from the breakdown of the wonderful stuff in the fluid—and produces many odors including hydrogen sulphide (H2S, a compound each of us can produce on our own). All college male freshman learn it is flammable, an experiment conducted in the dorm, not a lab. The compound breaks down fairly quickly in air.
Battery acid is sulphuric acid (H2SO4) and the gas may break down into hydrogen sulphide when in contact with air, but I don't know that for a fact. I believe when a battery is overheated, it gives off hydrogen which is explosive in the proper concentration. Perhaps some of the hydrogen binds with some of the sulphur to make H2S.
Then there is the BTEX group—butane, toluene, ethane, xylene. These are really nasty and quite poisonous. Methane is not poisonous and is sometimes found in drinking water. When gas companies drill for natural gas, sometimes people suddenly get a lot of methane in their well water and can light the water when they open the faucet. They may also get fracking fluids which are also nasty as well as the BTEX group.
This all comes from someone who had an awful time with basic chemistry and was so bad at it he didn't know what he didn't know or thought he knew things he didn't know.
At some campgrounds there is a powerful odor of sewer gas when I open the drain, at others, not so. I suspect it is a function of how well the system was installed and how well it is maintained. Some don't have threaded drains and we use rocks, boards and anything else we can come up with to create a positive seal—sometimes it works, sometimes it doesn't. Someone nearby may have a broken drain and sewer gas may be blowing your way.