Gas leaks and the explosive range. Episode thirty-five of the Gas Service Animated series.

A leak doesn't have to be breathed to kill — it burns. Natural gas becomes explosive when it reaches just five percent of the air in a room, and stays explosive up to fifteen percent. Between those two numbers, all it needs is a spark. That's why a gas leak is never a wait-and-see call.

Every fuel gas has a flammable range: a lower explosive limit, where the mixture first becomes ignitable, and an upper explosive limit, where it's momentarily too rich to burn. For natural gas the range runs from about five to fifteen percent gas in air. For propane it's narrower and lower — about two point one to nine point five percent — so it takes far less propane to reach an ignitable mixture. And too rich to burn is never safe: as a rich pocket dilutes back down, it passes right through the range again.

The most violent mixture isn't at the edge — it's in the middle. Methane burns perfectly at about nine and a half percent, and that's where an explosion hits its peak pressure. The lower explosive limit is where the danger starts, not where it's worst.

Where you look for the gas depends on which gas it is. Natural gas is lighter than air, about six-tenths its weight, so it rises — sweep high, at ceilings, upper floors, the top of a meter room. Propane is about one and a half times heavier than air, so it sinks and pools low — in basements, crawlspaces, pits, and floor drains, where it can sit undetected far from the leak. Same instrument, opposite direction.

Autoignition — the temperature gas lights with no spark — is high: about a thousand degrees for natural gas. But that number is a trap, because you'll never need it. Any ordinary ignition source lights a mixture that's already in range at room temperature: a light switch, a pilot, static, a doorbell, a phone. In a space that smells of gas, you don't create sparks — you make it safe first.

Gas is naturally colorless and odorless. The rotten-egg smell is an added odorant — usually a sulfur compound called mercaptan. Federal law requires it be strong enough to smell at one-fifth of the lower explosive limit — for natural gas, about one percent in air — giving a margin of warning before the mixture can burn.

But the nose is not an instrument. Odorant fades — it's absorbed and oxidized onto new pipe, onto rust, and as gas travels through soil, so gas that reaches you can arrive odorless. Your sense of smell also goes numb within minutes of steady exposure, other smells mask it, and some people can't smell it at all. Odor gets your attention. A calibrated meter confirms the leak.

So a gas leak is a life-safety emergency for one reason: an invisible, sometimes odorless, cloud is sitting somewhere between five and fifteen percent, waiting for a spark. The rest of this season is how you find it, grade it, and make it safe — starting with how you read it.

The explosive range in one breath. Natural gas: five to fifteen percent, rises, sweep high. Propane: two to nine and a half, sinks, sweep low. The middle of the range is the most dangerous. Any spark will do — so remove them. And never trust your nose over a meter. Next: the three scales a meter reads.