Three scales and the meters. Episode thirty-six of the Gas Service Animated series.

A combustible-gas reading can be shown three ways, and mixing them up is dangerous. Parts per million for pinpointing a trace leak. Percent of the lower explosive limit for judging explosion risk. And percent gas by volume when the gas is thick. Three numbers, one atmosphere — know which one you're looking at.

It all hangs on one anchor: for natural gas, the lower explosive limit is five percent by volume. So one hundred percent of the L-E-L equals five percent gas, which equals fifty thousand parts per million. Everything else is just arithmetic off that anchor.

Work it down. One percent of the L-E-L is one five-hundredth of that — about five hundred parts per million of methane. So a meter alarming at ten percent L-E-L is sensing about five thousand parts per million — half a percent gas. The general bridge for any gas: parts per million equals the L-E-L reading, times the gas's lower limit in percent, times one hundred. Thirty-five percent L-E-L on a gas whose limit is two percent is seven thousand parts per million.

You never wait for the explosive concentration. Industry practice acts at ten percent of the lower explosive limit — a tenth of the way to ignitable. Ten percent L-E-L is a stop-and-assess line, not a maybe.

The workhorse is the combustible gas indicator — a catalytic bead that burns a tiny sample and reports percent L-E-L. Because it burns the sample, it needs oxygen: in a very rich or oxygen-starved pocket it can read falsely low, or peak and roll backward — a dangerous quirk to know. It can be poisoned by silicones and sulfur, and it's calibrated to one gas, so it mis-reads another unless you apply a response factor.

Other tools fit other jobs. Flame ionization reads down to parts per billion — the walking-survey tool for mains and services. Thermal conductivity handles high gas, above the explosive limit, where a catalytic bead can't. Infrared reads methane without needing oxygen and can't be poisoned. And solid-state sniffers are cheap and sensitive but not selective. Pick the sensor for the atmosphere.

A meter you don't trust is worse than none. Bump-test it before each day's use — a quick squirt of gas to prove it responds and alarms. Calibrate it fully every three to six months, and immediately after any drop, big exposure, or a reading you doubt. Out of cal, out of service.

One line that saves lives: a combustible gas indicator is not a carbon monoxide tool. It measures explosion risk in percent L-E-L, not poison in parts per million. Carbon monoxide harms at levels far below anything a C-G-I will even notice. Flammable gas, C-G-I. Carbon monoxide, an electrochemical C-O meter. Never trade one for the other.

Three scales: ppm to pinpoint, percent L-E-L to judge risk, percent volume when it's thick. Anchor on five percent equals one hundred percent L-E-L equals fifty thousand ppm. Act at ten percent L-E-L. Trust a bumped, calibrated meter — and never a C-G-I for C-O. Next: finding the leak.