Leak investigation and the combustible gas indicator. Episode six of the Gas Service Animated series.

Natural gas has no color and no odor of its own. An odorant from the mercaptan family is added so even a small leak announces itself with a sharp sulfur smell. But smell is a warning, not a measurement. Odorant can be faint on new lines and a nose tires quickly, so trained investigators trust instruments. And the rule for everyone: if you ever smell gas, leave right away and call your utility and nine one one from outside.

Natural gas ignites only inside a narrow window, roughly five to fifteen percent gas in air. Leaner than that and there is too little fuel. Richer and there is too little air. Instruments report this two ways. Percent of gas in air reads the mixture directly. Percent of L E L reads how close you are to the bottom edge of the window, so one hundred percent of L E L is about five percent gas. And a too-rich pocket is never safe, because as it vents and thins it passes right back through the flammable window.

The combustible gas indicator is the investigator's eyes. A pump pulls a steady air sample across a sensor, and the screen reads percent of L E L or percent gas. The instrument only tells the truth when it is treated right: warmed up, zeroed in fresh outdoor air away from the leak, calibration checked, and set for the right gas. Alarm points demand action. But indoors the bar is stricter: any reading above zero is treated as serious.

Outside, the job is to draw a boundary called the leakage perimeter. The technician samples bar holes and available openings such as manholes and catch basins, working outward from the strongest reading until the positive readings are boxed in by two consecutive zero readings in every direction — one zero is not enough — and then rechecks that boundary over time. Watch why it matters: gas follows the loose backfill of the pipe trench, slips beneath pavement and frost, and can surface far from its source. Any indication of gas against or under a building extends the investigation inside.

Inside, sampling moves point to point: floor cracks, drains, and every place a pipe or utility passes through a wall or floor, working from the floor up toward the ceiling, because natural gas is lighter than air and collects high. Suspect fittings get a soap solution, and a swelling bubble pinpoints the leak. Confirmation is instrument and soap only. A flame is never, ever used to look for gas.

When the readings say danger, protection comes before diagnosis. Get the occupants out. Leave ignition sources untouched: no light switches, no doorbells, nothing that can spark. Trained crews may ventilate a building from the outside where company procedure directs it. Control the area so no one wanders back in, and call it in so the right resources respond. Every step here is company emergency procedure, never a do-it-yourself job.

Every confirmed leak receives a grade based on hazard: how close it is to buildings, whether it is migrating, and how strong the readings are. The most serious grade means an existing or probable hazard and demands an immediate emergency response. Lower grades still carry scheduled repairs or rechecks, so a graded leak is never simply forgotten. And the standing rule from the standards: if there is any doubt, classify the leak one grade higher.

Before the job closes, the instrument speaks one more time. After a repair, or once a hazard has been made safe, every test point is rechecked to zero, because a fading reading is not proof and nothing is assumed. A lower-grade leak does not end at zero: it is documented and submitted, and the repair or recheck is scheduled to follow. Either way, readings, locations, and the perimeter sketch go into the record for the crews that follow. And if a hazardous source cannot be confirmed, the technician does not walk away; more help gets called in.

Odorant makes gas announce itself, but instruments make the call. Gas burns only between roughly four and fifteen percent in air. The C G I earns trust through zeroing and calibration, and indoors any reading means act. Draw the perimeter, protect people first, grade the leak, and document everything — and after a repair or make-safe, verify zero before you leave. Next episode: appliance startup.