Oxygen and LEL Metering by Firefighters

When it comes to metering in the fire service, there’s a culture of “I’m no hazmat technician. Meters are for hazmat geeks.” However, competency in basic metering is a skill that first responders must have.

As technology progresses, engineers continually find innovative ways to use chemicals. With these technologies, threats to first responders increase tenfold.

Neither 1910.120 of the Code of Federal Regulations nor NFPA 475: Recommended Practice for Organizing, Managing, and Sustaining a Hazardous Materials/Weapons of Mass Destruction Response Program emphasize meter training programs for those who use the meters the most, the first responder. Many hazmat operations programs don’t provide hands-on meter training. That’s unfortunate, because a better understanding of four-gas meter configuration and how the sensors work is important.

The most common sensor configuration of a four-gas meter is oxygen (O₂), lower explosive limit (LEL), carbon monoxide (CO) and hydrogen sulfide (H₂S). The most important element? The one that’s between your ears doing the interpretation. Understanding what the readings mean is essential to your safety and health.

The oxygen sensor is arguably the most important. Every sensor’s readings are based on normal oxygen levels in air. If the oxygen sensor is faulty, all of the other sensor readings become suspect.

The oxygen sensor

On startup, the four-gas meter begins a series of self-checks of the sensors, alarms and calibration status. Once “warm,” the oxygen sensor should read 20.9 percent (sensors might drift from 20.8 percent–21.0 percent).

After self-checks, a fresh-air calibration is needed to establish a baseline atmosphere. This tells the sensors that what they now sense is clean air. Therefore, a fresh-air calibration must be done outside, in clean air. (The cab of your apparatus might be contaminated by exhaust fumes or dirty gear. If your meter displays negative numbers, you likely calibrated in a dirty atmosphere.)

The oxygen sensor has two alarms: low (19.5 percent) and high (23.5 percent). These alarm levels are considered IDLH (immediately dangerous to life and health) atmospheres. At 19.5 percent, you should utilize your SCBA because of possible toxicity, flammability or hypoxia hazards. At 23.5 percent, there’s an increased fire risk.

When you enter a structure, you should meter from the floor to the ceiling. When the air is still, although there can be no readings at face level, dangerous levels of gas can be above or below face level.

The bottom line: If your meter alarms, you must take action. Your department’s standard operating procedures (SOPs) will dictate your actions.

Regarding the reading of 19.5 percent oxygen: Air is approximately one-fifth oxygen. Because 1 percent air equals 10,000 ppm, one-fifth of that comprises 2,000 ppm of oxygen, with the other 8,000 ppm mostly nitrogen. When the oxygen level drops a full percent, 10,000 ppm of oxygen and 40,000 ppm of nitrogen were displaced by something. Therefore, when your oxygen sensor alarms at 19.5 percent, approximately 75,000 ppm of something else is in the air that you are metering. Once again, this could be a toxic, flammable or hypoxic atmosphere.

Many times, I have heard responders say, “My oxygen dropped, but the other sensors showed nothing. The oxygen sensor must be bad.”

The other sensors don’t see everything. Thus, your oxygen sensor can tell you that something else that’s in the air is displacing oxygen and that you should take action to protect yourself. The results of these situations are that you were lucky that day that it wasn’t a toxic, flammable or hypoxic atmosphere.

An oxygen reading of 23.5 percent means that the air is oxygen-enriched. Many things become much more flammable and more easily ignited. Thus, protective action should be taken, particularly SCBA and hazardous substance management systems.

The average lifespan of an oxygen sensor is approximately two years from the time that it’s exposed to air. The chemical reaction that measures the oxygen continues 24/7. Thus, even if the meter is used seldomly, the oxygen sensor degrades over time.

The LEL sensor

News reports of gas explosions and injuries to first responders are all too common. However, properly used, the LEL sensor reduces the risk from flammable atmospheres.

Most LEL sensors use a catalytic bead technology. The bead is heated to a set temperature; exposure to traces of flammable gas causes the bead to react, which is measured and interpreted into the LEL reading. In the case of methane (natural gas), the flammable range in normal air is 5 percent–15 percent. This equates to a flammable range of 50,000 ppm for LEL and 150,000 ppm for the upper explosive limit (UEL). Because the detection of the UEL means that the LEL, or flammable range, was exceeded, we’ll focus here on the LEL: We seek to detect the gas prior to the LEL to mitigate an incident safely.

The LEL sensor reads a percentage of the LEL from zero percent–100 percent, in 1 percent increments. Five percent methane equals 50,000 ppm, so an LEL reading of 1 percent on a meter means that the sensor is reading 500 ppm at that spot. LEL sensors on most meters are set for a low alarm of 10 percent of the LEL (5,000 ppm). Realize that a meter only samples air in about a fistful amount at any given time.

The Occupational Safety and Health Administration designates 10 percent of the LEL as an IDLH atmosphere (not 10 percent total gas). You should be in full PPE at this point.

The high alarm usually is set at 20 percent of the LEL (10,000 ppm). Alarm settings might vary per local SOPs.

When you move into greater levels of gas, the reading climbs. As it approaches 100 percent of the LEL, the sensor will shut itself off to prevent it from becoming an ignition source. The meter usually displays “OR,” for overrange. (This varies with different manufacturers.) The sensor won’t start up again until you follow the manufacturer’s procedures. Never restart your meter in a flammable atmosphere.

“I smell gas, but I have no readings.” From 1–499 ppm, your meter will read zero percent LEL even though methane is present. The additive mercaptan is what you smell, not methane. Mercaptan can be smelled in an amount as low as 1–2 parts per billion. An LEL sensor won’t register 1 percent until 500 ppm are present.

Metering pitfalls

Methane is lighter than air. It rises inside of a building and can be trapped above drop ceilings and in other enclosed spaces. Propane is heavier than air and gathers in a structure at floor level or works its way down stairwells and vents to lower elevations, particularly sumps and sewers. When you enter a structure, you should meter from the floor to the ceiling. When the air is still, although there can be no readings at face level, dangerous levels of gas above or below face level can be present.

Some meters read more slowly than others do—and even slower when they are cold. It can take as much as 20 seconds to get a reading in certain conditions. In other words, metering is a slow, deliberate process, so take your time. If you walk through a door too aggressively, the meter could be in alarm for where you were 20 seconds earlier. Consider that if you read 1 percent of the LEL at the door frame, higher concentrations likely are inside.

Underground leaks can pose a different hazard. Mercaptan can be scrubbed from the gas as it seeps through the ground. Don’t be led astray because you can’t smell mercaptan; it could cost you and your crew dearly.