By COLE KLEINWOLTERINK
You’re dispatched to a commercial building for an odor of natural gas. Upon arrival, your crew grabs the four-gas meter and begins investigating. As you approach the building, the meter starts registering combustible gas. The reading climbs to 5% LEL, then 8%, and eventually 10%.
One firefighter looks at the screen and says, “It’s only 10%. We’re good.” But are we?
Most firefighters have carried a four-gas meter at some point in their careers. We bump-test them, turn them on, and wait for the numbers to appear. We know that when the meter starts alarming, something isn’t right. But how well do we actually understand what those numbers mean? More importantly, do we understand what the meter is, and isn’t, telling us?
UNDERSTANDING LEL AND UEL
Before we can make sense of the numbers on our meter, we need to understand two terms: Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL). The Lower Explosive Limit is the minimum concentration of a combustible gas or vapor in the air that can ignite when an ignition source is introduced. Below that concentration, the mixture is considered too lean to burn. The Upper Explosive Limit is the maximum concentration at which a gas-air mixture can sustain combustion. Above that concentration, the mixture is considered too rich to burn. The area between those two limits is what we call the flammable range.
Let’s use methane, the primary component of natural gas, as our example. Methane has an LEL of approximately 5% and a UEL of approximately 15% by volume in air. That means when methane makes up between approximately 5% and 15% of the air, we have a mixture that can ignite if an ignition source is introduced. Pretty straightforward.
WHAT DOES 10% LEL ACTUALLY MEAN?
This is probably the most important part of this article, and something I think every firefighter should understand.
When your four-gas meter reads 10% LEL, it does NOT mean that 10% of the air around you is combustible gas. The percentage displayed on your meter represents a percentage of the gas’s Lower Explosive Limit, not the actual percentage of gas in the air. That’s assuming the meter is properly calibrated for that gas and operating correctly.
Let’s go back to methane. We know methane has an LEL of approximately 5% by volume. So, if our meter is calibrated for methane and reads 10% LEL, the actual concentration of methane in the air is approximately 0.5%.
At 20% LEL, we’re at approximately 1% methane by volume. At 50% LEL, we’re at approximately 2.5% methane by volume.
And at 100% LEL, we’re at approximately 5% methane by volume, which means we’ve reached the lower end of the flammable range.
Now think back to that initial scenario. Your meter is reading 10% LEL. Do you look at that number differently now?
Many meters have their first combustible gas alarm set at 10% LEL, with a second alarm at 20% LEL. Those settings can vary depending on the instrument and your department’s configuration.
YOUR GAS METERISN’T PERFECT
We put a tremendous amount of trust in our gas meters, but like any piece of equipment, they have limitations. Many traditional four-gas meters use catalytic-bead sensors to detect combustible gases. These sensors work by causing a small combustion reaction on a heated sensor element. The resulting change in temperature allows the instrument to estimate the amount of combustible gas present.
Here’s the catch. These sensors need oxygen to work properly. If you’re operating in an oxygen-deficient environment, your combustible gas sensor may not provide an accurate reading. In fact, it could significantly underestimate the amount of combustible gas present.
High gas concentrations can create problems as well. Depending on the meter, concentrations above its measurement range may trigger an over-range alarm or a protective sensor condition. That’s why understanding your specific meter is so important. Different manufacturers and sensor technologies have different capabilities and limitations.
Another consideration is calibration. Most combustible gas sensors are calibrated using a specific gas, commonly methane or pentane. However, that doesn’t mean the sensor responds the same way to every combustible gas it encounters. A meter calibrated for methane may respond differently when exposed to propane or another combustible gas. Manufacturers provide correction factors and guidance for these situations, but those factors depend on the specific instrument and sensor.
PUTTING IT INTO PRACTICE
First, know your equipment. What type of combustible gas sensor does your department use? What gas is it calibrated for? What are your alarm settings? How does it respond to an over-range condition? If you don’t know the answers, that’s a great place to start with your next company drill.
Second, make sure we’re actually checking these meters. A bump test verifies that the sensors respond to a known test gas and that the alarms function. It doesn’t replace calibration, but it’s an important part of making sure our equipment is ready when we need it.
Third, understand that gas concentrations can change throughout a building. Depending on the gas, it may collect near the ceiling, close to the floor, or in concealed spaces. Taking one reading at the front door doesn’t tell us what’s happening throughout the entire structure.
And finally, use the meter as part of your size-up, not as a replacement for it. We still need to consider the suspected gas, potential ignition sources, ventilation, isolation distances, and what the utility company can tell us.
Cole Kleinwolterink is a member of the Waukee Fire Department, Pleasant Hill Fire Department, Van Meter Volunteer Fire Department, and a Fire Science instructor at Des Moines Area Community College. Feel free to reach out to him at kleinwolterinkc@gmail.com with any questions, comments or inquiries. IFA training articles are archived at: bit.ly/IowaFirefighter