The Multigas Meter

MSA’s Sirius (left) and Drager’s XAM-7000 (right). (Photo by author.)



By Kevin Yoos

Metering the environment is no longer just a hazmat team’s or hazard assessment team’s responsibility as had been in years past. For many years now, technical rescue teams have been using meters to ensure safety of members working in confined space and trench operations. Firefighters from around the globe are now using meters daily to ensure the safety of other firefighters, civilians, and to protect the environment. Meters are now on most first-line apparatus for the daily responses, and firefighters are becoming more proficient in their operation. However, it is important to always refresh your memory by training and reviewing the proper operation of your meters to keep you and your responders safe.

Multigas meters are used daily by industry professionals to help protect workers. They are produced by many different manufacturers, but they all have the same basic operations. The gas sensors can be changed depending on what you are looking to monitor, but for this article, I will review the basic gases that firefighters and hazmat teams encounter. You must remember that you are monitoring for specific gases, and there is always the possibility of something present that you do not see on your metering device, or there may be a cross sensitivity issue.

For the most part, firefighters and hazmat teams use multigas meters to monitor four or five different gases in the environment. These are gases that we come across on a day-to-day basis. These gases usually include oxygen (O2), carbon monoxide (CO), hydrogen sulfide (H2S), and lower explosive limit (LEL). Some meters will include a fifth sensor, which monitors the ionization potential (IP) or sulfur dioxide (SO2). Each of these chemicals is potentially dangerous to civilians and first responders and should not be taken lightly. Anyone using monitoring equipment needs to know the hazards associated with these particular gases before picking up the meter, and he should always ensure he knows the baseline readings before starting any operation. Once operations begin, the meter operator should continuously monitor for any changes and adjust operations as needed before the alarm thresholds are reached.

Toxic gases are monitored in parts per million (ppm) because of the very low values that are dangerous to us. These gases include CO, H2S, SO2 and the IP that we monitor with our meter. O2 and LEL are measured in percentage. There is a relationship between ppm and percentage; normally, 10,000 ppm is equal to one percent. So, if a gas is flammable at four percent, then it is equal to 40,000 ppm, but when we look at O2, the numbers do change a bit. O2 is one-fifth of the atmosphere; the rest is mostly nitrogen and a mix of different gases. So, when we look at our O2 reading, any change (especially after conversion to ppm) will need to be multiplied by five. If your O2 reading drops one percent, from 20.8 to 19.8 percent, that would be equal to 50,000 ppm of something displacing the O2 (Note: our meter is not alarming!).

Research all of these chemicals and gases using different means; the National Institute for Occupational Safety and Health’s Guide, Cameo Chemicals, or WISER, to name a few, to get a better understanding of the hazards involved. Just having a meter is a good start, but if you don’t know how it works or the hazards involved, you won’t be operating safely, and you may be putting your firefighters in great danger.

Let’s look at some of the properties of the gases that we are monitoring.



This is very important to the function of sustaining human life, but it is also essential for the proper operation of the meter. Depending on the manufacturer, most will not function correctly once O2 levels drop below 16 percent. It may also alter the way other chemicals or gases behave in the environment. When related to flammability, the amount of O2 may increase or decrease the chances of ignition. Normal O2 content should be 20.8 percent at sea level, and the meter operator should constantly monitor for any changes in this value. As discussed in my last article, firefighters noticed a decrease in O2 value on their meter that was later determined to be displaced by a liquid CO2 leak. Alarm thresholds are usually set at 19.5 percent for the low alarm and 23 percent for the high alarm. Remember, a one-percent drop, which equals 50,000 ppm, will not cause the meter to alarm, but it could signal a very toxic environment that could kill you.


Lower Explosive Limit

This is always on our mind; if the environment is flammable, it could be extremely dangerous. Most meters are calibrated using pentane as the baseline gas for LEL. This gas falls right in the middle of the chart of most flammables and, with our alarm thresholds, it will keep us out of danger. Remember, you might be in a safe environment with low LELs or no readings, but the source of the leak could be at the LEL, and the explosion or ignition could still harm you. Alarm thresholds are usually set for a low alarm of 10 percent LEL for indoor spaces and 20 percent LEL for outdoor spaces. Since pentane is not the only gas we are looking for, there is always a conversion chart if you know the particular chemical and if you really want to determine the exact LEL in the environment. For the most part, you won’t need to know the true LEL present, but it can be determined if needed. These charts are in the meter’s operators manual and can vary depending on the manufacturer.


Hydrogen Sulfide

Also known as sewer gas, this comes from the decomposing or breaking down of organic matter. At low levels, it smells like rotten eggs and is colorless, but it is considered a flammable gas. You may find H2S in many confined spaces including manholes, drainage pipes, and basements. It is flammable at four percent (or 40,000 ppm) and is extremely toxic at low PPM [the immediately dangerous to life and health (IDLH) is only 100 ppm]. The low alarm value is usually set at 10 ppm, and the high alarm threshold is 20 ppm to ensure you stay in a safe environment. It is heavier than air and will be found in low lying areas, but remember, when you heat it up in a fire, it will rise and move throughout the structure.


Carbon Monoxide

As explained in my last article, CO is a flammable gas that is poisonous (toxic), colorless, odorless, tasteless, and slightly lighter than air at room temperature. CO is flammable at 12.5 percent, equal to 125,000 ppm. At 68°F, CO is slightly lighter than air at .968, compared to air at 1.0, but it has almost the same value and will be everywhere within the structure. Remember, any temperature change will quickly change its density. So, when you are using a hoseline to put a fire out, you will be cooling the area; the CO will then get heavier and sink to low-lying areas. The alarm thresholds are usually set at 35 ppm for the low alarm and 100 ppm or higher for the high alarm.


Sulfur Dioxide

Some meters have a fifth sensor which may exist for detecting SO2—another sewer gas which is non-flammable and heavier than air but very toxic (IDLH of 100 ppm)—or the IP. The IP seems to be why most departments have chosen to use the fifth sensor. IP looks for volatile organic compounds that may be present in the atmosphere. The meter uses a 10.6-electron volt lamp, which is the amount of energy required to remove an electron from the atom or molecule. So what does that mean?  The meter will get a reading on chemicals with an IP of 10.6 or below. The meter will not know the actual chemical, but it will at least let you know something is present that can harm you. It is looking for toxic chemicals and reads in ppm. The meter will help you pinpoint the source since the readings will increase as you get closer. If you get readings of 50 ppm or more, you should definitely be using self-contained breathing apparatus since there are many toxic chemicals that can harm you above this value. Alarm values are usually set at 50 ppm for the low alarm and 100 ppm for the high alarm.

It is also important to understand all the buttons and controls on the meter. Take the time to train with other firefighters and go over the operator’s manual. Understanding your readings, how to reset alarms, finding peak values, and cross sensitivities issues are all extremely important to ensure safe operations.

There is a lot you need to know if you put any type of metering device in your hands. Having the proper training and knowledge will protect you and your crew on a daily basis and should not be taken lightly. This article has only touched the surface of basic metering operations, but hopefully it will assist you in understanding the importance of your metering devices.


KEVIN YOOS is a 19-year veteran with the Fire Department of New York (FDNY) and a lieutenant assigned to Squad Company 270 in Queens. He has been working in the FDNY Special Operations Command (SOC) for the past 15 years, and prior assignments include Squad 61, Ladder 17, and Engine 50 in the Bronx. He has served twice as Chief of Department for the Setauket (NY) Fire Department on Long Island, is currently a commissioner of the Setauket Fire District, and has been an active firefighter and officer for the past 26 years. He is a nationally certified Fire Instructor II and is a Deputy Chief Instructor at the Suffolk County Fire Academy. He teaches many programs including hazmat, technical rescue, and shipboard firefighting for the FDNY and also instructs technical rescue programs for the New York State Division of Homeland Security and Emergency Services. He can be reached at


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