Four-Gas Monitoring for the Fire Service

The fire service is much different today than it was a decade ago. Our list of responsibilities and competencies has grown immensely, and this list will only continue to grow.

The public relies heavily on the fire department to respond to and mitigate a host of issues, including emergency events involving hazardous materials. Numerous recent events have underscored the need for increased training and proficiency with air-monitoring equipment.

Fire departments across America range in size and scope of duties; however, with the increase in technology and affordability, there really is no argument against each first-due unit having some level of air-monitoring equipment. There is a strong case to be made for a four-gas monitor with specific sensors based upon your response district. The exact sensors should be determined through a risk assessment and what specific hazards your department could face. 

For the case of this article, we will cover the basic sensors of four-gas monitors, which identify carbon monoxide (CO), hydrogen sulfide (H₂S), oxygen (O₂), and a combustible gas indicator or the ability to measure a percentage of the lower explosive limit (LEL). 

Increasing your proficiency with the four-gas monitor will allow you to make confident decisions in a host of different scenarios. 

Call types

Before we delve into the ins and outs of the monitoring technology and associated equipment, let’s first state our case. We are a fire department, and we are here to protect the public. We need to be able to respond to a wide range of issues and have an idea of what we face on scene.

There are a variety of “routine” calls where being proficient with the four-gas monitor will assist us in our effective response:

• CO issues
• Flammable liquid spills
• Natural gas emergencies

Further, there are several non-routine alarms for which proficiency in your four-gas monitor is a life-or-death situation:

• Workers overcome in the bottom of a confined space
• Leaking hazardous materials, especially an oxidizer
• Fuel tanker that has rolled over

Fire departments must train on how to mitigate all of these incidents.

How it works

Like most Type-A personalities, we as firefighters want to be given a piece of equipment, shown how to use it, and then go about our business. However, one critical aspect of proficiently operating a four-gas monitor is understanding how it works. The more we know about how the sensors create the numbers displayed, the safer we can keep our crews and the public.

There are two sensor technologies that we need to study to understand the four-gas monitor operation: catalytic combustion and electrochemical. The technology behind the sensors is what gives us the readings, which in turn we use to make critical decisions.

Catalytic combustion sensors

We have the capabilities of being in a unique position within the hazmat response field. We are THE department that deals with flammability and, as such, we should have a complete grasp and understanding of how the catalytic combustion sensor works.

The catalytic combustion part of the four-gas monitor is trying to tell us what percentage of the LEL of flammable vapor we are currently monitoring. How does it know if we are in a flammable vapor? Catalytic combustion sensors contain a heated wire that is treated and will burn gases that come in contact with it. As the wire temperature increases, the electrical resistance of the wire increases.

The number being displayed is the measurement of the change occurring within the sensor; the actual material causing the reaction can be different. In order to account for the widest range of different flammable gas, the four-gas monitor is calibrated to a flammable vapor that is midline in relation to others. 

We need to understand the numbers associated with the LEL. This is probably the most commonly misunderstood reading on the four-gas monitor and also right up there with the most critical. What does the percentage of LEL mean? Let’s start by looking at a flammable gas that is common across the nation.

Let’s say your engine company responds to a leak of propane within a structure in your response area. On the way to the call, you think, “I know the general hazards of propane, but I better check some figures.” You pull out the NIOSH Pocket Guide to Chemicals and look up propane. You view the chemical information and find that propane is a colorless, odorless gas with an LEL of 2.1 percent and an upper explosive limit (UEL) of 9.5 percent. You take out your four-gas monitor and let it run through its checks, complete a bump test to ensure the sensors are reacting, and complete a fresh air zero.

As you approach the building, you notice that your oxygen readings are not changing; however, you notice an increase in your combustible gas indicator (CGI). As you enter the structure, your meter starts to alarm at 10 percent of the LEL. You are now responsible for reading, understanding and directing activities based upon the reading.

So where are we actually at in the flammable range? The meter says 10 percent. In this particular case, we know that our LEL of propane is 2.1 percent, and with our meter reading 10 percent, that means we are at 0.21 percent of propane in the atmosphere, as the meter is indicating 10 percent of the LEL. Note: This is where things often go wrong with readings, as someone might read 10 percent as the concentration of propane in the atmosphere. We need to be able to comprehend this measurement to be able to make confident decisions in our actions. The action level of 10 percent of the LEL gives us a safety factor for our protection, as it allows us time to evacuate and attempt to change the environment before reaching the LEL.

When looking at the flammable range, the LEL and UEL are significant factors to comprehend. The difference between the LEL and UEL is the range of concentration in the environment that will burn. If the meter shows 10 percent, then we need to take actions to change the environment. Some of these actions could include ventilating the structure, ensuring proper PPE for flammability, ensuring a hoseline is in place, and finding the source of the leak.

The deeper you go into the inner workings of the four-gas monitor, you will find that the midline gas for calibration will also require a correction factor for other gases. In the case of propane, our actual reading may be a touch higher than the display. Here is one thing to keep in mind: We are a fire department, not a laboratory. We don’t carry super-accurate laboratory equipment; we carry devices that have a little wider range of acceptance and, thus, our readings are not as accurate. The four-gas monitor for the fire service is designed to be tough, dependable and allow us to make choices in the incident.

Note: When we think of the CGI on our four-gas monitor, remember 10 percent of the LEL for your action level. In other words, we must take action to affect the situation. In this case, safely identifying the source of the leak and ventilating the structure would be appropriate actions. Due to the flammability of propane, having a charged hoseline in place is another action.

Electrochemical sensors

Electrochemical sensors relay information related to oxygen, hydrogen sulfide and carbon monoxide levels. 

Oxygen (O₂)—Our knowledge of oxygen levels in the fire service usually comes from the ability to wear an SCBA. We can all remember our time back at the academy when the instructor was drilling in our heads the acceptable levels of oxygen in the environment. This level of understanding translates to the four-gas monitor very well.

The levels of oxygen are not just important to our PPE selection but also to the effectiveness of the meter and the readings you are trying to decipher. For a quick refresher, the acceptable oxygen levels in an ambient environment are 19.5 percent on the low side and 23.5 percent on the high side. The normal reading for your meter should be close to 20.8 percent oxygen. 

The 0₂ sensor is an electrochemical sensor much like the H₂S and CO sensors. In the most basic form, the sensor sees the gas through a membrane and produces an electrical current that is used to translate the reading to the screen that you view.

The acceptable range of oxygen is crucial to obtaining quality readings with your meter. The last point to remember is that with a 1 percent drop in oxygen readings, it is actually 50,000 parts per million (ppm) of something filling that void. Pay close attention to the oxygen readings!

Note: When we think of the oxygen level on our four-gas monitor, think of the number 20.8 percent. If this moves in any direction, start thinking what might be displacing it.

Hydrogen sulfide (H₂S)—Why is hydrogen sulfide so important to us in the fire service? To answer this question, unfortunately, all you have to do is review recent fire department responses that ended in injury or death of emergency responders. Hydrogen sulfide is extremely toxic as an inhalation hazard.

One number we need to remember is 10 ppm, as this is the lower alarm level for our four-gas monitor. The IDLH is only 100 ppm, which represents how little H₂S has to be in the air to be deadly. The most common space we will find hydrogen sulfide is in confined spaces. H₂S is heavy and will be found in low-lying areas. 

Note: When we think of the H₂S sensor on our four-gas monitor, think of the number 10 ppm for your action level. Due to the toxicity of H₂S, it is very important to ensure respiratory protection is used. If the incident is in a confined space, ventilation is a very important action for response. 

Carbon monoxide (CO)—This is one of the most routine hazmat calls that fire departments run in today’s age. Whether it’s a CO alarm in a residence, a furnace failure, or a car or piece of equipment running in a building, the public expects trained and qualified responders to assist them. The four-gas monitor will allow you to respond and make educated decisions based from your readings. CO is a colorless, odorless, and toxic gas. It also has flammable properties when it approaches 125,000 ppm. The alarm limits should be known and trained on regularly.

Note: When we think of CO on our four-gas monitor, think of the number 35 ppm for our low alarm and 100 ppm for our action level. Your department probably has a specific procedure for CO incidents. In most cases, the actions of evacuation will be based upon the levels of CO you record on your response. Proper ventilation of the structure is another action step for CO response. 

Practical application

Now let’s take a look at some practical response scenarios and how the four-gas monitor technology fits in your first-due response. As I stated in my last hazmat-focused article, “First-Due Hazardous Materials Size-Up” (firehouse.com/12259512), your first-due engine will be faced with numerous decisions, and their ability to conduct air monitoring from the start will lead to a more effective response.

LP tractor-trailer scenario

Your company is dispatched to an accident involving a liquefied petroleum (LP) tractor-trailer and a pickup truck. When you arrive, you conduct a size-up and notice that the trailer is stenciled NON-ODORIZED. You have an unconscious patient in the vehicle and begin air monitoring while the patient is being removed to ensure the safety of the response.

As discussed earlier, you know that your four-gas monitor’s CGI will measure the LEL. As you approach the truck, your sensor shows 5 percent of the LEL. You determine that the saddle tanks on the truck are intact. Then as you approach the trailer, the readings get higher. You are now confident that you have a leak involving the tanker. As such, you order a crew to pull a charged hoseline for scene protection. With any rise in the LEL, we would begin to evacuate the area around the tanker; however, the more it rises, the more actions we must take to ensure life safety of the surrounding area.

If we arrived and were getting readings of 50 percent of the LEL, for example, we would ensure our crews were in firefighter turnout gear, very quickly evacuate the area, and find and manage all sources of ignition. If we arrived and were getting minimal readings only inches from the tanker, we would set up a small perimeter, but the speed and intensity of our operation would not be near what it would be at 50 percent. The tricky part is when it reaches 100 percent of the LEL. In these scenarios, the meter doesn’t go to 110 percent; the highest reading you will see is 100. So are we at the LEL, in the middle of the flammable range or above the flammable range? This is why the safety factor is so important.

Further, the O₂ sensor will tell us if there is an oxygen deficiency in the environment or, worse in this case, an oxidizer present. In other words, we need to monitor the environment to ensure there isn’t something displacing the oxygen or increasing the likelihood of ignition.

Confined space scenario

Your engine is dispatched to a report of a board of public works worker who is not responding at the bottom of a manhole. You arrive on scene, conduct a size-up and are met on scene by three anxious coworkers of the worker in the bottom of the manhole. You decide to place the monitor in service and get a reading of 150 ppm of H₂S.

Your research of H₂S in the NIOSH pocket guide shows the IDLH at 100 ppm. Your monitor is reading well above the IDLH. You also know that H₂S would most likely collect at the bottom of this manhole. How does this monitor reading correlate with your PPE choice and tactics? You choose SCBA for the respiratory protection, continual air monitoring and ventilation of the confined space for rescue. Your knowledge of the four-gas monitor allowed you to be confident in your tactical decisions.

In sum

Having the knowledge and understanding of your four-gas monitor will allow you to make more effective decisions on many of your incidents. Each department is different, so be sure to review your departments operating procedures. The technical information that comes with your equipment is also a great resource for training. The four-gas meter is like any other piece of equipment, the more you use it, the more proficient you will become in air monitoring techniques.

References

National Institute for Occupational Safety and Health. 2016. NIOSH Pocket Guide to Chemical Hazards. Retrieved from www.cdc.gov/niosh/npg.