Trust your meter

The basics of four gas air monitoring

CarolinaFireJournal - Glenn Clapp
Glenn Clapp CHMM, CFPS
01/11/2010 -

Sometimes in the hazmat community (and in a larger scale throughout the emergency response community) we become wrapped up in the latest technology and new “toys” of the trade, and fail to revisit the formative building blocks of our discipline. This is not to say that new equipment and technologies should not be considered or utilized, but rather that we should maintain our grasp of the basics that will see us through in many hazmat situations.


One such basic building block in the realm of air monitoring is the four gas meter.  A properly operating, properly calibrated four gas meter can not only provide us with beneficial information regarding the four functions the meter is set up for, but also can clue us in to the possible presence of other contaminants in the atmosphere.  In addition, this topic now not only applies to personnel at the hazmat technician level, but also to personnel at the operations level with our responses to natural gas leaks, carbon monoxide detector activations, and the additional responsibilities now allowed of personnel at the Operations Level. With the preceding having been said, let us now take a closer look at the four gas meter.

Although four gas meters can be set up to monitor a wide array of parameters, the vast majority of such pieces of air monitoring equipment are set up in a similar configuration to monitor the following parameters:

  • Oxygen Concentration (in percent O2)
  • Flammability (in percentage of lower explosive limit)
  • Carbon Monoxide Concentration (in parts per million [ppm])
  • Hydrogen Sulfide Concentration (also in ppm)

The preceding configuration allows us to not only utilize the meter at our most common types of hazmat responses, but also in confined space entry settings. Such usefulness of the four gas meter underscores the need for taking a closer look at the four parameters discussed above.

The percentage of oxygen concentration shown by the four gas meter is a valuable piece of information that does triple duty for us, as it displays not only the possible existence of an oxygen-deficient atmosphere, but also the possibility of an oxygen-enriched atmosphere or possible contamination of the atmosphere by other products. The O2 sensor operates on an electrochemical principle, with an inherent chemical reaction establishing an electrical current proportional to the concentration of O2 being measured. As we all know from our hazmat and basic fire behavior training, the normal percentage concentration of oxygen in air is 20.9 percent. We define an oxygen-deficient atmosphere as containing less than 19.5 percent oxygen, and an oxygen-enriched atmosphere as containing greater than 23.5 percent oxygen. Although we often recognize the pitfalls of inhabiting an oxygen-deficient atmosphere without proper respiratory protection (i.e. unconsciousness and even death), we sometimes forget the hazards presented by an oxygen-enriched atmosphere. The existence of an oxygen concentration exceeding 23.5 percent naturally tells us that something is adding to the oxygen concentration and therefore may be enhancing our flammability concerns, which should lead us to exit the area of concern and determine the source of the enrichment.

As stated earlier, the four gas meter (and specifically the oxygen sensor) can also warn us of the possible contamination of our environment by other substances. If our oxygen concentration is less than 20.9 percent, we can surmise that something is lowering the concentration of, or displacing the oxygen in our environment. While we may encounter situations in which processes such as oxidation (i.e. rust), combustion, or microbial action can lower the oxygen concentration; oftentimes another contaminant is present and is displacing the oxygen present. We should not rely solely on our oxygen sensor to indicate the concentration of other contaminants; however the percentage decrease from the normal 20.9 percent O2 concentration can give us a “ballpark” estimate as to the concentration of any contaminants. We only need to remember the conversion of one percent equals 10,000 parts per million (ppm) to enable us to make the aforementioned “ballpark” estimate of the concentration of other contaminants in parts per million, by converting our percentage displacement into ppm and then multiplying by five (since O2 comprises one fifth of the atmosphere) to determine the estimated concentration of the contaminant, or contaminants present.

The four gas meter displays flammability in terms of the percentage of lower explosive limit (LEL). In simple terms, our percentage of LEL tells us how close we are getting to the point at which a substance is guaranteed to ignite or explode if an ignition source is present. The percentage of LEL is determined through the use of a combustible gas indicator (CGI) that utilizes oxidation (combustion) to produce a differential resistance between two filaments —one with a catalytic bead and one without —which is then displayed as the percentage of LEL. Our levels of concern for flammability are greater than 25 percent LEL for non-confined spaces and greater than 10 percent LEL for confined spaces. These levels of concern do give us a built-in “safety margin,” as we then still have a cushion prior to reaching the LEL itself.

One concept of the CGI that is often overlooked time and time again is that the CGI exhibits what is termed a “relative response” when monitoring any substance other than that which the sensor is calibrated to. For example, if your CGI is calibrated to methane and you are monitoring another substance, the number indicated on the CGI is not the percentage of LEL, but is rather meter units. Meter units are simply a non-dimensional number that must be multiplied by a correction factor (CF) to determine the actual percentage of LEL. Correction factors can be found in the literature accompanying the meter or many times can be found online at the manufacturer’s Website.

In other words, remember your correction factors when using your CGI! For example, let us say that you are in an atmosphere (non-confined space) in which the CGI is displaying 10 percent LEL (or so you think). You therefore surmise that you are not in an area with an excessive flammability hazard. For the sake of argument, however, we will say that you are monitoring a substance that has a CF of three. If you have forgotten to apply the CF, you are therefore looking at 10 meter units and not 10 percent LEL. When the CF is applied, you are actually in a 30 percent LEL environment, which is above our level of concern. As you can see, the correction factor must be applied in such situations and the substance we are monitoring has to be correctly identified to determine the proper CF to be utilized.

The remaining two sensors in the standard four gas meter setup —those used to monitor the concentration of carbon monoxide (CO) and hydrogen sulfide (H2S) —are electrochemical sensors that operate in much the same manner as the previously discussed oxygen sensor, but are designed to detect the concentration of the respective substance in parts per million (ppm). The use of CO detection capability of the four gas meter is steadily increasing in such settings as the determination of CO levels in which it is safe to remove SCBA’s during overhaul operations at structure fires and in responses involving CO detector activations. Due to the fact that CO is a colorless, odorless gas that oftentimes imparts flu-like symptoms at lower concentrations, and can be fatal at higher concentrations, competent monitoring skills for CO detection are a must, even for those at the operations level. The levels of concern for CO are generally accepted as nine ppm for indoor residential areas (derived from EPA and World Health Organization guidance for outdoor air over an eight hour time frame and American Society of Heating, Refrigeration and Air Conditioning Engineers [ASHRAE] guidance for indoor air) and 35 ppm as the NIOSH Recommended Exposure Limit (REL) in a time weighted average format, which is the threshold below which many fire departments allow personnel to remove their SCBA’s during overhaul activities. Some departments, however, have lowered the 35 ppm threshold to maintain an even greater safety factor.

The final sensor in our usual four sensor suite is the H2S sensor briefly mentioned above. H2S sensing capabilities are many times utilized in confined space or below grade settings, as the existence of hydrogen sulfide (better known as sewer gas) in such settings in sufficient concentrations can be fatal to personnel making entry without performing sufficient air monitoring. In terms of a level of concern for H2S, 10 ppm is the NISOH REL in a ceiling format and is the generally accepted level at which actions should be taken.

Now that we have discussed the sensors normally present in standard four gas meters, let us now discuss how the utilization of proper air monitoring techniques can enhance the accuracy of our monitoring efforts. First of all, we need to consciously remember to monitor in a slow and methodical manner.  With the lag time inherent in most four gas meters (i.e. the time required to either pump the air sample through the sensor array or allow the air sample to diffuse across the sensors; and the time required for the sensor itself to “recognize” the contaminant and display the correct information), personnel can actually misdiagnose the location of the contaminant if monitoring in too rapid a fashion.  In addition, we also need to remember the characteristics of the substances we are monitoring for, namely vapor density. The vapor density of a gas allows us to surmise whether the substance will ascend (vapor density less than one), descend (vapor density greater than one), or remain neutrally buoyant (vapor density equaling one) in the air column. While the vapor density does give us a general idea as to where the substance will be found (i.e. high or low), we must remember that certain atmospheric conditions can cause a gas to behave in a manner not indicated by its vapor density.

The question often arises as to what is the proper order for the monitoring of hazards.  The acronym RCOFT serves as a method for remembering the widely accepted order of monitoring, with the rationale behind each step as follows:

  • Radiation —It may be present, especially in today’s terrorism climate
  • Corrosivity —Corrosives may destroy our sensors (detected by wetted pH paper)
  • Oxygen Concentration —O2 deficient atmospheres can reduce the accuracy of flammability readings
  • Flammability —Due to the hazards presented to responders, even in proper personal protective equipment  
  • Toxicity —Due to the toxicological effects imparted to personnel


It is important to note that the specific order of monitoring may need to be adjusted according to the conditions presented.

The final element of four gas air monitoring to be discussed is the proper calibration and testing of our meters. Calibration is simply the process of exposing the sensors to known concentrations of gases in order for the sensors to be adjusted to those values. Many meters allow for a multi-gas calibration, in which concentrations of all four gases are applied at once from a calibration gas cylinder to simplify the process. Specifically relating to the oxygen sensor, some personnel are now utilizing nitrogen to “blow down” the O2 sensor to zero to exercise the sensor over a larger operating range. Following the exposure to known gas concentrations, a fresh air calibration is then performed in a non-contaminated area to set the “zeros” for LEL, CO, and H2S sensors and the 20.9 percent value for the O2 sensor. Such a calibration is performed on a monthly basis by most jurisdictions.

If we should calibrate our meters on a monthly basis, what should we do prior to every use? The textbook answer is that we should perform a “bump test” before using the meter. No, a bump test is not throwing the meter on the ground to see if it will survive (although some personnel sometimes seem like they are doing so). In all seriousness, a bump test consists of exposing the sensors to known concentrations of gases from a calibration gas cylinder so that the readings from the sensors can be verified as being within an acceptable range.  Unlike a calibration, a bump test does not modify the values presented to the user. If it is not possible to perform a bump test prior to the use of the meter, a fresh air calibration should be performed as a minimum element of initializing the meter for use as a stopgap measure. The performance of a bump test should be a standard operating procedure for any jurisdiction.

As can be witnessed in our preceding discussion, the four gas meter is a vital building block of our air monitoring capabilities. Not only can such a meter display the concentrations of the gases corresponding to the sensors commonly installed; a properly operating, properly calibrated meter can also give us clues as to the presence of other contaminants. We should not turn our back on new technologies that are constantly being developed in the realm of air monitoring, but we should also never forget the basic components of our air monitoring heritage.

In closing, stay safe out there and be sure to visit the new North Carolina Association of Hazardous Materials Responders Webpage at

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