AN OVERVIEW OF FOUR GAS AIR MONITORING
A properly operating, properly calibrated four gas meter can provide us with beneficial information regarding the possible presence of contaminants in the atmosphere where we are about to perform some type of work. This information can then be used to formulate hazard elimination/control plan prior to starting our task. 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)
This configuration allows us to not only utilize the meter at our most common types of hot work, excavating work and hazmat incidents 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 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.
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 level of concern for flammability greater than 10 percent LEL. This level of concern gives us a built-in “safety margin,” as we then still have a cushion prior to reaching the LEL itself.
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).
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. The levels of concern for CO are generally accepted as 35ppm.
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 generally accepted level at which actions should be taken.
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.
- Oxygen Concentration —O2 deficient atmospheres can reduce the accuracy of flammability readings
- Flammability —Due to the hazards presented to first responders, even in proper personal protective equipment
- Toxicity —Due to the toxicological effects imparted to personnel
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. 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.
If we should calibrate our meters on a monthly basis, what should we do prior to every use? We should perform a “bump test” before using the meter. Always perform a function (bump) test in the field before use. Never perform a function (bump) test in the suspected atmosphere. The performance of a bump test should be a standard operating procedure before the use of the meter.
In conclusion, 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 accurate information about the potential atmospheric hazards of the job we are about to perform.