The first of a two-part series on confined space gas monitoring: Unlike a raging river or pounding surf, the dangers of confined spaces are often silent and unseen. Unless a dangerous atmosphere creates a visible cloud of vapor, there isn’t any way for you to see what hazards lurk within a confined space. This is precisely the reason monitoring equipment is so vital to confined space entry. While it isn’t possible within this format to provide a comprehensive guide for the use of all the different types of gas monitors out there, we will try to cover some of the monitoring theories that will apply to many types of the confined spaces. Only by consulting your user instructions from the manufacturer and training in non-emergency situations with your device can you hope to become proficient in understanding exactly what the information it provides means in terms of safety for you and your team. To really know what sort of atmospheric hazards are present, your gas monitor must be functioning properly and calibrated. If you are new to confined space entry, or have not used a particular type of monitor before, consult your user manual and make sure all sensors have been properly treated or burned in and are ready to test for hazards. Before you use your monitor to sample the air, it’s important to know what sorts of atmospheric hazards can exist in confined spaces and how your monitor will react to them. Most modern 4-gas monitors will check for LEL, 0xygen, carbon monoxide and hydrogen sulfide. Specialized monitors can also be specifically configured to check for other dangerous gases present in your industry. When measuring for lower explosive limit (LEL) remember that not only do you have to understand how to properly interpret the information provided by your monitor, but you also have to understand that depending upon the density of the gas you are trying to monitor, concentrations may vary within the space. A gas that is heavier than air such as hydrogen sulfide will tend to accumulate near the bottom of a space, while lighter vapors like methane will rise. That is why samples must be taken from at least the top, middle and bottom levels of the tank. Oxygen levels in a neutral location will be 20.9%. Higher or lower concentrations present serious hazards and must be closely watched. There are a number of conditions that might lower oxygen levels such as decomposing organic material in a sealed space or welding operations. The levels might also be lowered intentionally as with the addition of nitrogen gas to create an inert space, or carbon dioxide for fire suppression. In any case, in an area with less than 19.5% O2 content (the threshold for oxygen deficiency) a confined space entrant should not be allowed in without a positive pressure self-contained breathing apparatus or positive pressure supplied air respirator. An oxygen-rich environment has a whole other set of problems. Atmospheres with greater than 23.5% oxygen lend fuel to combustive materials, and items that might normally be difficult to ignite may burn easily. In particular, pay attention to things like flammable liquids (with FP over 200 degrees F), electrical equipment and clothing that can create static. As with LELs, oxygen must also be measured at various levels within a confined space. The danger here is that you measure in the middle of the tank, and it is fine, but then when you drop your screwdriver and bend down to pick it up, you breathe in a heavy dose of H2S, and that can put you on the ground in two seconds flat.