1910.146 (b) of OSHA’s Permit-Required Confined Space standard defines a hazardous atmosphere as follows: “Flammable gas, vapor, or mist in excess of 10 percent of its lower flammable limit (LFL);”. However, whenever I conduct onsite confined training courses for clients, I always recommend establishing a maximum of 0% LFL (or LEL). One exception is if the work task requires working with materials that emit flammable vapors, such as one of my clients whose technicians perform work inside underground storage tanks utilizing a styrene monomer. The rationale to my recommendation of 0% LFL/LEL is if your combustible gas meter detects any flammable vapor reading – some flammable material remains inside the tank. An additional reason is depending upon the calibration gas for your combustible gas sensor and the flammable gas vapor present in the confined space, the combustible gas sensor may actually be under-reading the actual concentration of the flammable vapor inside the confined space.

Consider the following example: A combustible gas meter calibrated on methanol is reading 10% LEL in a pentane atmosphere. To find actual % LEL pentane, use the following chart by Industrial Scientific. The multiplier is 1.9, resulting in an actual % LEL for pentane of 19%. This is above OSHA’s limit of 10% LEL.

To summarize, restricting LEL/LFL to 0% is the best practice. Whenever necessary to perform work emitting flammable vapors inside the space, purchase a sensor specific to the specific flammable vapor, if possible. Otherwise, utilize the applicable correction factor to confirm you are not exceeding acceptable LFL/LEL levels.

Jon