Before an employee enters the space, the internal atmosphere shall be tested, with a calibrated direct-reading instrument, for oxygen content, for flammable gases and vapors, and for potential toxic air contaminants, in that order.
We test for oxygen before testing for flammability (LEL) for two (2) critical reasons:
- One is a technological limitation of the equipment, and
- The other is an immediate life-safety priority
Here is the breakdown of why this sequence is mandatory in confined space entry and industrial hygiene.
The most common type of LEL sensor used in multi-gas meters is the catalytic bead sensor (also known as a pellistor). These sensors detect combustible gases by literally burning them. They contain a small, heated ceramic bead coated with a catalyst (usually platinum or palladium). When a combustible gas comes into contact with the heated bead, it oxidizes (burns). This combustion raises the temperature of the bead, which changes its electrical resistance. The meter translates this change in resistance into an LEL percentage. Because the sensor relies on actual combustion, it requires a minimum concentration of oxygen to work properly—typically around 10% to 15% by volume, depending on the manufacturer.
If we test a confined space that is oxygen-deficient (for example, a vessel that has been inerted with nitrogen or argon) without checking the oxygen levels first, the combustible gas inside will not burn on the catalytic bead. The LEL meter will read 0%, giving a false sense of security. If a worker then enters the space and introduces fresh air (or if ventilation is turned on), the sudden introduction of oxygen into the fuel-rich environment can instantly create an explosive atmosphere.
(Note: Infrared (IR) LEL sensors do not require oxygen to operate, but catalytic bead sensors remain the industry standard for general-purpose multi-gas meters because IR sensors cannot detect certain gases like hydrogen).
Because of these equipment limitations and physiological realities, standard confined space protocols (including OSHA’s Permit-Required Confined Spaces standard, 1910.146) strictly mandate the following order for atmospheric testing:
- Oxygen Content: Ensures the atmosphere can sustain life and allows the LEL sensor to function accurately.
- Flammable Gases/Vapors (LEL): Checks for explosion risks now that oxygen levels are confirmed sufficient for accurate readings.
- Potential Toxic Air Contaminants: Checks for specific poisons (like H2S or CO) after immediate asphyxiation and explosion hazards are ruled out.