📖LFL / UFL terminology

LFL (Lower Flammability Limit)

The lowest concentration of a flammable gas in air at which combustion can be sustained given an ignition source. Below LFL there is too little fuel for the flame front to propagate.

For hydrogen, the classical value is 4%, measured in closed, pre-mixed, static conditions; it does not translate directly to open, dynamic conditions such as inhaler operation. For the inhalation environment (open, dynamic conditions), an empirical value of 10% has been reported in the literature and is the figure referenced in practice for inhalation applications (→ Inhalation concentration). The classical 4% and the empirical inhalation 10% are distinct values with different premises and should not be conflated.

UFL (Upper Flammability Limit)

The highest concentration of a flammable gas in air at which combustion can be sustained. Above UFL there is too little oxidizer (oxygen).

For hydrogen this is approximately 75%. However, even at 100% pure-hydrogen output, the concentration gradient at the device–air boundary necessarily includes a layer in the 4–75% flammable range (→ UFL 75% paradox).

Ignition sources

Combustion requires a flammable mixture, oxidizer, AND an ignition source. Hydrogen has an exceptionally low minimum ignition energy (≈ 0.017 mJ) — about one-tenth that of methane or propane. Mundane phenomena such as airflow static electricity, mucosal friction, and electrical sparks can therefore initiate ignition.

Related terms

• MIE (Minimum Ignition Energy)
• LFL / UFL — lower / upper flammability limit (above)
• Stoichiometric concentration — the fuel concentration at which complete combustion occurs in theory; ≈ 29.5% for hydrogen in air

See Inhalation concentration for the operational implications.