Respo Newsletter | Catalytic Methane Sensors: Why They Read 0

Catalytic Methane Sensors: Why They Read 0–100% LEL Without a Flame

Catalytic (pellistor) sensors don’t wait for methane to “catch fire.” A hot platinum/palladium bead catalyzes surface oxidation far below the Lower Explosive Limit (LEL), producing heat that’s converted into an electrical signal and scaled to %LEL.

What actually happens inside the sensor

Heated catalyst, no open flame: A tiny bead with a platinum coil and Pt/Pd catalyst is held at ~300–500 °C. Methane oxidises on the bead’s surface, not in the bulk gas.
Catalysis lowers the activation barrier: The catalyst enables CH₄ + 2 O₂ → CO₂ + 2 H₂O at concentrations well below the LEL—so you get a measurable reaction long before 5% vol.
Heat becomes a signal: The reaction’s heat slightly raises the bead temperature, changing the coil resistance. A Wheatstone bridge compares this to a non-catalytic reference bead and outputs a signal proportional to gas concentration.
Why does it read 0–100% LEL: For methane, 100% LEL ≡ 5% vol by definition. The instrument calibrates the bridge output so that 5% vol shows 100% LEL. The sensor responds from near-zero up to that point because surface oxidation is happening the whole time.

Safety by design

Flame arrestor & geometry: The sintered metal flame arrestor and tiny thermal mass let oxidation happen on the bead while preventing flame propagation outside the sensor.

Field realities & best practices

Oxygen is required: Low-O₂ atmospheres can cause under-reading; pellistors are not for inerted spaces.
Poisoning/inhibition: Silicones, sulfur, lead, and some chlorinated compounds can poison or inhibit the catalyst—use filters, poison-resistant beads, or frequent bump tests.
Linearity and drift: Response is nearly linear to ~100% LEL for methane, but diffusion limits, temperature control, and ageing can introduce drift—follow the maker’s bump/cal protocols.
Cross-sensitivity: Other combustibles contribute to the reading; understand your process gas mix and calibration gas selection.

Bottom line

The palladium/platinum bead doesn’t burn first—it enables controlled oxidation at the surface across the 0–100% LEL range. The heat released is what you measure; %LEL is simply the scale we use to map that signal for methane safety decisions.