* Question
What is the detection method for the lean gas sensor?
* Answer
Primary methods used in certified %LEL instruments
Catalytic combustion (pellistor)
Principle: Gas diffuses to a heated catalyst bead and oxidizes; the heat rise changes bead resistance measured in a Wheatstone bridge (vs. a reference bead).
Strengths: Broad response (includes H₂), fast, cost-effective.
Limitations: Needs ~≥10% O₂ to oxidize; susceptible to poisons (silicones, sulfides, lead); can drift and requires periodic bump tests/calibration.
Infrared absorption (NDIR)
Principle: Measures IR attenuation at hydrocarbon bands (e.g., C–H stretch); dual-channel optics compensate drift.
Strengths: Works in low-oxygen or inert atmospheres; resistant to catalyst poisons; stable long-term.
Limitations: Does not detect hydrogen; optical path affected by condensation/dust—needs clean optics and proper filtering.
Other methods sometimes seen (less common for certified LEL alarms)
Thermal conductivity (TCD): Compares thermal conductivity to a reference; better for %vol ranges/binary mixes; poor selectivity at %LEL.
Semiconductor (MOS, e.g., SnO₂): Gas adsorption changes film resistance; low cost/compact but higher cross-sensitivity and drift than pellistor/NDIR for safety use.
Rule of thumb
Need wide combustible coverage including H₂ and oxygen is present → choose pellistor.
Need stability in low-O₂/harsh environments and target hydrocarbons → choose NDIR.
Key specs to check (selection checklist)
Measuring range & units (0–100 %LEL; gas factors for CH₄/propane).
Response/recovery (t90/t10).
Temperature/pressure/humidity limits and compensation.
Poisoning/contamination tolerance (pellistor) or optical fouling handling (NDIR).
Certifications (e.g., ATEX/IECEx/FM/UL).
Maintenance: bump-test frequency and calibration interval.
Maintenance basics
Perform regular bump tests with a known test gas; calibrate per manufacturer guidance (pellistors more frequently than NDIR).
Protect from silicones/solvents (pellistor) and keep optics/filter membranes clean (NDIR).
COMMENTS