Gas detection instruments: principles, selection and limitations
Gas detection instruments: principles, selection and limitations
This domain tests how sensors work, which one to choose, and why a reading may be wrong.
Sensor principles
- Catalytic bead (pellistor): measures flammable gas in %LEL by burning it on a heated bead. It needs oxygen, is poisoned by silicones, lead and sulphur compounds, and can be saturated by high gas concentrations. It reads relative to the gas used for calibration.
- Infrared (NDIR): measures gases that absorb infrared light, such as methane, propane and CO2. It works in inert atmospheres and does not need oxygen, but it cannot see symmetrical molecules such as hydrogen, nitrogen or oxygen.
- Electrochemical: used for oxygen and toxic gases (CO, H2S, SO2, Cl2). It has a limited life, cross-sensitivities and temperature effects.
- Photoionisation (PID): detects volatile organic compounds in ppm. Lamp energy determines what it can ionise; a standard 10.6 eV lamp will not detect methane.
- Flame ionisation (FID): laboratory-grade VOC measurement using hydrogen fuel.
- Colorimetric tubes: useful spot checks but have limited accuracy and cross-sensitivity.
Selection
Match the sensor to the target gas, range and atmosphere; confirm ATEX/IECEx certification for the hazardous area; consider pumped versus diffusion sampling, response time (T90), sample line length and filtration. A detector that is not certified for the zone is itself an ignition source.
Worked example
A vessel has been purged with nitrogen and you need to check for methane before introducing air. Reasoning: a pellistor needs oxygen and will not work in the inert atmosphere. Use an infrared methane detector (or thermal conductivity) and an oxygen meter. Re-check the pellistor's suitability once the vessel is aerated.
Traps
Using a pellistor in an oxygen-free space; expecting a PID to detect methane; ignoring cross-sensitivity and response factors; forgetting that sample lines delay the reading; using a detector without the correct area certification.