Sensor Types and Their Limits
Every sensor has a blind spot; knowing it is the point.
13 min
| Type | Measures | Key limitation |
|---|---|---|
| Catalytic bead (pellistor) | Flammable gas, %LEL | Needs oxygen to work; poisoned by silicones, sulphides and lead; can read low or zero in very rich atmospheres |
| Infrared | Flammable gas and CO₂ | Works without oxygen and is not poisoned, but cannot detect hydrogen |
| Electrochemical | Toxic gases and oxygen, in ppm | Cross-sensitive to other gases; limited life; affected by temperature and humidity |
| Photoionisation (PID) | Volatile organic compounds, ppm | Does not detect methane; affected by humidity; needs correction factors per compound |
| Paramagnetic | Oxygen | Sensitive to vibration and position |
Two traps that have killed people
- Catalytic beads need oxygen. In an inerted or oxygen-deficient atmosphere a pellistor can read low or zero while the atmosphere is thoroughly flammable. If oxygen is depleted, the flammable reading is not trustworthy.
- Over-range roll-over. In an atmosphere above the upper explosive limit, a pellistor can rise off scale and then fall back toward zero as the sensor is starved of oxygen. A falling reading is not always good news.
This is why testing order matters: oxygen first, then flammable, then toxic.