This infographic is a perfect cheat-sheet for gas detection — one of the most critical controls in HSE.

 This infographic is a perfect cheat-sheet for gas detection — one of the most critical controls in HSE.

It boils down to one idea at the top: The right detector, the right gas, the right place, at the right time.

What is a gas detector?

A safety device that continuously senses what gases are in the air and how much, then alerts you when levels become dangerous.

It helps to:

Prevent exposure to toxic gases

Avoid fire and explosion risks

Protect life and property

The 5 Types of Gas Detectors

1. Fixed Gas Detectors

Installed permanently in one location. They monitor 24/7 and send alarms to a control system or sound a local alarm.

Commonly detects: Combustible gases (LEL), Toxic gases like CO, H2S, SO2, and Oxygen (O2)

Example: A fixed detector on a wall in a plant or outside a confined space.

2. Portable Gas Detectors

Handheld devices you carry for personal protection and spot checks. They alert with audible, visual, and vibration alarms.

Commonly detects: Same as fixed — LEL, toxic gases, O2

Example: The handheld 4-gas detector you see workers wear entering a vessel — usually O2, LEL, CO, H2S.

3. Single Gas Detectors

Designed to detect just ONE specific gas. Simple, compact, cost-effective.

Commonly detects: CO, H2S, O2, or LEL individually

Example: CO single gas detector clipped to a worker near boilers or engines.

4. Multi-Gas Detectors

Detects multiple gases at the same time. This is the standard for high-risk environments.

Commonly detects: LEL + Toxic gases + O2 together

Example: 4-gas multi-detector (O2, LEL, CO, H2S) — mandatory for confined space entry, refineries, and chemical plants.

5. Diffusion and Pumped (Draw) Detectors

This is about how the gas gets to the sensor.

Diffusion type: Uses natural air movement to reach the sensor. Simple, no pump, but shorter detection range.

Pumped (Draw) type: Uses a small pump with a probe to pull air from a distance. Longer reach, perfect for checking tanks, pits, and ducts before you enter.

Example: Diffusion personal monitor vs. pumped detector with a probe you lower into a manhole.

Key Gases Detected & Their Definitions

COMBUSTIBLE GASES (LEL): Gases that can ignite and cause fire or explosion. Measured in % LEL — Lower Explosive Limit. e.g. Methane, Propane, Butane.

TOXIC GASES: Harmful gases that can cause serious health effects or death even at low levels. e.g. CO, H2S, SO2, NH3, Cl2.

OXYGEN (O2): Normal air is 20.9%. Too low is dangerous, too high is dangerous. Deficiency is <19.5% — risk of asphyxiation. Enrichment is >23.5% — risk of fire.

CARBON MONOXIDE (CO): Colourless, odourless, very toxic gas produced by incomplete combustion. e.g. engines, boilers, fires.

HYDROGEN SULPHIDE (H2S): Toxic, flammable gas with a rotten egg smell at low levels, but you lose the ability to smell it at high levels. Very common in sewage, oil & gas, and refineries.

The bottom line of the poster says it all: DETECT GASES | TAKE ACTION | STAY SAFE — Small device... Big difference!

In practice, you choose based on the job: fixed for continuous area monitoring, portable/multi-gas for personal entry, single-gas for a known specific hazard, and pumped for remote sampling before entry.