4-Gas Monitor Essentials: Understanding O2, H2S, CO, and LEL

↠Part of the Series: Confined Space Entry 2026: The Essential Safety & Equipment Compliance Guide

4-Gas Monitor Essentials: Understanding O2, H2S, CO, and LEL

The standard "Personal Monitor" on a UK site is a 4-Gas unit (like the Crowcon T4 or BW Ultra). But what are those numbers on the screen? And when should you run?

1. Oxygen (O2)

Normal: 20.9%
Alarm Low: 19.5% (Suffocation risk).
Alarm High: 23.5% (Fire risk).
The Danger: Rust inside a steel tank consumes oxygen. You enter, pass out, and never wake up.

2. Hydrogen Sulfide (H2S)

Source: Rotting sewage.
Smell: Rotten eggs (at low levels).
The Trap: At high levels (100ppm), H2S paralyses your sense of smell. You think the smell has gone. It hasn't. You are about to die.
Limit: 5ppm (Short Term Limit).

3. Carbon Monoxide (CO)

Source: Petrol generators, vehicle exhaust.
Danger: It binds to your blood cells instead of oxygen.
Limit: 30ppm.

4. LEL (Flammables/Methane)

Measurement: % LEL (Lower Explosive Limit).
Meaning: If the metre reads 100% LEL, the air is now explosive. A spark will kill you.
Alarm: usually at 10% or 20% LEL. If it goes off, get out immediately.

Conclusion

Respect the beep. If your alarm sounds, don't "finish the job". Evacuate.

---

Why Professional Equipment Matters

In the field of utility surveying and safety, "cheap" equipment is arguably the most expensive mistake you can make. False readings leading to a cable strike, or a failed gas monitor in a sewer, can cost lives and millions in liability.

At Cable Locators & Survey, we stock only the verified industry standards Radiodetection, C.Scope, Abtech, and Leica. Every unit is checked, calibrated, and field-ready.

Competence & Training: The Forgotten Variable

You can buy the most expensive equipment on the market, but if the operator is untrained, it is a paperweight. Health & Safety guidance HSG47 makes it clear: equipment must be used by competent people.

We recommend a tiered training approach:

• Level 1 (Basic): Manufacturer-led familiarisation (turning it on, self-tests).
• Level 2 (User): EUSR or CITB recognised courses for Genny usage and swing technique.
• Level 3 (Manager): Data log analysis and permit-to-dig management.

Don't just tick the box ensure your team understands the physics behind the beep.

Looking After Your Lifeline

Gas Monitor Sensors

The sensors in your 4-gas monitor are chemical sponges. They can be "poisoned" by specific contaminants. Avoid using silicone sprays, WD40, or cleaning solvents near the unit, as these can permanently coat the LEL sensor, rendering it blind to methane.

Winch Cable Inspection

Pull out the full length of your fall arrest winch cable weekly. partial deployment often leads to "bird-nesting" inside the drum. Run it through a gloved hand to check for broken wire strands (meat hooks).