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4 Gas Monitors

A 4-gas monitor measures the four hazards most often found in confined spaces: flammable gas, oxygen, hydrogen sulphide and carbon monoxide. It is the standard instrument for sewers, manholes, chambers, tanks, vessels and utility pits.

 

Why these four gases

  • LEL (flammable gas): methane from sewers and landfill, fuel vapours and other flammables, measured as % of the lower explosive limit.

  • Oxygen: deficiency from rusting, biological activity or inert gases, and enrichment from leaking oxygen supplies.

  • Hydrogen sulphide: from decaying organic matter in sewers, water treatment and tanks; UK limits 5 ppm (8-hour) and 10 ppm (15-minute).

  • Carbon monoxide: from engines, combustion and generators; UK limits 20 ppm (8-hour) and 100 ppm (15-minute).

 

Typical alarm set points

  • LEL: low 10% LEL, high 20% LEL

  • Oxygen: low 19.5% vol, high 23.5% vol

  • H2S: low 5 ppm, high 10 ppm, plus TWA and STEL alarms

  • CO: low 20 ppm, high 100 ppm, plus TWA and STEL alarms

These are common settings; your risk assessment decides the final values, and we set alarms to your requirements before dispatch.

 

Pumped or diffusion

Diffusion monitors sample the air around the wearer and suit continuous personal monitoring. Pre-entry testing needs a pump (built in or as a module), with tubing long enough to reach the bottom of the space. Allow time for the sample to travel through the tubing before reading.

 

Disposable or serviceable

Two-year maintenance-free 4-gas monitors avoid calibration costs and suit fleets where units are lost or damaged. Serviceable rechargeable monitors take pumps, have replaceable sensors, and over a longer life usually cost less per year. Either way, bump test daily.

 

Docking and bump test stations

For larger fleets, a docking station bump tests and calibrates automatically and keeps the records your auditors ask for. We can advise on stations for the brands you use.

 

Compliance

The Confined Spaces Regulations 1997 require a risk assessment, a safe system of work, which normally includes atmospheric testing, and emergency arrangements before anyone enters. See confined space gas detection for the pre-entry testing sequence.

 

Buy, hire or calibrate

 

How to choose a 4-gas monitor

  • Run time: check the monitor lasts a full shift with the pump running, including in cold weather.

  • Display and alarms: a clear screen, loud audible alarm and bright visual alarm matter when wearing PPE.

  • Robustness: look for an IP rating suitable for wet and dusty sites.

  • Data logging: event and exposure logs help investigations and audits.

  • Running cost: compare sensor life, calibration cost and whether a docking station is available.

 

Using a 4-gas monitor correctly

Switch the monitor on and let it zero in clean air, bump test it, and wear it in the breathing zone. For confined space entry, sample with a pump and tube before entry, at the top, middle and bottom of the space, allowing the extra sampling time the manufacturer specifies for the length of tube. Keep it running for the whole time people are inside.

 

Bump testing and calibration

A bump test exposes the detector briefly to a known gas to prove that the sensors respond and the alarms sound. It takes about a minute and should be done before each day's use. A full calibration adjusts each sensor against certified gas and is normally needed at least every six months, or sooner if a bump test fails, the monitor has been dropped or it has seen a high exposure.

Our calibration service covers any brand of portable or fixed detector at standard rates by instrument type: from £40 for a single gas monitor, £50 for a 4-gas monitor, £90 for a multigas monitor and £140 for a PID. Each instrument comes back with a traceable calibration certificate.

 

Training your team

A detector only protects people who know how to use it. Our gas detection training covers start-up checks, bump testing, reading alarms and what to do when one sounds, plus pre-entry testing for confined spaces. We run on-site courses anywhere in the UK from £750 a day, or a three-hour online session from £350.

Talk to our engineers on 01489 326031 or email sales@7solutions.co.uk. We supply, hire, calibrate and train across the UK.

 

Frequently asked questions

What gases does a 4-gas monitor detect?

Flammable gas as a percentage of the lower explosive limit (LEL), oxygen (O2), hydrogen sulphide (H2S) and carbon monoxide (CO).

Do I need a pumped 4-gas monitor?

For pre-entry testing of a confined space, yes. A pump draws a sample through tubing from the bottom, middle and top of the space before anyone enters. A diffusion monitor is fine for continuous personal monitoring once inside.

What are typical 4-gas alarm settings?

Common settings are 10% and 20% LEL for flammables, 19.5% and 23.5% for oxygen, 5 ppm and 10 ppm for H2S, and 20 ppm and 100 ppm for CO, with TWA and STEL alarms based on EH40. Your risk assessment may require different levels.

Disposable or rechargeable?

Two-year disposable 4-gas monitors need no calibration or sensor changes but still need bump testing. Rechargeable serviceable monitors cost more up front but last longer, accept pumps and can be repaired.

How often does a 4-gas monitor need calibrating?

Typically every six months, with a bump test before each day's use. We calibrate any brand of 4-gas monitor for £50.

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