4 Gas Monitors Explained: Which Gases, Which Monitor, and How to Keep It Compliant

The 4 gas monitor is the most widely used portable gas detector in UK industry. You will find a 4 gas detector clipped to workers in sewers, tanks, utility chambers, ship holds and plant rooms. This guide explains which gases a standard 4 gas monitor measures, how to choose the right one, and what is needed to keep it compliant.

What is a 4 gas monitor?

A 4 gas monitor is a portable detector fitted with four sensors, chosen to cover the most common atmospheric hazards found in confined spaces and industrial work areas. A typical 4 gas monitor measures:

• Oxygen (O2): alarms on both oxygen deficiency and oxygen enrichment

• Flammable gases (LEL): measured as a percentage of the lower explosive limit, usually referenced to methane

• Carbon monoxide (CO): a toxic gas produced by combustion, engines and some industrial processes

• Hydrogen sulphide (H2S): a toxic gas commonly associated with sewage, decaying organic matter and some process industries

Because these four hazards cover so many confined space scenarios, a 4 gas detector is often simply called a confined space monitor.

Choosing the right 4 gas detector

Most 4 gas monitors measure the same four gases, but the instruments differ in ways that matter on site.

Diffusion or pumped

Diffusion monitors sample the air around the wearer and are ideal as personal monitors. Pumped monitors, or diffusion monitors with a pump attachment, draw a sample through a tube so the atmosphere in a confined space can be tested from outside before anyone enters. If your work involves pre-entry testing, make sure the monitor supports sampling.

LEL sensor type

Most 4 gas monitors use catalytic LEL sensors, while some use infrared sensors. Each has strengths for different gases and environments, so it is worth checking that the sensor is suitable for the flammable gases you expect.

Practical features

Consider battery life against your shift length, how robust and water resistant the instrument is, whether it has datalogging, and whether it can be used with a docking station for automated bump testing and calibration. Some models are maintenance-light with fixed lifespans, while others are fully serviceable with replaceable sensors.

When four gases are not enough

If your risk assessment identifies other hazards, such as volatile organic compounds, ammonia or chlorine, you may need a multi-gas monitor with additional sensors, or a PID monitor for VOCs. Where only one gas is a concern, a single gas monitor may be enough.

Keeping your 4 gas monitor compliant

A 4 gas monitor is only as reliable as its maintenance. Most manufacturers recommend:

• A bump test before each day's use, to confirm the sensors respond and the alarms work

• Full calibration at least every six months, or more often if your risk assessment or the manufacturer requires it

• Calibration after a failed bump test, exposure to high gas concentrations, or damage

The Confined Spaces Regulations 1997 require a risk assessment for work in confined spaces, and where atmospheric monitoring forms part of your safe system of work, the monitor needs to be properly maintained. A dated calibration certificate for each instrument is the simplest evidence of that. Our article on calibration and bump testing covers the difference between the two checks.

Buy, hire or service a 4 gas monitor with 7 Solutions

We supply 4 gas monitors for purchase, and for short projects or to cover instruments away for service, we offer 1 week hire of a 4-gas confined space monitor.

Our manufacturer-trained engineers calibrate and service 4 gas monitors from Honeywell (BW and RAE), Crowcon, Dräger, MSA, Industrial Scientific, WatchGas, GMI, Oldham and more. Calibration of any 4-gas confined space monitor costs £50 plus VAT, and every instrument comes back with a calibration certificate. Send instruments to our Hampshire service centre in Waltham Chase, Southampton, or book an on-site visit; our Northern Service Centre covers the North East and Yorkshire.

Frequently asked questions

What gases does a standard 4 gas monitor detect?

Typically oxygen, flammable gases (LEL), carbon monoxide and hydrogen sulphide. Other sensor combinations are available, so always check the specification.

Is a 4 gas monitor the same as a confined space monitor?

In most cases, yes. The term confined space monitor usually refers to a 4 gas detector, often with a pump for pre-entry sampling.

How often should a 4 gas detector be calibrated?

Most manufacturers recommend calibration at least every six months, with a bump test before each day's use.

Can you calibrate any brand of 4 gas monitor?

We calibrate a wide range of brands. See gas detector brands we calibrate and repair or get in touch to check your model.

Talk to us about 4 gas monitors

Whether you need to buy, hire or calibrate a 4 gas monitor, call 01489 326031, email info@7solutions.co.uk or book a calibration through our customer portal at portal.7solutions.co.uk.