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What are gas monitors?

Gas monitors measure levels of gases that are normally present and are installed to warn of raised levels of gases for example carbon dioxide or reduced levels of oxygen.

What are gas detectors?

Gas detectors are installed to warn of the presence of a gas that is not normally present in air in significant quantities, such as hydrogen.

Where should the monitor or detector be installed?

In general, if the gas is lighter than air, then the monitor or detector should be mounted well above the breathing zone, and if it's heavier than air, it should be mounted well below the breathing zone. (The breathing zone is defined as within a 10-inch radius of the worker’s nose and mouth.)

Repeater unit

Repeater Unit

A repeater unit can be mounted outside the room to mimic the alarm and warn against entry.

What should you do if it keeps sounding?

If the monitor or detector alarm sounds frequently during routine operations DO NOT SWITCH IT OFF. Review the system of work and the location of the monitor or detector.

On all occasions when the alarm sounds, the reason for the alarm and the actions that were taken to rectify the situation should be recorded. RiskNET UCL's incident reporting system should be used for this purpose.


The risk assessment and the resultant arrangements for using compressed gases must take account of the possibility of the alarm sounding out-of-hours and the consequences of for example security staff entering areas of low oxygen when investigating the reason for the alarm. Other vulnerable groups would be cleaning and maintenance staff.

do not enter

A sign must be displayed on the door describing the action to take if the monitor or detector alarm is sounding. i.e. 'do not enter if alarm is sounding'. Include the contact details of who should be told if the alarm is sounding.



Monitors and detectors should be calibrated, maintained and serviced at intervals specified by the manufacturer and resources (funding) should be allocated for this purpose.

And finally always remember! Gas monitors and detectors are not an alternative to the provision of a safe system of work but are an additional measure, which give an early warning that something has gone wrong.




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