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Gas Detection in a Confined Space


What is a confined space?
A space, room or building is called a confined space when there is a possible presence of the following;
>Flammable airborne contaminants
>Toxic airborne contaminants
>Hazardous level of Oxygen

Examples of a confined space include:
  • Storage tanks and vessels
  • Sewers and manholes
  • Agriculture silos
  • Marine vessel tanks
  • Tunnels

Monitoring confined spaces for Atmospheric Hazards
Monitoring the air inside a confined space is required prior to entering.
Testing a confined space for atmospheric hazards should be done remotely before entering and it is recommended that it is done in this order:  
Remember – O.C.T- for Oxygen / Combustible Gases / Toxic Gases


Ensure that proper oxygen levels are present.

Potential Effects of Oxygen Enriched and Deficient Atmospheres
Oxygen Content
(% by Vol.)
Effects and Symptoms (At Atmospheric Pressure)
> 23.5 %
Oxygen enriched, extreme fire hazard
20.9 %
Oxygen concentration in normal air
19.5 %
Minimum permissible oxygen level
15 % to 19 %
Decreased ability to work strenuously; may impair coordination and may cause early symptoms for persons of coronary, pulmonary or circulatory problems
10 % to 12 %
Respiration further increases in rate and depth; poor judgment, blue lips
8 % to 10 %
Mental failure, fainting, unconsciousness, ashen face, nausea, and vomiting
6 % to 8 %
Recovery still possible after four to five minutes.
50% fatal after six minutes.
Fatal after eight minutes.
4 % to 6 %
Coma in 40 seconds, convulsions, respiration ceases, death
These values are approximate and may vary with each individual


Ensure that combustible gases are not present.

The lowest concentration (air-fuel mixture) at which a gas can ignite is called lower explosive limit (LEL). Concentrations below this limit are too lean to burn.
The highest concentration that can be ignited is its upper explosive limit (UEL). Above that concentration, the mixture is too rich to burn.
Most confined space entry gas detectors measure in the lower explosive (LEL) range.


Ensure that toxic gases are below the permissible workplace exposure limits.

Common toxic gases in a confined space are hydrogen sulphide (H2S) and carbon monoxide (CO), but other toxic compounds could be present.  
Check New Zealand’s workplace exposure standards

In a confined space it is important to take samples at the top, middle and bottom to locate varying concentrations of gases and vapours. Highly concentrated gases can accumulate at the top or bottom of a confined space depending on whether they are heavier or lighter than air.
Dilute gases and vapours in the ppm (parts per million) range distribute evenly throughout a confined space.   
The ABC of Gases in Industry
Courtesy of Rae Systems Application Note AP-202
Talk to Teltherm about Rae systems wireless gas detection solutions
Ammonia: NH3 n-Butane*: C4H10
   Colorless gas with a pungent suffocating odor
Fertilizer Plants
Fish & Meat Packing Plants
Industrial Refrigeration and Cold Storage
Semiconductor Industry
Water and Wastewater Treatment Plants
   Colorless gas with a gasoline-like odor
Aerosol Propellant Filling Docks
Feed stock for Chemical Processes
Storage Tanks and Filling Docks
Benzene: C6H6 Carbon Dioxide: CO2
   Colorless liquid with aromatic odor
Oil & Gas Distribution
Feed Stock for Chemical Production
Solvent Distribution Centers
Gas Stations
   Colorless, odorless gas
Breweries and Wineries
Carbonated Beverage Bottling Plants
Dry Ice Plants, Food Processing Plants, Fruit Storage and Ripening Chambers
Greenhouses, Indoor Air Quality Studies, and Ventilation Control
Mushroom Farms Stack Gas
Oil Well Injection
Carbon Monoxide: CO Chlorine: Cl2
   Colorless, odorless gas - most abundant toxic gas
Gasoline Generators/Engines
Grain Storage Silos
Lumber Drying Kilns
Mining and Metals
Parking Garages
   Green-yellow gas with a pungent, irritating odor
Mining & Metals Industry
Nuclear Reactors
Pulp & Paper Mills
PVC Plastics Manufacturing
Semiconductor Water Etching Facilities
Swimming Pool Chlorinization Plants
Water Treatment Plants
Chlorine Gas Manufacturing
Hydrogen Cyanide: HCN Hydrogen Sulfide: H2S  
   Colorless gas with a bitter, almond-like odor
Plating and Mining
Nylon Manufacturing
   Colorless gas with a strong odor of rotten eggs
Leather Tanneries and Paper Mills
Mining and Metals Industry
Oil Fields and Refineries
Sewage Treatment Plants
Sewer Maintenance
Methane: CH4 Nitric Oxide: NO  
   Colorless, odorless gas – odorized with mercaptans
Primary component of natural gas
No exposure limits – Simple asphyxiant
Oil & Gas Distribution & Refining
Mining Industry
   Colorless gas
Semiconductor Plants
Nitrogen Dioxide: NO2 Oxygen Deficiency: O2  
   Reddish-brown with a pungent odor
Boilers and Furnaces
Diesel Emissions
Semiconductor Plants
Mining Industry
   Colorless, odorless gas
Cargo Holds and Storage Tanks
Grain Storage Silos (Inerted Atmospheres)
Liquid Nitrogen Storage
Liquid Nitrogen Cooled Laser Facilities
Liquid Nitrogen Cooled Telescope
Sewer Maintenance
Sewer Treatment Facilities
Underground Vaults (Utilities)

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