Atmospheric testing confirms, before and during entry, that the air in a confined space has enough oxygen and does not contain flammable or toxic gases at dangerous levels. It must be done by a trained person with a working, bump-tested detector, from outside the space first, at every level where gases could collect. Your senses cannot do this job: many dangerous gases have no smell, or stop being noticed at high concentrations.
What to test for
The hazard assessment for each space lists the gases to test for. Most programs test at least the following:
- Oxygen. Too little oxygen can cause collapse without warning. Too much oxygen greatly increases the risk of fire.
- Flammable gases and vapours. Measured as a percentage of the lower explosive limit (LEL), the lowest concentration at which a gas can ignite.
- Toxic gases. Commonly hydrogen sulphide and carbon monoxide, plus any others specific to the space, such as ammonia, chlorine, sulphur dioxide or solvent vapours.
A four-gas detector covering oxygen, LEL, hydrogen sulphide and carbon monoxide is the usual starting point, but it is not enough if the space can contain other gases. Some vapours need different sensor types or separate instruments.
Acceptable limits
Your province sets the acceptable range for oxygen, the maximum percentage of the LEL allowed for entry and work, and occupational exposure limits for toxic substances. These numbers are not identical across Alberta, British Columbia, Ontario and other provinces, and some limits are updated over time. Put the limits that apply to your sites on the permit form so the tester can compare readings directly, and check them against the current legislation when you review your program.
The testing sequence
A widely used order is oxygen first, then flammables, then toxics. There are practical reasons. Many flammable gas sensors need adequate oxygen to give an accurate reading, and an oxygen-deficient result tells you something has displaced the air. Modern multi-gas detectors read all sensors at once, but the tester should still interpret the results in that order.
- Confirm the detector has been bump tested and is within its calibration date.
- Turn it on and let it complete start-up in fresh air away from the space.
- Test from outside the space using a sampling pump and probe or hose. Do not enter to test.
- Sample at the top, middle and bottom of the space, and in corners or areas where gas could pocket.
- Allow enough time at each point for the sample to travel through the hose and the sensors to respond, following the manufacturer's guidance.
- Record each reading, its location, the time and the tester's name on the permit.
Why sampling height matters
Gases separate in still air. Hydrogen sulphide and many solvent vapours are heavier than air and collect near the bottom. Methane is lighter and rises. Carbon monoxide is close to the density of air and can be found throughout. A single reading at the hatch can miss a lethal layer a metre below. Testing at several levels, and repeating after ventilation has run for a while, gives a truer picture.
Ventilation can hide a problem. Test before ventilating to understand what the space contains, then test again with ventilation running. If readings rise when the fan stops, there is an ongoing source and entry should be reconsidered.
Continuous monitoring
Pre-entry tests describe the space at one moment. Conditions change when sludge is disturbed, when hot work or coatings begin, when a valve leaks or when ventilation fails. For this reason, many programs require continuous monitoring with a detector in the breathing zone of each entrant, and some provinces require it in certain conditions. The attendant should also be able to see or hear the readings or alarms. Periodic re-testing at set intervals is the minimum where continuous monitoring is not used.
Detector care and limitations
- Bump test before each day's use, or as the manufacturer and your program require, and calibrate on schedule. See Gas detector bump testing and calibration.
- Know your sensor limits. Catalytic LEL sensors can be damaged or misled by silicones, some sulphur compounds and low oxygen.
- Respect alarm set points and never silence or disable an alarm to keep working.
- Keep sampling hoses clean and intact, and check for leaks that would dilute the sample.
- Train every tester on the specific model in use and on what to do with each alarm.
What to do when readings are out of range
Nobody enters. If people are already inside, they leave immediately. The supervisor then finds the cause: incomplete isolation, residues, a leaking line, inadequate ventilation or a detector fault. Entry resumes only after the cause is corrected, the space is re-tested and the permit is updated. If a worker inside collapses, the attendant calls the rescue team and does not enter. Rescue planning is covered in Confined space rescue planning.
How ehZee can help
We review which gases each space can contain, the detectors you use and how your testers record results, and we write testing procedures into your permit system. Learn more on our confined space programs page.
Common questions
Can we rely on smell to detect hydrogen sulphide?
No. Hydrogen sulphide has a rotten egg smell at low levels, but at higher concentrations it quickly deadens the sense of smell, so the danger can seem to disappear just as it becomes life-threatening.
Who is allowed to test the atmosphere?
A worker trained in the detector's use and in interpreting the results, as required by your program and province. The tester should understand alarm points and what to do when one is reached.