Atmospheric testing in permit required confined spaces is critical for employee safety and health during entry. OSHA requires under the Permit Required Confined Space Standard (29 CFR 1910.146) that we test for oxygen, LEL, and toxic chemicals prior to and periodically during entry. I have noticed that many employers are concerned about what to test for and how to test. Or they may think that using a confined space monitor covers every issue. It may not be good enough to just use a three or four gas monitor to test the confined space prior to entry.
How do we know what to test for? Think about the following:
- What is or has been inside the tank? Think about products, raw materials, gases produced in decomposition, nitrogen purges, etc.
- What is near the confined space? Think about vehicle traffic, work practices like hot work performed nearby, and other processes producing gases and vapors that may be drawn into the confined space. Also consider where your ventilation air is being drawn from and what may contaminate that air.
- What work will be performed inside the confined space? Think about welding and other hot work, by-products of combustion, use of chemicals like degreasers, and chemical reactions that could occur inside the tank based on what is in the confined space and what is being brought in. Be aware that hot work in a confined space can consume oxygen faster that breathing.
Now you are ready to test the confined space. Your standard three gas monitor will usually have sensors for oxygen, LEL, and carbon monoxide or hydrogen sulfide. You may have a four gas monitor that has sensors for all four of these. Review the list above and make sure that you have considered all of the actual and potential atmospheric hazards that your employees or contractors may encounter in the confined space. You may need to add additional monitoring to cover all the bases and ensure that employees and contractors are protected.
You should test prior to entry and periodically (at least hourly unless conditions change) during the entry. Where possible, have the entrant wear a personal monitor inside the confined space so that continuous testing is possible. You should also ensure that your ventilation is not pulling contaminated air into the confined space. Test these areas also if necessary to ensure that the ventilation is effective.
Air does not always mix well inside a confined space. You should test at the top, middle, and bottom of the confined space. When you only test the top of a confined space, you may not discover toxic conditions that exist elsewhere in the space. Ventilation can also help you avoid these situations.
Another thing to remember is that the monitoring equipment is only accurate when used by qualified individuals who fully understand how to use it and when it is properly calibrated. You should always follow the manufacturer’s recommendations on use and calibration of these instruments.