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Confined Space Entry – Atmospheric Testing

The atmosphere within a permit-required confined space must be tested prior to entry and when the space is vacated for short periods.  

The atmosphere must be tested in the following order:

  • Oxygen.
  • Combustible Gases (LEL or LFL).
  • Toxic gases and vapors. Examples of toxic gases or vapors may include carbon monoxide (CO), phosphine (PH3), ammonia (NH3), and hydrogen sulfide (H2S), to name a few. 

The testing results are documented on the permit near the levels identified for safety entry.

It is important to allow enough time for the air from the space to be drawn into the equipment or over the sensors during atmospheric testing.  

This is referred to as the minimum response time. The monitor’s manufacturer will note the “minimum response time” in the user manual. 

The user manual may require additional response time, if tubing or a probe extension are attached to the air inlet or intake.  

The additional time is needed to allow air from the different depths of the space to enter the equipment’s air inlet.

Some permit spaces have significant depth, while others have remote areas that lead away from the entry point.  

Some gases are heavier or lighter causing the atmosphere to stratify or form layers.  

In these instances, testing must be done in the area where the worker will be positioned and the areas where he or she may move.  

This is considered four feet in the direction of travel and to each side of the entrant.  

Sampling probes or remote monitoring allow the monitor’s tubing inlet to be positioned in each area or layer.   

If a sample probe is used for testing, the worker must move slowly and keep the equipment’s response time in mind.


Source: Joe Mlynek is president of Progressive Safety Services LLC, Gates Mills, OH;,and content creation expert for Safety Made Simple Inc., Olathe, KS;


This Safety Tip of the Week is sponsored by:
M&M Specialty Services, LLC



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Safety Tip of the Week is edited by Managing Editor Tucker Scharfenberg and published each Monday by Grain Journal, Decatur, IL

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