Nancy Pearce

Selecting the right gas monitor for confined space entry

Blog Post created by Nancy Pearce Employee on Feb 4, 2013

Gas monitoring with proper equipment is probably the number one most important step you can take to protect against confined space fatalities. But selecting the right gas monitor for entry is not without its challenges.   Gas monitoring for confined space entry can be straightforward or it can be complicated, depending on the type of space being entered and the work that will be performed.  All gas monitoring for confined space entry must include tests for both oxygen (%) and flammables (% LEL).   Beyond these two tests, it is up to the employer to determine what other toxic gas monitoring needs be done.    A typical four gas meter will include carbon monoxide and hydrogen sulfide as the additional toxic gases.  But confined space entry is not a “one size fits all” type of operation.  If you simply use the “standard” four gas meter, and do not carefully consider the need for monitoring other potentially toxic gases that may be present, you may not obtain the results you need to determine if the space is safe for entry.  The error could lead to catastrophic results.

All toxic gases that are likely to be in the space should be identified prior to entry.  But determining what these toxic gases may be requires some detective work.  Not only do you need to know what was in the space previously, but you also need to know what toxic gases may be of concern as a result of the work that will be done in the space.   Reactions between materials and byproducts further complicates the identification. If there is decaying material in the bottom of the tank, you may need to evaluate byproducts of the decay such as hydrogen sulfide.  If there were chemicals previously stored in the tank, the vapors may not be released initially, however residual vapor pockets in the bottom of the tank may release trapped vapors when stirred up during entry or cleaning.   Work such as welding on stainless steel can lead to exposures to hexavalent chromium and nickel, both carcinogenic agents.  All potential sources of atmospheric hazards must be considered and then evaluated using a careful selected monitoring device.   

If available, Material Safety Data Sheets (MSDSs or SDSs) on products previously stored in a confined space can provide useful information on toxicity, flammability, reactivity and decomposition products.   Material Safety Data Sheets can also be reviewed for all materials that will be brought INTO the space such as cleaning chemicals or welding gases. Chemical ingredients that are listed on the MSDS and have OSHA or other recognized exposure limits must be tested and compared to these limits.  Direct reading sensors that can be incorporated into the gas monitor may not be commercially available for all contaminants that you need to evaluate.   Other test methods such as photoionization detectors or colorimetric detector tubes may be needed to screen for the presence of these toxic materials.   Photoionization detectors can provide generic information on the presence of volatile organic compounds (VOCs) however they can only be used as a sort of screening tool for the presence of VOCs unless you know exactly what contaminant you are measuring.    Detector tubes that change color in proportion to the air contaminant concentration can also be used.  Detector tubes are not particularly accurate (+/-25 % error is typical) but can provide some initial screening for the presence of a particular toxic material and an estimate of the amount present. 

A comprehensive identification of the toxic materials expected to be present in a confined space, and then selection of the proper gas monitor by a qualified individual, is necessary to insure that the proper atmospheric tests have been done prior to and during entry.  However, there are some contaminants that cannot be easily evaluated by gas monitoring of any type.   Properly placed continuous ventilation can provide dilution of all contaminants within a confined space.  It is critical that entrants are always attached to non-entry rescue equipment and are wearing breathing air while entering a confined space where there may be unknown or “unmeasured” toxic atmospheres.  

The National Fire Protection Association is developing aBest Practices document for confined space entry. This document will address gaps in existing standards and will be more prescriptive in describing things like how to identify potentially toxic atmospheres and select the proper gas monitor for entry.    This is a document that is looking to go beyond the minimum standards and will provide those looking to develop a “gold star” confined space entry program with the information they need to do so.  If you have any suggestions for what you would like to see in this type of document related to gas monitoring or other issues you think should be included in a best practices document,  please email me at npearce@nfpa.org and/or leave a comment below for discussion.  I look forward to hearing from you!

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